Re: [PATCH, RFC] v7 scalable classic RCU implementation

[Paul E. McKenney]

On Fri, Oct 17, 2008 at 02:04:52PM +0530, Gautham R Shenoy wrote:
> On Fri, Oct 10, 2008 at 09:09:30AM -0700, Paul E. McKenney wrote:
> > Hello!
> Hi Paul,
> 
> Looks interesting. Couple of minor nits. Comments interspersed. Search for "=>"

Thank you for looking this over, and especially for noting several issues!

Responses interspersed.

							Thanx, Paul

> > This patch fixes a long-standing performance bug in classic RCU that
> > results in massive lock contention on the internal RCU lock on systems
> > with more than a few hundred CPUs.  Although this patch creates a
> > separate flavor of RCU for easy of review and patch maintenance, it
> > is intended to replace classic RCU.
> > 
> > Still experimental, not for inclusion, but getting quite close.  I expect
> > to have it in shape for 2.6.29.  Definitely ready for -serious- testing
> > and abuse.  In particular, experience on an actual 1000+ CPU machine
> > would be most welcome, and still appears to be forthcoming...
> > 
> > Updates from v6 (http://lkml.org/lkml/2008/9/23/448):
> > 
> > o	Fix a number of checkpatch.pl complaints.
> > 
> > o	Apply review comments from Ingo Molnar and Lai Jiangshan
> > 	on the stall-detection code.
> > 
> > o	Fix several bugs in !CONFIG_SMP builds.
> > 
> > o	Fix a misspelled config-parameter name so that RCU now announces
> > 	at boot time if stall detection is configured.
> > 
> > o	Run tests on numerous combinations of configurations parameters,
> > 	which after the fixes above, now build and run correctly.
> > 
> > Updates from v5 (http://lkml.org/lkml/2008/9/15/92, bad subject line):
> > 
> > o	Fix a compiler error in the !CONFIG_FANOUT_EXACT case (blew a
> > 	changeset some time ago, and finally got around to retesting
> > 	this option).
> > 
> > o	Fix some tracing bugs in rcupreempt that caused incorrect
> > 	totals to be printed.
> > 
> > o	I now test with a more brutal random-selection online/offline
> > 	script (attached).  Probably more brutal than it needs to be
> > 	on the people reading it as well, but so it goes.
> > 
> > o	A number of optimizations and usability improvements:
> > 
> > 	o	Make rcu_pending() ignore the grace-period timeout when
> > 		there is no grace period in progress.
> > 
> > 	o	Make force_quiescent_state() avoid going for a global
> > 		lock in the case where there is no grace period in
> > 		progress.
> > 
> > 	o	Rearrange struct fields to improve struct layout.
> > 
> > 	o	Make call_rcu() initiate a grace period if RCU was
> > 		idle, rather than waiting for the next scheduling
> > 		clock interrupt.
> > 
> > 	o	Invoke rcu_irq_enter() and rcu_irq_exit() only when
> > 		idle, as suggested by Andi Kleen.  I still don't
> > 		completely trust this change, and might back it out.
> > 
> > 	o	Make CONFIG_RCU_TRACE be the single config variable
> > 		manipulated for all forms of RCU, instead of the prior
> > 		confusion.
> > 
> > 	o	Document tracing files and formats for both rcupreempt
> > 		and rcutree.
> > 
> > Updates from v4 for those missing v5 given its bad subject line:
> > 
> > o	Separated dynticks interface so that NMIs and irqs call separate
> > 	functions, greatly simplifying it.  In particular, this code
> > 	no longer requires a proof of correctness.  ;-)
> > 
> > o	Separated dynticks state out into its own per-CPU structure,
> > 	avoiding the duplicated accounting.
> > 
> > o	The case where a dynticks-idle CPU runs an irq handler that
> > 	invokes call_rcu() is now correctly handled, forcing that CPU
> > 	out of dynticks-idle mode.
> > 
> > o	Review comments have been applied (thank you all!!!).
> > 	For but one example, fixed the dynticks-ordering issue that
> > 	Manfred pointed out, saving me much debugging.  ;-)
> > 
> > o	Adjusted rcuclassic and rcupreempt to handle dynticks changes.
> > 
> > Attached is an updated patch to Classic RCU that applies a
> > hierarchy, greatly reducing the contention on the top-level lock
> > for large machines.  This passes 10-hour concurrent rcutorture and
> > online-offline testing on 128-CPU ppc64 without dynticks enabled,
> > and exposes some timekeeping bugs in presence of dynticks (exciting
> > working on a system where "sleep 1" hangs until interrupted...).
> > It is OK for experimental work, but not yet ready for inclusion.
> > See also Manfred Spraul's recent patches (or his earlier work from
> > 2004 at http://marc.info/?l=linux-kernel&m=108546384711797&w=2).
> > We will converge onto a common patch in the fullness of time, but are
> > currently exploring different regions of the design space.  That said,
> > I have already gratefully stolen quite a few of Manfred's ideas.
> > 
> > This patch provides CONFIG_RCU_FANOUT, which controls the bushiness
> > of the RCU hierarchy.  Defaults to 32 on 32-bit machines and 64 on
> > 64-bit machines.  If CONFIG_NR_CPUS is less than CONFIG_RCU_FANOUT,
> > there is no hierarchy.  By default, the RCU initialization code will
> > adjust CONFIG_RCU_FANOUT to balance the hierarchy, so strongly NUMA
> > architectures may choose to set CONFIG_RCU_FANOUT_EXACT to disable
> > this balancing, allowing the hierarchy to be exactly aligned to the
> > underlying hardware.  Up to two levels of hierarchy are permitted
> > (in addition to the root node), allowing up to 16,384 CPUs on 32-bit
> > systems and up to 262,144 CPUs on 64-bit systems.  I just know that I
> > am going to regret saying this, but this seems more than sufficient
> > for the foreseeable future.  (Some architectures might wish to set
> > CONFIG_RCU_FANOUT=4, which would limit such architectures to 64 CPUs.
> > If this becomes a real problem, additional levels can be added, but I
> > doubt that it will make a significant difference on real hardware.)
> > 
> > In the common case, a given CPU will manipulate its private rcu_data
> > structure and the rcu_node structure that it shares with its immediate
> > neighbors.  This can reduce both lock and memory contention by multiple
> > orders of magnitude, which should eliminate the need for the strange
> > manipulations that are reported to be required when running Linux on
> > very large systems.
> > 
> > Some shortcomings:
> > 
> > o	Some of the NR_CPUS need to be eliminated.  That said, some
> > 	will remain.
> > 
> > o	There is a bit of debug code in place.  This will be removed.
> > 
> > o	There are probably hangs, rcutorture failures, &c.  Seems
> > 	quite stable on a 128-CPU machine, but that is kind of small
> > 	compared to 4096 CPUs.
> > 
> > o	There is not yet a human-readable design document.  One is now
> > 	close to completion.
> > 
> > Credits:
> > 
> > o	Manfred Spraul for ideas, review comments, and bugs spotted,
> > 	as well as some good friendly competition.  ;-)
> > 
> > o	Josh Triplett, Ingo Molnar, Peter Zijlstra, Mathieu Desnoyers,
> > 	Lai Jiangshan, Andi Kleen, Andy Whitcroft, and Andrew Morton
> > 	for reviews and comments.
> > 
> > o	Thomas Gleixner for much-needed help with some timer issues
> > 	(see patches below).
> > 
> > o	Jon M. Tollefson, Tim Pepper, Andrew Theurer, Jose R. Santos,
> > 	Andy Whitcroft, Darrick Wong, Nishanth Aravamudan, Anton
> > 	Blanchard, and Nathan Lynch for keeping machines alive despite
> > 	my heavy abuse^Wtesting.
> > 
> > To build, start with 2.6.27-rc7, and apply:
> > 
> > 	http://www.rdrop.com/users/paulmck/patches/2.6.27-rc3-treeRCU-20.patch
> > 	http://tglx.de/~tglx/gack.patch
> > 	http://tglx.de/~tglx/clockevents-keep-tick-next-period-up-to-date.patch
> > 
> > Thoughts?
> 
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > ---
> > 
> >  Documentation/RCU/00-INDEX             |    2 
> >  Documentation/RCU/trace.txt            |  398 ++++++++
> >  arch/powerpc/platforms/pseries/rtasd.c |    4 
> >  include/linux/hardirq.h                |   14 
> >  include/linux/rcupdate.h               |   10 
> >  include/linux/rcutree.h                |  325 +++++++
> >  init/Kconfig                           |   18 
> >  kernel/Kconfig.preempt                 |   62 +
> >  kernel/Makefile                        |    6 
> >  kernel/rcupreempt.c                    |   10 
> >  kernel/rcupreempt_trace.c              |   10 
> >  kernel/rcutree.c                       | 1510 +++++++++++++++++++++++++++++++++
> >  kernel/rcutree_trace.c                 |  232 +++++
> >  kernel/softirq.c                       |   15 
> >  lib/Kconfig.debug                      |   13 
> >  15 files changed, 2595 insertions(+), 34 deletions(-)
> > 
> > diff --git a/Documentation/RCU/00-INDEX b/Documentation/RCU/00-INDEX
> > index 461481d..7dc0695 100644
> > --- a/Documentation/RCU/00-INDEX
> > +++ b/Documentation/RCU/00-INDEX
> > @@ -16,6 +16,8 @@ RTFP.txt
> >  	- List of RCU papers (bibliography) going back to 1980.
> >  torture.txt
> >  	- RCU Torture Test Operation (CONFIG_RCU_TORTURE_TEST)
> > +trace.txt
> > +	- CONFIG_RCU_TRACE debugfs files and formats
> >  UP.txt
> >  	- RCU on Uniprocessor Systems
> >  whatisRCU.txt
> > diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt
> > new file mode 100644
> > index 0000000..d25110c
> > --- /dev/null
> > +++ b/Documentation/RCU/trace.txt
> > @@ -0,0 +1,398 @@
> > +CONFIG_RCU_TRACE debugfs Files and Formats
> > +
> > +
> > +The rcupreempt and rcutree implementations of RCU provide debugfs trace
> > +output that summarizes counters and state.  This information is useful for
> > +debugging RCU itself, and can sometimes also help to debug abuses of RCU.
> > +Note that the rcuclassic implementation of RCU does not provide debugfs
> > +trace output.
> > +
> > +The following sections describe the debugfs files and formats for
> > +preemptable RCU (rcupreempt) and hierarchical RCU (rcutree).
> > +
> > +
> > +Preemptable RCU debugfs Files and Formats
> > +
> > +This implementation of RCU provides three debugfs files under the
> > +top-level directory RCU: rcu/rcuctrs (which displays the per-CPU
> > +counters used by preemptable RCU) rcu/rcugp (which displays grace-period
> > +counters), and rcu/rcustats (which internal counters for debugging RCU).
> > +
> > +The output of "cat rcu/rcuctrs" looks as follows:
> > +
> > +CPU last cur F M
> > +  0    5  -5 0 0
> > +  1   -1   0 0 0
> > +  2    0   1 0 0
> > +  3    0   1 0 0
> > +  4    0   1 0 0
> > +  5    0   1 0 0
> > +  6    0   2 0 0
> > +  7    0  -1 0 0
> > +  8    0   1 0 0
> > +ggp = 26226, state = waitzero
> > +
> > +The per-CPU fields are as follows:
> > +
> > +o	"CPU" gives the CPU number.  Offline CPUs are not displayed.
> > +
> > +o	"last" gives the value of the counter that is being decremented
> > +	for the current grace period phase.  In the example above,
> > +	the counters sum to 4, indicating that there are still four
> > +	RCU read-side critical sections still running that started
> > +	before the last counter flip.
> > +
> > +o	"cur" gives the value of the counter that is currently being
> > +	both incremented (by rcu_read_lock()) and decremented (by
> > +	rcu_read_unlock()).  In the example above, the counters sum to
> > +	1, indicating that there is only one RCU read-side critical section
> > +	still running that started after the last counter flip.
> > +
> > +o	"F" indicates whether RCU is waiting for this CPU to acknowledge
> > +	a counter flip.  In the above example, RCU is not waiting on any,
> > +	which is consistent with the state being "waitzero" rather than
> > +	"waitack".
> > +
> > +o	"M" indicates whether RCU is waiting for this CPU to execute a
> > +	memory barrier.  In the above example, RCU is not waiting on any,
> > +	which is consistent with the state being "waitzero" rather than
> > +	"waitmb".
> > +
> > +o	"ggp" is the global grace-period counter.
> > +
> > +o	"state" is the RCU state, which can be one of the following:
> > +
> > +	o	"idle": there is no grace period in progress.
> > +
> > +	o	"waitack": RCU just incremented the global grace-period
> > +		counter, which has the effect of reversing the roles of
> > +		the "last" and "cur" counters above, and is waiting for
> > +		all the CPUs to acknowledge the flip.  Once the flip has
> > +		been acknowledged, CPUs will no longer be incrementing
> > +		what are now the "last" counters, so that their sum will
> > +		decrease monotonically down to zero.
> > +
> > +	o	"waitzero": RCU is waiting for the sum of the "last" counters
> > +		to decrease to zero.
> > +
> > +	o	"waitmb": RCU is waiting for each CPU to execute a memory
> > +		barrier, which ensures that instructions from a given CPU's
> > +		last RCU read-side critical section cannot be reordered
> > +		with instructions following the memory-barrier instruction.
> > +
> > +The output of "cat rcu/rcugp" looks as follows:
> > +
> > +oldggp=48870  newggp=48873
> > +
> > +Note that reading from this file provokes a synchronize_rcu().  The
> > +"oldggp" value is that of "ggp" from rcu/rcuctrs above, taken before
> > +executing the synchronize_rcu(), and the "newggp" value is also the
> > +"ggp" value, but taken after the synchronize_rcu() command returns.
> > +
> > +
> > +The output of "cat rcu/rcugp" looks as follows:
> > +
> > +na=1337955 nl=40 wa=1337915 wl=44 da=1337871 dl=0 dr=1337871 di=1337871
> > +1=50989 e1=6138 i1=49722 ie1=82 g1=49640 a1=315203 ae1=265563 a2=49640
> > +z1=1401244 ze1=1351605 z2=49639 m1=5661253 me1=5611614 m2=49639
> > +
> > +These are counters tracking internal preemptable-RCU events, however,
> > +some of them may be useful for debugging algorithms using RCU.  In
> > +particular, the "nl", "wl", and "dl" values track the number of RCU
> > +callbacks in various states.  The fields are as follows:
> > +
> > +o	"na" is the total number of RCU callbacks that have been enqueued
> > +	since boot.
> > +
> > +o	"nl" is the number of RCU callbacks waiting for the previous
> > +	grace period to end so that they can start waiting on the next
> > +	grace period.
> > +
> > +o	"wa" is the total number of RCU callbacks that have started waiting
> > +	for a grace period since boot.  "na" should be roughly equal to
> > +	"nl" plus "wa".
> > +
> > +o	"wl" is the number of RCU callbacks currently waiting for their
> > +	grace period to end.
> > +
> > +o	"da" is the total number of RCU callbacks whose grace periods
> > +	have completed since boot.  "wa" should be roughly equal to
> > +	"wl" plus "da".
> > +
> > +o	"dr" is the total number of RCU callbacks that have been removed
> > +	from the list of callbacks ready to invoke.  "dr" should be roughly
> > +	equal to "da".
> > +
> > +o	"di" is the total number of RCU callbacks that have been invoked
> > +	since boot.  "di" should be roughly equal to "da", though some
> > +	early versions of preemptable RCU had a bug so that only the
> > +	last CPU's count of invocations was displayed, rather than the
> > +	sum of all CPU's counts.
> > +
> > +o	"1" is the number of calls to rcu_try_flip().  This should be
> > +	roughly equal to the sum of "e1", "i1", "a1", "z1", and "m1"
> > +	described below.  In other words, the number of times that
> > +	the state machine is visited should be equal to the sum of the
> > +	number of times that each state is visited plus the number of
> > +	times that the state-machine lock acquisition failed.
> > +
> > +o	"e1" is the number of times that rcu_try_flip() was unable to
> > +	acquire the fliplock.
> > +
> > +o	"i1" is the number of calls to rcu_try_flip_idle().
> > +
> > +o	"ie1" is the number of times rcu_try_flip_idle() exited early
> > +	due to the calling CPU having no work for RCU.
> > +
> > +o	"g1" is the number of times that rcu_try_flip_idle() decided
> > +	to start a new grace period.  "i1" should be roughly equal to
> > +	"ie1" plus "g1".
> > +
> > +o	"a1" is the number of calls to rcu_try_flip_waitack().
> > +
> > +o	"ae1" is the number of times that rcu_try_flip_waitack() found
> > +	that at least one CPU had not yet acknowledge the new grace period
> > +	(AKA "counter flip").
> > +
> > +o	"a2" is the number of time rcu_try_flip_waitack() found that
> > +	all CPUs had acknowledged.  "a1" should be roughly equal to
> > +	"ae1" plus "a2".  (This particular output was collected on
> > +	a 128-CPU machine, hence the smaller-than-usual fraction of
> > +	calls to rcu_try_flip_waitack() finding all CPUs having already
> > +	acknowledged.)
> > +
> > +o	"z1" is the number of calls to rcu_try_flip_waitzero().
> > +
> > +o	"ze1" is the number of times that rcu_try_flip_waitzero() found
> > +	that not all of the old RCU read-side critical sections had
> > +	completed.
> > +
> > +o	"z2" is the number of times that rcu_try_flip_waitzero() finds
> > +	the sum of the counters equal to zero, in other words, that
> > +	all of the old RCU read-side critical sections had completed.
> > +	The value of "z1" should be roughly equal to "ze1" plus
> > +	"z2".
> > +
> > +o	"m1" is the number of calls to rcu_try_flip_waitmb().
> > +
> > +o	"me1" is the number of times that rcu_try_flip_waitmb() finds
> > +	that at least one CPU has not yet executed a memory barrier.
> > +
> > +o	"m2" is the number of times that rcu_try_flip_waitmb() finds that
> > +	all CPUs have executed a memory barrier.
> > +
> > +
> > +Hierarchical RCU debugfs Files and Formats
> > +
> > +This implementation of RCU provides three debugfs files under the
> > +top-level directory RCU: rcu/rcudata (which displays fields in struct
> > +rcu_data), rcu/rcugp (which displays grace-period counters), and
> > +rcu/rcuhier (which displays the struct rcu_node hierarchy).
> > +
> > +The output of "cat rcu/rcudata" looks as follows:
> > +
> > +rcu:
> > +  0 c=1985 g=1986 pq=1 pqc=1985 qp=0 dt=26097 dn=2 df=9102 of=0 ri=11 ql=2 b=10
> > +  1 c=1985 g=1986 pq=1 pqc=1985 qp=0 dt=30421 dn=2 df=6608 of=0 ri=2 ql=39 b=10
> > +  2 c=1982 g=1982 pq=1 pqc=1982 qp=0 dt=10934 dn=2 df=9612 of=0 ri=0 ql=0 b=10
> > +  3 c=1985 g=1986 pq=1 pqc=1985 qp=0 dt=30139 dn=2 df=6043 of=0 ri=0 ql=58 b=10
> > +  4 c=1960 g=1960 pq=1 pqc=1960 qp=1 dt=1202 dn=2 df=30470 of=0 ri=3 ql=0 b=10
> > +  5 c=1985 g=1986 pq=1 pqc=1985 qp=0 dt=15341 dn=2 df=5350 of=0 ri=0 ql=25 b=10
> > +  6 c=1983 g=1984 pq=1 pqc=1983 qp=1 dt=516 dn=2 df=31950 of=0 ri=0 ql=0 b=10
> > +  7 c=1985 g=1986 pq=1 pqc=1985 qp=0 dt=8205 dn=2 df=7465 of=0 ri=0 ql=28 b=10
> > +rcu_bh:
> > +  0 c=375 g=375 pq=1 pqc=375 qp=0 dt=26097 dn=2 df=0 of=0 ri=0 ql=0 b=10
> > +  1 c=375 g=375 pq=1 pqc=375 qp=0 dt=30421 dn=2 df=162 of=0 ri=0 ql=0 b=10
> > +  2 c=375 g=375 pq=1 pqc=375 qp=1 dt=10934 dn=2 df=162 of=0 ri=0 ql=0 b=10
> > +  3 c=375 g=375 pq=1 pqc=375 qp=0 dt=30139 dn=2 df=107 of=0 ri=0 ql=0 b=10
> > +  4 c=375 g=375 pq=1 pqc=375 qp=1 dt=1202 dn=2 df=174 of=0 ri=0 ql=0 b=10
> > +  5 c=375 g=375 pq=1 pqc=375 qp=0 dt=15341 dn=2 df=122 of=0 ri=0 ql=0 b=10
> > +  6 c=375 g=375 pq=1 pqc=375 qp=1 dt=516 dn=2 df=117 of=0 ri=0 ql=0 b=10
> > +  7 c=375 g=375 pq=1 pqc=375 qp=0 dt=8205 dn=2 df=127 of=0 ri=0 ql=0 b=10
> > +
> > +The first section lists the rcu_data structures for rcu, the second for
> > +rcu_bh.  Each section has one line per CPU, or eight for this 8-CPU system.
> > +The fields are as follows:
> > +
> > +o	The number at the beginning of each line is the CPU number.
> > +	CPUs numbers followed by an exclamation mark are offline,
> > +	but have been online at least once since boot.	There will be
> > +	no output for CPUs that have never been online, which can be
> > +	a good thing in the surprisingly common case where NR_CPUS is
> > +	substantially larger than the number of actual CPUs.
> > +
> > +o	"c" is the count of grace periods that this CPU believes have
> > +	completed.  CPUs in dynticks idle mode may lag quite a ways
> > +	behind, for example, CPU 4 under "rcu" above, which has slept
> > +	through the past 25 RCU grace periods.	It is not unusual to
> > +	see CPUs lagging by thousands of grace periods.
> > +
> > +o	"g" is the count of grace periods that this CPU believes have
> > +	started.  Again, CPUs in dynticks idle mode may lag behind.
> > +	If the "c" and "g" values are equal, this CPU has already
> > +	reported a quiescent state for the last RCU grace period that
> > +	it is aware of, otherwise, the CPU believes that it owes RCU a
> > +	quiescent state.
> > +
> > +o	"pq" indicates that this CPU has passed through a quiescent state
> > +	for the current grace period.  It is possible for "pq" to be
> > +	"1" and "c" different than "g", which indicates that although
> > +	the CPU has passed through a quiescent state, either (1) this
> > +	CPU has not yet reported that fact, (2) some other CPU has not
> > +	yet reported for this grace period, or (3) both.
> > +
> > +o	"pqc" indicates which grace period the last-observed quiescent
> > +	state for this CPU corresponds to.  This is important for handling
> > +	the race between CPU 0 reporting an extended dynticks-idle
> > +	quiescent state for CPU 1 and CPU 1 suddenly waking up and
> > +	reporting its own quiescent state.  If CPU 1 was the last CPU
> > +	for the current grace period, then the CPU that loses this race
> > +	will attempt to incorrectly mark CPU 1 as having checked in for
> > +	the next grace period!
> > +
> > +o	"qp" indicates that RCU still expects a quiescent state from
> > +	this CPU.
> > +
> > +o	"dt" is the current value of the dyntick counter that is incremented
> > +	when entering or leaving dynticks idle state, either by the
> > +	scheduler or by irq.
> > +
> > +	This field is displayed only for CONFIG_NO_HZ kernels.
> > +
> > +o	"dn" is the current value of the dyntick counter that is incremented
> > +	when entering or leaving dynticks idle state via NMI.  If both
> > +	the "dt" and "dn" values are even, then this CPU is in dynticks
> > +	idle mode and may be ignored by RCU.  If either of these two
> > +	counters is odd, then RCU must be alert to the possibility of
> > +	an RCU read-side critical section running on this CPU.
> > +
> > +	This field is displayed only for CONFIG_NO_HZ kernels.
> > +
> > +o	"df" is the number of times that some other CPU has forced a
> > +	quiescent state on behalf of this CPU due to this CPU being in
> > +	dynticks-idle state.
> > +
> > +	This field is displayed only for CONFIG_NO_HZ kernels.
> > +
> > +o	"of" is the number of times that some other CPU has forced a
> > +	quiescent state on behalf of this CPU due to this CPU being
> > +	offline.  In a perfect world, this might neve happen, but it
> > +	turns out that offlining and onlining a CPU can take several grace
> > +	periods, and so there is likely to be an extended period of time
> > +	when RCU believes that the CPU is online when it really is not.
> > +	Please note that erring in the other direction (RCU believing a
> > +	CPU is offline when it is really alive and kicking) is a fatal
> > +	error, so it makes sense to err conservatively.
> > +
> > +o	"ri" is the number of times that RCU has seen fit to send a
> > +	reschedule IPI to this CPU in order to get it to report a
> > +	quiescent state.
> > +
> > +o	"ql" is the number of RCU callbacks currently residing on
> > +	this CPU.  This is the total number of callbacks, regardless
> > +	of what state they are in (new, waiting for grace period to
> > +	start, waiting for grace period to end, ready to invoke).
> > +
> > +o	"b" is the batch limit for this CPU.  If more than this number
> > +	of RCU callbacks is ready to invoke, then the remainder will
> > +	be deferred.
> > +
> > +
> > +The output of "cat rcu/rcudata" looks as follows:
> > +
> > +rcu: completed=33062  gpnum=33063
> > +rcu_bh: completed=464  gpnum=464
> > +
> > +Again, this output is for both "rcu" and "rcu_bh".  The fields are
> > +taken from the rcu_state structure, and are as follows:
> > +
> > +o	"completed" is the number of grace periods that have completed.
> > +	It is comparable to the "c" field from rcu/rcudata in that a
> > +	CPU whose "c" field matches the value of "completed" is aware
> > +	that the corresponding RCU grace period has completed.
> > +
> > +o	"gpnum" is the number of grace periods that have started.  It is
> > +	comparable to the "g" field from rcu/rcudata in that a CPU
> > +	whose "g" field matches the value of "gpnum" is aware that the
> > +	corresponding RCU grace period has started.
> > +
> > +	If these two fields are equal (as they are for "rcu_bh" above),
> > +	then there is no grace period in progress, in other words, RCU
> > +	is idle.  On the other hand, if the two fields differ (as they
> > +	do for "rcu" above), then an RCU grace period is in progress.
> > +
> > +
> > +The output of "cat rcu/rcuhier" looks as follows, with very long lines:
> > +
> > +rcu:
> > +c=33184 g=33185 s=0 jfq=1 nfqs=61601/nfqsng=28011(33590)
> > +1/1 0:127 ^0    
> > +1/3 0:35 ^0    0/0 36:71 ^1    0/0 72:107 ^2    0/0 108:127 ^3    
> > +14/3f 0:5 ^0    0/3 6:11 ^1    0/0 12:17 ^2    0/0 18:23 ^3    0/0 24:29 ^4    0/0 30:35 ^5    0/0 36:41 ^0    0/0 42:47 ^1    0/0 48:53 ^2    0/0 54:59 ^3    0/0 60:65 ^4    0/0 66:71 ^5    0/0 72:77 ^0    0/0 78:83 ^1    0/0 84:89 ^2    0/0 90:95 ^3    0/0 96:101 ^4    0/0 102:107 ^5    0/0 108:113 ^0    0/0 114:119 ^1    0/0 120:125 ^2    0/0 126:127 ^3    
> > +rcu_bh:
> > +c=470 g=470 s=0 jfq=2 nfqs=62302/nfqsng=62027(275)
> > +0/1 0:127 ^0    
> > +0/3 0:35 ^0    0/0 36:71 ^1    0/0 72:107 ^2    0/0 108:127 ^3    
> > +0/3f 0:5 ^0    0/3 6:11 ^1    0/0 12:17 ^2    0/0 18:23 ^3    0/0 24:29 ^4    0/0 30:35 ^5    0/0 36:41 ^0    0/0 42:47 ^1    0/0 48:53 ^2    0/0 54:59 ^3    0/0 60:65 ^4    0/0 66:71 ^5    0/0 72:77 ^0    0/0 78:83 ^1    0/0 84:89 ^2    0/0 90:95 ^3    0/0 96:101 ^4    0/0 102:107 ^5    0/0 108:113 ^0    0/0 114:119 ^1    0/0 120:125 ^2    0/0 126:127 ^3    
> > +
> > +This is once again split into "rcu" and "rcu_bh" portions.  The fields are
> > +as follows:
> > +
> > +o	"c" is exactly the same as "completed" under rcu/rcugp.
> > +
> > +o	"g" is exactly the same as "gpnum" under rcu/rcugp.
> > +
> > +o	"s" is the "signaled" state that drives force_quiescent_state()'s
> > +	state machine.
> > +
> > +o	"jfq" is the number of jiffies remaining for this grace period
> > +	before force_quiescent_state() is invoked to help push things
> > +	along.  Note that CPUs in dyntick-idle mode thoughout the grace
> > +	period will not report on their own, but rather must be check by
> > +	some other CPU via force_quiescent_state().
> > +
> > +o	"nfqs" is the number of calls to force_quiescent_state() since
> > +	boot.
> > +
> > +o	"nfqsng" is the number of useless calls to force_quiescent_state(),
> > +	where there wasn't actually a grace period active.  This can
> > +	happen due to races.  The number in parentheses is the difference
> > +	between "nfqs" and "nfqsng", or the number of times that
> > +	force_quiescent_state() actually did some real work.
> > +
> > +o	Each element of the form "1/1 0:127 ^0" represents one struct
> > +	rcu_node.  Each line represents one level of the hierarchy, from
> > +	root to leaves.  It is best to think of the rcu_data structures
> > +	as forming yet another level after the leaves.  Note that there
> > +	might be either one, two, or three levels of rcu_node structures,
> > +	depending on the relationship between CONFIG_RCU_FANOUT and
> > +	CONFIG_NR_CPUS.
> > +	
> > +	o	The numbers separated by the "/" are the qsmask followed
> > +		by the qsmaskinit.  The qsmask will have one bit
> > +		set for each entity in the next lower level that
> > +		has not yet checked in for the current grace period.
> > +		The qsmaskinit will have one bit for each entity that is
> > +		currently expected to check in during each grace period.
> > +		The value of qsmaskinit is assigned to that of qsmask
> > +		at the beginning of each grace period.
> > +
> > +		For example, for "rcu", the qsmask of the first entry
> > +		of the lowest level is 0x14, meaning that we are still
> > +		waiting for CPUs 2 and 4 to check in for the current
> > +		grace period.
> > +
> > +	o	The numbers separated by the ":" are the range of CPUs
> > +		served by this struct rcu_node.  This can be helpful
> > +		in working out how the hierarchy is wired together.
> > +
> > +		For example, the first entry at the lowest level shows
> > +		"0:5", indicating that it covers CPUs 0 through 5.
> > +
> > +	o	The number after the "^" indicates the bit in the
> > +		next higher level rcu_node structure that this
> > +		rcu_node structure corresponds to.
> > +
> > +		For example, the first entry at the lowest level shows
> > +		"^0", indicating that it corresponds to bit zero in
> > +		the first entry at the middle level.
> > diff --git a/arch/powerpc/platforms/pseries/rtasd.c b/arch/powerpc/platforms/pseries/rtasd.c
> > index c9ffd8c..d8e784a 100644
> > --- a/arch/powerpc/platforms/pseries/rtasd.c
> > +++ b/arch/powerpc/platforms/pseries/rtasd.c
> > @@ -208,6 +208,7 @@ void pSeries_log_error(char *buf, unsigned int err_type, int fatal)
> >  		break;
> >  	case ERR_TYPE_KERNEL_PANIC:
> >  	default:
> > +		WARN_ON_ONCE(!irqs_disabled()); /* @@@ DEBUG @@@ */
> >  		spin_unlock_irqrestore(&rtasd_log_lock, s);
> >  		return;
> >  	}
> > @@ -227,6 +228,7 @@ void pSeries_log_error(char *buf, unsigned int err_type, int fatal)
> >  	/* Check to see if we need to or have stopped logging */
> >  	if (fatal || !logging_enabled) {
> >  		logging_enabled = 0;
> > +		WARN_ON_ONCE(!irqs_disabled()); /* @@@ DEBUG @@@ */
> >  		spin_unlock_irqrestore(&rtasd_log_lock, s);
> >  		return;
> >  	}
> > @@ -249,11 +251,13 @@ void pSeries_log_error(char *buf, unsigned int err_type, int fatal)
> >  		else
> >  			rtas_log_start += 1;
> > 
> > +		WARN_ON_ONCE(!irqs_disabled()); /* @@@ DEBUG @@@ */
> >  		spin_unlock_irqrestore(&rtasd_log_lock, s);
> >  		wake_up_interruptible(&rtas_log_wait);
> >  		break;
> >  	case ERR_TYPE_KERNEL_PANIC:
> >  	default:
> > +		WARN_ON_ONCE(!irqs_disabled()); /* @@@ DEBUG @@@ */
> >  		spin_unlock_irqrestore(&rtasd_log_lock, s);
> >  		return;
> >  	}
> > diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
> > index 181006c..9b70b92 100644
> > --- a/include/linux/hardirq.h
> > +++ b/include/linux/hardirq.h
> > @@ -118,13 +118,17 @@ static inline void account_system_vtime(struct task_struct *tsk)
> >  }
> >  #endif
> > 
> > -#if defined(CONFIG_PREEMPT_RCU) && defined(CONFIG_NO_HZ)
> > +#if defined(CONFIG_NO_HZ) && !defined(CONFIG_CLASSIC_RCU)
> >  extern void rcu_irq_enter(void);
> >  extern void rcu_irq_exit(void);
> > +extern void rcu_nmi_enter(void);
> > +extern void rcu_nmi_exit(void);
> >  #else
> >  # define rcu_irq_enter() do { } while (0)
> >  # define rcu_irq_exit() do { } while (0)
> > -#endif /* CONFIG_PREEMPT_RCU */
> > +# define rcu_nmi_enter() do { } while (0)
> > +# define rcu_nmi_exit() do { } while (0)
> > +#endif /* #if defined(CONFIG_NO_HZ) && !defined(CONFIG_CLASSIC_RCU) */
> > 
> >  /*
> >   * It is safe to do non-atomic ops on ->hardirq_context,
> > @@ -134,7 +138,6 @@ extern void rcu_irq_exit(void);
> >   */
> >  #define __irq_enter()					\
> >  	do {						\
> > -		rcu_irq_enter();			\
> >  		account_system_vtime(current);		\
> >  		add_preempt_count(HARDIRQ_OFFSET);	\
> >  		trace_hardirq_enter();			\
> > @@ -153,7 +156,6 @@ extern void irq_enter(void);
> >  		trace_hardirq_exit();			\
> >  		account_system_vtime(current);		\
> >  		sub_preempt_count(HARDIRQ_OFFSET);	\
> > -		rcu_irq_exit();				\
> >  	} while (0)
> > 
> >  /*
> > @@ -161,7 +163,7 @@ extern void irq_enter(void);
> >   */
> >  extern void irq_exit(void);
> > 
> > -#define nmi_enter()		do { lockdep_off(); __irq_enter(); } while (0)
> > -#define nmi_exit()		do { __irq_exit(); lockdep_on(); } while (0)
> > +#define nmi_enter()		do { lockdep_off(); rcu_nmi_enter(); __irq_enter(); } while (0)
> > +#define nmi_exit()		do { __irq_exit(); rcu_nmi_exit(); lockdep_on(); } while (0)
> > 
> >  #endif /* LINUX_HARDIRQ_H */
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index e8b4039..f8544ae 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -52,11 +52,15 @@ struct rcu_head {
> >  	void (*func)(struct rcu_head *head);
> >  };
> > 
> > -#ifdef CONFIG_CLASSIC_RCU
> > +#if defined(CONFIG_CLASSIC_RCU)
> >  #include <linux/rcuclassic.h>
> > -#else /* #ifdef CONFIG_CLASSIC_RCU */
> > +#elif defined(CONFIG_TREE_RCU)
> > +#include <linux/rcutree.h>
> > +#elif defined(CONFIG_PREEMPT_RCU)
> >  #include <linux/rcupreempt.h>
> > -#endif /* #else #ifdef CONFIG_CLASSIC_RCU */
> > +#else
> > +#error "Unknown RCU implementation specified to kernel configuration"
> > +#endif /* #else #if defined(CONFIG_CLASSIC_RCU) */
> > 
> >  #define RCU_HEAD_INIT 	{ .next = NULL, .func = NULL }
> >  #define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT
> > diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
> > new file mode 100644
> > index 0000000..00f8be2
> > --- /dev/null
> > +++ b/include/linux/rcutree.h
> > @@ -0,0 +1,325 @@
> > +/*
> > + * Read-Copy Update mechanism for mutual exclusion (tree-based version)
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License as published by
> > + * the Free Software Foundation; either version 2 of the License, or
> > + * (at your option) any later version.
> > + *
> > + * This program is distributed in the hope that it will be useful,
> > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> > + * GNU General Public License for more details.
> > + *
> > + * You should have received a copy of the GNU General Public License
> > + * along with this program; if not, write to the Free Software
> > + * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
> > + *
> > + * Copyright IBM Corporation, 2008
> > + *
> > + * Author: Dipankar Sarma <dipankar@xxxxxxxxxx>
> > + *	   Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Hierarchical algorithm
> > + *
> > + * Based on the original work by Paul McKenney <paulmck@xxxxxxxxxx>
> > + * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
> > + *
> > + * For detailed explanation of Read-Copy Update mechanism see -
> > + * 	Documentation/RCU
> > + */
> > +
> > +#ifndef __LINUX_RCUTREE_H
> > +#define __LINUX_RCUTREE_H
> > +
> > +#include <linux/cache.h>
> > +#include <linux/spinlock.h>
> > +#include <linux/threads.h>
> > +#include <linux/percpu.h>
> > +#include <linux/cpumask.h>
> > +#include <linux/seqlock.h>
> > +
> > +/*
> > + * Define shape of hierarchy based on NR_CPUS and CONFIG_RCU_FANOUT.
> > + * In theory, it should be possible to add more levels straightforwardly.
> > + * In practice, this has not been tested, so there is probably some
> > + * bug somewhere.
> > + */
> > +#define MAX_RCU_LVLS 3
> > +#define RCU_FANOUT	      (CONFIG_RCU_FANOUT)
> > +#define RCU_FANOUT_SQ	      (RCU_FANOUT * RCU_FANOUT)
> > +#define RCU_FANOUT_CUBE	      (RCU_FANOUT_SQ * RCU_FANOUT)
> > +
> > +#if (NR_CPUS) <= RCU_FANOUT
> > +#  define NUM_RCU_LVLS	      1
> > +#  define NUM_RCU_LVL_0	      1
> > +#  define NUM_RCU_LVL_1	      (NR_CPUS)
> > +#  define NUM_RCU_LVL_2	      0
> > +#  define NUM_RCU_LVL_3	      0
> > +#elif (NR_CPUS) <= RCU_FANOUT_SQ
> > +#  define NUM_RCU_LVLS	      2
> > +#  define NUM_RCU_LVL_0	      1
> > +#  define NUM_RCU_LVL_1	      (((NR_CPUS) + RCU_FANOUT - 1) / RCU_FANOUT)
> > +#  define NUM_RCU_LVL_2	      (NR_CPUS)
> > +#  define NUM_RCU_LVL_3	      0
> > +#elif (NR_CPUS) <= RCU_FANOUT_CUBE
> > +#  define NUM_RCU_LVLS	      3
> > +#  define NUM_RCU_LVL_0	      1
> > +#  define NUM_RCU_LVL_1	      (((NR_CPUS) + RCU_FANOUT_SQ - 1) / RCU_FANOUT_SQ)
> > +#  define NUM_RCU_LVL_2	      (((NR_CPUS) + (RCU_FANOUT) - 1) / (RCU_FANOUT))
> > +#  define NUM_RCU_LVL_3	      NR_CPUS
> > +#else
> > +# error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
> > +#endif /* #if (NR_CPUS) <= RCU_FANOUT */
> > +
> > +#define RCU_SUM (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
> > +#define NUM_RCU_NODES (RCU_SUM - NR_CPUS)
> > +
> > +/*
> > + * Dynticks per-CPU state.
> > + */
> > +struct rcu_dynticks {
> > +	int dynticks_nesting;	/* Track nesting level, sort of. */
> > +	int dynticks;		/* Even value for dynticks-idle, else odd. */
> > +	int dynticks_nmi;	/* Even value for either dynticks-idle or */
> > +				/*  not in nmi handler, else odd.  So this */
> > +				/*  remains even for nmi from irq handler. */
> > +};
> > +
> > +/*
> > + * Definition for node within the RCU grace-period-detection hierarchy.
> > + */
> > +struct rcu_node {
> > +	spinlock_t lock;
> > +	unsigned long qsmask;	/* CPUs or groups that need to switch in */
> > +				/*  order for current grace period to proceed.*/
> > +	unsigned long qsmaskinit;
> > +				/* Per-GP initialization for qsmask. */
> > +	unsigned long grpmask;	/* Mask to apply to parent qsmask. */
> > +	int	grplo;		/* lowest-numbered CPU or group here. */
> > +	int	grphi;		/* highest-numbered CPU or group here. */
> > +	u8	grpnum;		/* CPU/group number for next level up. */
> > +	u8	level;		/* root is at level 0. */
> > +	struct rcu_node *parent;
> > +} ____cacheline_internodealigned_in_smp;
> > +
> > +/* Index values for nxttail array in struct rcu_data. */
> > +#define RCU_DONE_TAIL		0	/* Also RCU_WAIT head. */
> > +#define RCU_WAIT_TAIL		1	/* Also RCU_NEXT_READY head. */
> > +#define RCU_NEXT_READY_TAIL	2	/* Also RCU_NEXT head. */
> > +#define RCU_NEXT_TAIL		3
> > +#define RCU_NEXT_SIZE		4
> > +
> > +/* Per-CPU data for read-copy update. */
> > +struct rcu_data {
> > +	/* 1) quiescent-state and grace-period handling : */
> > +	long		completed;	/* Track rsp->completed gp number */
> > +					/*  in order to detect GP end. */
> > +	long		gpnum;		/* Highest gp number that this CPU */
> > +					/*  is aware of having started. */
> > +	long		passed_quiesc_completed;
> > +					/* Value of completed at time of qs. */
> > +	bool		passed_quiesc;	/* User-mode/idle loop etc. */
> > +	bool		qs_pending;	/* Core waits for quiesc state. */
> > +	bool		beenonline;	/* CPU online at least once. */
> > +	struct rcu_node *mynode;	/* This CPU's leaf of hierarchy */
> > +	unsigned long grpmask;		/* Mask to apply to leaf qsmask. */
> > +
> > +	/* 2) batch handling */
> > +	/*
> > +	 * If nxtlist is not NULL, it is partitioned as follows.
> > +	 * Any of the partitions might be empty, in which case the
> > +	 * pointer to that partition will be equal to the pointer for
> > +	 * the following partition.  When the list is empty, all of
> > +	 * the nxttail elements point to nxtlist, which is NULL.
> > +	 *
> > +	 * [*nxttail[RCU_NEXT_READY_TAIL], NULL = *nxttail[RCU_NEXT_TAIL]):
> > +	 *	Entries that might have arrived after current GP ended
> > +	 * [*nxttail[RCU_WAIT_TAIL], *nxttail[RCU_NEXT_READY_TAIL]):
> > +	 *	Entries known to have arrived before current GP ended
> > +	 * [*nxttail[RCU_DONE_TAIL], *nxttail[RCU_WAIT_TAIL]):
> > +	 *	Entries that batch # <= ->completed - 1: waiting for current GP
> > +	 * [nxtlist, *nxttail[RCU_DONE_TAIL]):
> > +	 *	Entries that batch # <= ->completed
> > +	 *	The grace period for these entries has completed, and
> > +	 *	the other grace-period-completed entries may be moved
> > +	 *	here temporarily in rcu_process_callbacks().
> > +	 */
> > +	struct rcu_head *nxtlist;
> > +	struct rcu_head **nxttail[RCU_NEXT_SIZE];
> > +	long		qlen; 	 	/* # of queued callbacks */
> > +	long		blimit;		/* Upper limit on a processed batch */
> > +
> > +	/* 3) rcu-barrier functions */
> > +	struct rcu_head barrier;
> > +
> > +#ifdef CONFIG_NO_HZ
> > +	/* 4) dynticks interface (see http://lwn.net/Articles/279077/) */
> > +	struct rcu_dynticks *dynticks;	/* Shared per-CPU dynticks state. */
> > +	int dynticks_snap;		/* Per-GP tracking for dynticks. */
> > +	int dynticks_nmi_snap;		/* Per-GP tracking for dynticks_nmi. */
> > +#endif /* #ifdef CONFIG_NO_HZ */
> > +
> > +	/* 5) reasons this CPU needed to be kicked by force_quiescent_state */
> > +#ifdef CONFIG_NO_HZ
> > +	unsigned long dynticks_fqs;	/* Kicked due to dynticks idle. */
> > +#endif /* #ifdef CONFIG_NO_HZ */
> > +	unsigned long offline_fqs;	/* Kicked due to being offline. */
> > +	unsigned long resched_ipi;	/* Sent a resched IPI. */
> > +
> > +	int cpu;
> > +};
> > +
> > +/* Values for signaled field in struc rcu_data. */
> 				   ^^^^^^^^^^^^^^^^^^
> =>			   should be struct rcu_state.

Fixed!

> > +#define RCU_SAVE_DYNTICK	0	/* Need to scan dyntick state. */
> > +#define RCU_FORCE_QS		1	/* Need to force quiescent state. */
> > +#ifdef CONFIG_NO_HZ
> > +#define RCU_SIGNAL_INIT		RCU_SAVE_DYNTICK
> > +#else /* #ifdef CONFIG_NO_HZ */
> > +#define RCU_SIGNAL_INIT		RCU_FORCE_QS
> > +#endif /* #else #ifdef CONFIG_NO_HZ */
> > +
> > +#define RCU_JIFFIES_TILL_FORCE_QS	 3	/* for rsp->jiffies_force_qs */
> > +#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
> > +#define RCU_SECONDS_TILL_STALL_CHECK	(3 * HZ)  /* for rsp->jiffies_stall */
> > +#define RCU_SECONDS_TILL_STALL_RECHECK (30 * HZ)  /* for rsp->jiffies_stall */
> > +#define RCU_STALL_RAT_DELAY		2	  /* Allow other CPUs time */
> > +						  /*  to take at least one */
> > +						  /*  scheduling clock irq */
> > +						  /*  before ratting on them. */
> > +
> > +#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
> > +
> > +/*
> > + * RCU global state, including node hierarchy.  This hierarchy is
> > + * represented in "heap" form in a dense array.  The root (first level)
> > + * of the hierarchy is in ->node[0] (referenced by ->level[0]), the second
> > + * level in ->node[1] through ->node[m] (->node[1] referenced by ->level[1]),
> > + * and the third level in ->node[m+1] and following (->node[m+1] referenced
> > + * by ->level[2]).  The number of levels is determined by the number of
> > + * CPUs and by CONFIG_RCU_FANOUT.  Small systems will have a "hierarchy"
> > + * consisting of a single rcu_node.
> > + */
> > +struct rcu_state {
> > +	struct rcu_node node[NUM_RCU_NODES];	/* Hierarchy. */
> > +	struct rcu_node *level[NUM_RCU_LVLS];	/* Hierarchy levels. */
> > +	u32 levelcnt[MAX_RCU_LVLS + 1];		/* # nodes in each level. */
> > +	u8 levelspread[NUM_RCU_LVLS];		/* kids/node in each level. */
> > +	struct rcu_data *rda[NR_CPUS];		/* array of rdp pointers. */
> > +
> > +	/* The following fields are guarded by the root rcu_node's lock. */
> > +
> > +	u8	signaled ____cacheline_internodealigned_in_smp;
> > +						/* Force QS state. */
> > +	long	gpnum;				/* Current gp number. */
> > +	long	completed;			/* # of last completed gp. */
> > +	spinlock_t onofflock;			/* exclude on/offline and */
> > +						/*  starting new GP. */
> > +	spinlock_t fqslock;			/* Only one task forcing */
> > +						/*  quiescent states. */
> > +	unsigned long jiffies_force_qs;		/* Time at which to invoke */
> > +						/*  force_quiescent_state(). */
> > +	unsigned long n_force_qs;		/* Number of calls to */
> > +						/*  force_quiescent_state(). */
> > +	unsigned long n_force_qs_ngp;		/* Number of calls leaving */
> > +						/*  due to no GP active. */
> > +#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
> > +	unsigned long gp_start;			/* Time at which GP started, */
> > +						/*  but in jiffies. */
> > +	unsigned long jiffies_stall;		/* Time at which to check */
> > +						/*  for CPU stalls. */
> > +#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
> > +#ifdef CONFIG_NO_HZ
> > +	long dynticks_completed;		/* Value of completed @ snap. */
> > +#endif /* #ifdef CONFIG_NO_HZ */
> > +};
> > +
> > +extern struct rcu_state rcu_state;
> > +DECLARE_PER_CPU(struct rcu_data, rcu_data);
> > +
> > +extern struct rcu_state rcu_bh_state;
> > +DECLARE_PER_CPU(struct rcu_data, rcu_bh_data);
> > +
> > +/*
> > + * Increment the quiescent state counter.
> > + * The counter is a bit degenerated: We do not need to know
> > + * how many quiescent states passed, just if there was at least
> > + * one since the start of the grace period. Thus just a flag.
> > + */
> > +static inline void rcu_qsctr_inc(int cpu)
> > +{
> > +	struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
> > +	rdp->passed_quiesc = 1;
> > +	rdp->passed_quiesc_completed = rdp->completed;
> > +}
> > +static inline void rcu_bh_qsctr_inc(int cpu)
> > +{
> > +	struct rcu_data *rdp = &per_cpu(rcu_bh_data, cpu);
> > +	rdp->passed_quiesc = 1;
> > +	rdp->passed_quiesc_completed = rdp->completed;
> > +}
> > +
> > +extern int rcu_pending(int cpu);
> > +extern int rcu_needs_cpu(int cpu);
> > +
> > +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> > +extern struct lockdep_map rcu_lock_map;
> > +# define rcu_read_acquire()	\
> > +			lock_acquire(&rcu_lock_map, 0, 0, 2, 1, _THIS_IP_)
> > +# define rcu_read_release()	lock_release(&rcu_lock_map, 1, _THIS_IP_)
> > +#else
> > +# define rcu_read_acquire()	do { } while (0)
> > +# define rcu_read_release()	do { } while (0)
> > +#endif
> > +
> > +static inline void __rcu_read_lock(void)
> > +{
> > +	preempt_disable();
> > +	__acquire(RCU);
> > +	rcu_read_acquire();
> > +}
> > +static inline void __rcu_read_unlock(void)
> > +{
> > +	rcu_read_release();
> > +	__release(RCU);
> > +	preempt_enable();
> > +}
> > +static inline void __rcu_read_lock_bh(void)
> > +{
> > +	local_bh_disable();
> > +	__acquire(RCU_BH);
> > +	rcu_read_acquire();
> > +}
> > +static inline void __rcu_read_unlock_bh(void)
> > +{
> > +	rcu_read_release();
> > +	__release(RCU_BH);
> > +	local_bh_enable();
> > +}
> > +
> > +#define __synchronize_sched() synchronize_rcu()
> > +
> > +#define call_rcu_sched(head, func) call_rcu(head, func)
> > +
> > +static inline void rcu_init_sched(void)
> > +{
> > +}
> > +
> > +extern void __rcu_init(void);
> > +extern void rcu_check_callbacks(int cpu, int user);
> > +extern void rcu_restart_cpu(int cpu);
> > +
> > +extern long rcu_batches_completed(void);
> > +extern long rcu_batches_completed_bh(void);
> > +
> > +#ifdef CONFIG_NO_HZ
> > +void rcu_enter_nohz(void);
> > +void rcu_exit_nohz(void);
> > +#else /* CONFIG_NO_HZ */
> > +static inline void rcu_enter_nohz(void)
> > +{
> > +}
> > +static inline void rcu_exit_nohz(void)
> > +{
> > +}
> > +#endif /* CONFIG_NO_HZ */
> > +
> > +#endif /* __LINUX_RCUTREE_H */
> > diff --git a/init/Kconfig b/init/Kconfig
> > index b678803..6fdca78 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -914,10 +914,16 @@ source "block/Kconfig"
> >  config PREEMPT_NOTIFIERS
> >  	bool
> > 
> > -config CLASSIC_RCU
> > -	def_bool !PREEMPT_RCU
> > +config TREE_RCU_TRACE
> > +	def_bool RCU_TRACE && TREE_RCU
> > +	select DEBUG_FS
> >  	help
> > -	  This option selects the classic RCU implementation that is
> > -	  designed for best read-side performance on non-realtime
> > -	  systems.  Classic RCU is the default.  Note that the
> > -	  PREEMPT_RCU symbol is used to select/deselect this option.
> > +	  This option provides tracing for the TREE_RCU implementation,
> > +	  permitting Makefile to trivially select kernel/rcutree_trace.c.
> > +
> > +config PREEMPT_RCU_TRACE
> > +	def_bool RCU_TRACE && PREEMPT_RCU
> > +	select DEBUG_FS
> > +	help
> > +	  This option provides tracing for the PREEMPT_RCU implementation,
> > +	  permitting Makefile to trivially select kernel/rcupreempt_trace.c.
> > diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> > index 9fdba03..463f297 100644
> > --- a/kernel/Kconfig.preempt
> > +++ b/kernel/Kconfig.preempt
> > @@ -52,10 +52,29 @@ config PREEMPT
> > 
> >  endchoice
> > 
> > +choice
> > +	prompt "RCU Implementation"
> > +	default CLASSIC_RCU
> > +
> > +config CLASSIC_RCU
> > +	bool "Classic RCU"
> > +	help
> > +	  This option selects the classic RCU implementation that is
> > +	  designed for best read-side performance on non-realtime
> > +	  systems.
> > +	  
> > +	  Select this option if you are unsure.
> > +
> > +config TREE_RCU
> > +	bool "Tree-based hierarchical RCU"
> > +	help
> > +	  This option selects the RCU implementation that is
> > +	  designed for very large SMP system with hundreds or
> > +	  thousands of CPUs.
> > +
> >  config PREEMPT_RCU
> >  	bool "Preemptible RCU"
> >  	depends on PREEMPT
> > -	default n
> >  	help
> >  	  This option reduces the latency of the kernel by making certain
> >  	  RCU sections preemptible. Normally RCU code is non-preemptible, if
> > @@ -64,16 +83,47 @@ config PREEMPT_RCU
> >  	  now-naive assumptions about each RCU read-side critical section
> >  	  remaining on a given CPU through its execution.
> > 
> > -	  Say N if you are unsure.
> > +endchoice
> > 
> >  config RCU_TRACE
> > -	bool "Enable tracing for RCU - currently stats in debugfs"
> > -	depends on PREEMPT_RCU
> > -	select DEBUG_FS
> > -	default y
> > +	bool "Enable tracing for RCU"
> > +	depends on TREE_RCU || PREEMPT_RCU
> >  	help
> >  	  This option provides tracing in RCU which presents stats
> >  	  in debugfs for debugging RCU implementation.
> > 
> >  	  Say Y here if you want to enable RCU tracing
> >  	  Say N if you are unsure.
> > +
> > +config RCU_FANOUT
> > +	int "Tree-based hierarchical RCU fanout value"
> > +	range 2 64 if 64BIT
> > +	range 2 32 if !64BIT
> > +	depends on TREE_RCU
> > +	default 64 if 64BIT
> > +	default 32 if !64BIT
> > +	help
> > +	  This option controls the fanout of hierarchical implementations
> > +	  of RCU, allowing RCU to work efficiently on machines with
> > +	  large numbers of CPUs.  This value must be at least the cube
> > +	  root of NR_CPUS, which allows NR_CPUS up to 32,768 for 32-bit
> > +	  systems and up to 262,144 for 64-bit systems.
> > +
> > +	  Select a specific number if testing RCU itself.
> > +	  Take the default if unsure.
> > +
> > +config RCU_FANOUT_EXACT
> > +	bool "Disable tree-based hierarchical RCU auto-balancing"
> > +	depends on TREE_RCU
> > +	default n
> > +	help
> > +	  This option forces use of the exact RCU_FANOUT value specified,
> > +	  regardless of imbalances in the hierarchy.  This is useful for
> > +	  testing RCU itself, and might one day be useful on systems with
> > +	  strong NUMA behavior.
> > +
> > +	  Without RCU_FANOUT_EXACT, the code will balance the hierarchy.
> > +
> > +	  Say n if unsure.
> > +
> > +	
> > diff --git a/kernel/Makefile b/kernel/Makefile
> > index 4e1d7df..101e880 100644
> > --- a/kernel/Makefile
> > +++ b/kernel/Makefile
> > @@ -74,10 +74,10 @@ obj-$(CONFIG_GENERIC_HARDIRQS) += irq/
> >  obj-$(CONFIG_SECCOMP) += seccomp.o
> >  obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
> >  obj-$(CONFIG_CLASSIC_RCU) += rcuclassic.o
> > +obj-$(CONFIG_TREE_RCU) += rcutree.o
> >  obj-$(CONFIG_PREEMPT_RCU) += rcupreempt.o
> > -ifeq ($(CONFIG_PREEMPT_RCU),y)
> > -obj-$(CONFIG_RCU_TRACE) += rcupreempt_trace.o
> > -endif
> > +obj-$(CONFIG_TREE_RCU_TRACE) += rcutree_trace.o
> > +obj-$(CONFIG_PREEMPT_RCU_TRACE) += rcupreempt_trace.o
> >  obj-$(CONFIG_RELAY) += relay.o
> >  obj-$(CONFIG_SYSCTL) += utsname_sysctl.o
> >  obj-$(CONFIG_TASK_DELAY_ACCT) += delayacct.o
> > diff --git a/kernel/rcupreempt.c b/kernel/rcupreempt.c
> > index 2782793..6bc8489 100644
> > --- a/kernel/rcupreempt.c
> > +++ b/kernel/rcupreempt.c
> > @@ -559,6 +559,16 @@ void rcu_irq_exit(void)
> >  	}
> >  }
> > 
> > +void rcu_nmi_enter(void)
> > +{
> > +	rcu_irq_enter();
> > +}
> > +
> > +void rcu_nmi_exit(void)
> > +{
> > +	rcu_irq_exit();
> > +}
> > +
> >  static void dyntick_save_progress_counter(int cpu)
> >  {
> >  	struct rcu_dyntick_sched *rdssp = &per_cpu(rcu_dyntick_sched, cpu);
> > diff --git a/kernel/rcupreempt_trace.c b/kernel/rcupreempt_trace.c
> > index 5edf82c..def42e8 100644
> > --- a/kernel/rcupreempt_trace.c
> > +++ b/kernel/rcupreempt_trace.c
> > @@ -149,12 +149,12 @@ static void rcupreempt_trace_sum(struct rcupreempt_trace *sp)
> >  		sp->done_length += cp->done_length;
> >  		sp->done_add += cp->done_add;
> >  		sp->done_remove += cp->done_remove;
> > -		atomic_set(&sp->done_invoked, atomic_read(&cp->done_invoked));
> > +		atomic_add(atomic_read(&cp->done_invoked), &sp->done_invoked);
> >  		sp->rcu_check_callbacks += cp->rcu_check_callbacks;
> > -		atomic_set(&sp->rcu_try_flip_1,
> > -			   atomic_read(&cp->rcu_try_flip_1));
> > -		atomic_set(&sp->rcu_try_flip_e1,
> > -			   atomic_read(&cp->rcu_try_flip_e1));
> > +		atomic_add(atomic_read(&cp->rcu_try_flip_1),
> > +			   &sp->rcu_try_flip_1);
> > +		atomic_add(atomic_read(&cp->rcu_try_flip_e1),
> > +			   &sp->rcu_try_flip_e1);
> >  		sp->rcu_try_flip_i1 += cp->rcu_try_flip_i1;
> >  		sp->rcu_try_flip_ie1 += cp->rcu_try_flip_ie1;
> >  		sp->rcu_try_flip_g1 += cp->rcu_try_flip_g1;
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > new file mode 100644
> > index 0000000..d0852c8
> > --- /dev/null
> > +++ b/kernel/rcutree.c
> > @@ -0,0 +1,1510 @@
> > +/*
> > + * Read-Copy Update mechanism for mutual exclusion
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License as published by
> > + * the Free Software Foundation; either version 2 of the License, or
> > + * (at your option) any later version.
> > + *
> > + * This program is distributed in the hope that it will be useful,
> > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> > + * GNU General Public License for more details.
> > + *
> > + * You should have received a copy of the GNU General Public License
> > + * along with this program; if not, write to the Free Software
> > + * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
> > + *
> > + * Copyright IBM Corporation, 2008
> > + *
> > + * Authors: Dipankar Sarma <dipankar@xxxxxxxxxx>
> > + *	    Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
> > + *	    Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Hierarchical version
> > + *
> > + * Based on the original work by Paul McKenney <paulmck@xxxxxxxxxx>
> > + * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
> > + *
> > + * For detailed explanation of Read-Copy Update mechanism see -
> > + * 	Documentation/RCU
> > + */
> > +#include <linux/types.h>
> > +#include <linux/kernel.h>
> > +#include <linux/init.h>
> > +#include <linux/spinlock.h>
> > +#include <linux/smp.h>
> > +#include <linux/rcupdate.h>
> > +#include <linux/interrupt.h>
> > +#include <linux/sched.h>
> > +#include <asm/atomic.h>
> > +#include <linux/bitops.h>
> > +#include <linux/module.h>
> > +#include <linux/completion.h>
> > +#include <linux/moduleparam.h>
> > +#include <linux/percpu.h>
> > +#include <linux/notifier.h>
> > +#include <linux/cpu.h>
> > +#include <linux/mutex.h>
> > +#include <linux/time.h>
> > +
> > +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> > +static struct lock_class_key rcu_lock_key;
> > +struct lockdep_map rcu_lock_map =
> > +	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key);
> > +EXPORT_SYMBOL_GPL(rcu_lock_map);
> > +#endif
> > +
> > +/* Data structures. */
> > +
> > +#define RCU_STATE_INITIALIZER(name) { \
> > +	.level = { &name.node[0] }, \
> > +	.levelcnt = { \
> > +		NUM_RCU_LVL_0,  /* root of hierarchy. */ \
> > +		NUM_RCU_LVL_1, \
> > +		NUM_RCU_LVL_2, \
> > +		NUM_RCU_LVL_3, /* == MAX_RCU_LVLS */ \
> > +	}, \
> > +	.signaled = RCU_SIGNAL_INIT, \
> > +	.gpnum = -300, \
> > +	.completed = -300, \
> > +	.onofflock = __SPIN_LOCK_UNLOCKED(&name.onofflock), \
> > +	.fqslock = __SPIN_LOCK_UNLOCKED(&name.fqslock), \
> > +	.n_force_qs = 0, \
> > +	.n_force_qs_ngp = 0, \
> > +}
> > +
> > +struct rcu_state rcu_state = RCU_STATE_INITIALIZER(rcu_state);
> > +DEFINE_PER_CPU(struct rcu_data, rcu_data);
> > +
> > +struct rcu_state rcu_bh_state = RCU_STATE_INITIALIZER(rcu_bh_state);
> > +DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);
> > +
> > +#ifdef CONFIG_NO_HZ
> > +DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks);
> > +#endif /* #ifdef CONFIG_NO_HZ */
> > +
> > +static int blimit = 10;		/* Maximum callbacks per softirq. */
> > +static int qhimark = 10000;	/* If this many pending, ignore blimit. */
> > +static int qlowmark = 100;	/* Once only this many pending, use blimit. */
> > +
> > +static void force_quiescent_state(struct rcu_state *rsp, int relaxed);
> > +
> > +/*
> > + * Return the number of RCU batches processed thus far for debug & stats.
> > + */
> > +long rcu_batches_completed(void)
> > +{
> > +	return rcu_state.completed;
> > +}
> > +EXPORT_SYMBOL_GPL(rcu_batches_completed);
> > +
> > +/*
> > + * Return the number of RCU BH batches processed thus far for debug & stats.
> > + */
> > +long rcu_batches_completed_bh(void)
> > +{
> > +	return rcu_bh_state.completed;
> > +}
> > +EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);
> > +
> > +/*
> > + * Does the CPU have callbacks ready to be invoked?
> > + */
> > +static int
> > +cpu_has_callbacks_ready_to_invoke(struct rcu_data *rdp)
> > +{
> > +	return &rdp->nxtlist != rdp->nxttail[RCU_DONE_TAIL];
> > +}
> > +
> > +/*
> > + * Does the current CPU require a yet-as-unscheduled grace period?
> > + */
> > +static int
> > +cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
> > +{
> > +	/* ACCESS_ONCE() because we are accessing outside of lock. */
> > +	return *rdp->nxttail[RCU_DONE_TAIL] &&
> > +	       ACCESS_ONCE(rsp->completed) == ACCESS_ONCE(rsp->gpnum);
> > +}
> > +
> > +/*
> > + * Return the root node of the specified rcu_state structure.
> > + */
> > +static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
> > +{
> > +	return &rsp->node[0];
> > +}
> > +
> > +#ifdef CONFIG_SMP
> > +
> > +/*
> > + * If the specified CPU is offline, tell the caller that it is in
> > + * a quiescent state.  Otherwise, whack it with a reschedule IPI.
> > + * Grace periods can end up waiting on an offline CPU when that
> > + * CPU is in the process of coming online -- it will be added to the
> > + * rcu_node bitmasks before it actually makes it online.
> =>
>        This can also happen when a CPU has just gone offline,
>        but RCU hasn't yet marked it as offline. However, it's impact
>        on delaying the grace period may not be high as in the
>        CPU-online case.

Good point -- I updated the comment to include the going-offline case.

> > + * Because this
> > + * race is quite rare, we check for it after detecting that the grace
> > + * period has been delayed rather than checking each and every CPU
> > + * each and every time we start a new grace period.
> > + */
> > +static int rcu_implicit_offline_qs(struct rcu_data *rdp)
> > +{
> > +	/*
> > +	 * If the CPU is offline, it is in a quiescent state.  We can
> > +	 * trust its state not to change because interrupts are disabled.
> > +	 */
> > +	if (cpu_is_offline(rdp->cpu)) {
> > +		rdp->offline_fqs++;
> > +		return 1;
> > +	}
> > +
> > +	/* The CPU is online, so send it a reschedule IPI. */
> > +	if (rdp->cpu != smp_processor_id())
> =>
> 	This check is safe here since this callpath is invoked
> 	from a softirq, and thus the system cannot do a stop_machine()
> 	as yet. This implies that the cpu in question cannot go offline
> 	until we're done.

Yep!  I note that in the comment preceding the cpu_is_offline() above.
Is that sufficient, or should I reiterate that point in another comment
here?

> > +		smp_send_reschedule(rdp->cpu);
> > +	else
> > +		set_need_resched();
> > +	rdp->resched_ipi++;
> > +	return 0;
> > +}
> > +
> > +#endif /* #ifdef CONFIG_SMP */
> > +
> > +#ifdef CONFIG_NO_HZ
> > +static DEFINE_RATELIMIT_STATE(rcu_rs, 10 * HZ, 5);
> > +
> > +/*
> > + * Enter nohz mode, in other words, -leave- the mode in which RCU
> > + * read-side critical sections can occur.  (Though RCU read-side
> > + * critical sections can occur in irq handlers in nohz mode, a possibility
> > + * handled by rcu_irq_enter() and rcu_irq_exit()).
> > + */
> > +void rcu_enter_nohz(void)
> > +{
> > +	unsigned long flags;
> > +	struct rcu_dynticks *rdtp;
> > +
> > +	smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
> > +	local_irq_save(flags);
> > +	rdtp = &__get_cpu_var(rcu_dynticks);
> > +	rdtp->dynticks++;
> > +	rdtp->dynticks_nesting++;
> > +	WARN_ON_RATELIMIT(__get_cpu_var(rcu_dynticks).dynticks & 0x1, &rcu_rs);
> > +	local_irq_restore(flags);
> > +}
> > +
> > +/*
> > + * Exit nohz mode.
> > + */
> > +void rcu_exit_nohz(void)
> > +{
> > +	unsigned long flags;
> > +	struct rcu_dynticks *rdtp;
> > +
> > +	local_irq_save(flags);
> > +	rdtp = &__get_cpu_var(rcu_dynticks);
> > +	rdtp->dynticks++;
> > +	rdtp->dynticks_nesting--;
> > +	WARN_ON_RATELIMIT(!(__get_cpu_var(rcu_dynticks).dynticks & 0x1),
> > +			  &rcu_rs);
> > +	local_irq_restore(flags);
> > +	smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
> > +}
> > +
> > +/**
> > + * rcu_nmi_enter - Called from NMI
> > + *
> > + * If the CPU was idle with dynamic ticks active, and there is no
> > + * irq handler running, this updates rdtp->dynticks_nmi to let the
> > + * RCU grace-period handling know that the CPU is active.
> > + */
> > +void rcu_nmi_enter(void)
> > +{
> > +	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
> > +
> > +	if (rdtp->dynticks & 0x1)
> > +		return;
> > +	rdtp->dynticks_nmi++;
> > +	WARN_ON_RATELIMIT(!(rdtp->dynticks_nmi & 0x1), &rcu_rs);
> > +}
> > +
> > +/**
> > + * rcu_nmi_exit - Called from NMI
> > + *
> > + * If the CPU was idle with dynamic ticks active, and there is no
> > + * irq handler running, this updates rdtp->dynticks_nmi to let the
> > + * RCU grace-period handling know that the CPU is no longer active.
> > + */
> > +void rcu_nmi_exit(void)
> > +{
> > +	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
> > +
> > +	if (rdtp->dynticks & 0x1)
> > +		return;
> > +	rdtp->dynticks_nmi++;
> > +	WARN_ON_RATELIMIT(rdtp->dynticks_nmi & 0x1, &rcu_rs);
> > +}
> > +
> > +/**
> > + * rcu_irq_enter - Called from hard irq handlers
> > + *
> > + * If the CPU was idle with dynamic ticks active, this updates the
> > + * rdtp->dynticks to let the RCU handling know that the CPU is active.
> > + */
> > +void rcu_irq_enter(void)
> > +{
> > +	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
> > +
> > +	if (rdtp->dynticks_nesting++)
> > +		return;
> > +	rdtp->dynticks++;
> > +	WARN_ON_RATELIMIT(!(rdtp->dynticks & 0x1), &rcu_rs);
> > +}
> > +
> > +/**
> > + * rcu_irq_exit - Called when exiting hard irq context.
> > + *
> > + * If the CPU was idle with dynamic ticks active, update the rdp->dynticks
> > + * to put let the RCU handling be aware that the CPU is going back to idle
> > + * with no ticks.
> > + */
> > +void rcu_irq_exit(void)
> > +{
> > +	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
> > +
> > +	if (--rdtp->dynticks_nesting)
> > +		return;
> > +	rdtp->dynticks++;
> > +	WARN_ON_RATELIMIT(rdtp->dynticks & 0x1, &rcu_rs);
> > +
> > +	/* If the interrupt queued a callback, get out of dyntick mode. */
> > +	if (__get_cpu_var(rcu_data).nxtlist ||
> > +	    __get_cpu_var(rcu_bh_data).nxtlist)
> > +		set_need_resched();
> 
> =>	Just wondering, can't NMI handlers queue callbacks? If yes,
> 	isn't this check needed in rcu_nmi_exit() as well ?

NMI handlers are forbidden to queue callbacks, as the current call_rcu()
implementation is not NMI-safe.  It would be possible to create an
NMI-safe implementation, but there needs to be someone who needs it
-really- badly to justify the added complexity.  ;-)

> > +}
> > +
> > +/*
> > + * Record the specified "completed" value, which is later used to validate
> > + * dynticks counter manipulations.  Specify "rsp->complete - 1" to
> =>					       ^^^^^^^^^^^^^^^^^^^
> 					       "rsp->completed - 1" ?

Good catch!  Fixed.

> > + * unconditionally invalidate any future dynticks manipulations (which is
> > + * useful at the beginning of a grace period).
> > + */
> > +static void dyntick_record_completed(struct rcu_state *rsp, int comp)
> > +{
> > +	rsp->dynticks_completed = comp;
> > +}
> > +
> > +#ifdef CONFIG_SMP
> > +
> > +/*
> > + * Recall the previously recorded value of the completion for dynticks.
> > + */
> > +static long dyntick_recall_completed(struct rcu_state *rsp)
> > +{
> > +	return rsp->dynticks_completed;
> > +}
> > +
> > +/*
> > + * Snapshot the specified CPU's dynticks counter so that we can later
> > + * credit them with an implicit quiescent state.  Return 1 if this CPU
> > + * is already in a quiescent state courtesy of dynticks idle mode.
> > + */
> > +static int dyntick_save_progress_counter(struct rcu_data *rdp)
> > +{
> > +	int ret;
> > +	int snap;
> > +	int snap_nmi;
> > +
> > +	snap = rdp->dynticks->dynticks;
> > +	snap_nmi = rdp->dynticks->dynticks_nmi;
> > +	smp_mb();	/* Order sampling of snap with end of grace period. */
> > +	rdp->dynticks_snap = snap;
> > +	rdp->dynticks_nmi_snap = snap_nmi;
> > +	ret = ((snap & 0x1) == 0) && ((snap_nmi & 0x1) == 0);
> > +	if (ret)
> > +		rdp->dynticks_fqs++;
> > +	return ret;
> > +}
> > +
> > +/*
> > + * Return true if the specified CPU has passed through a quiescent
> > + * state by virtue of being in or having passed through an dynticks
> > + * idle state since the last call to dyntick_save_progress_counter()
> > + * for this same CPU.
> > + */
> > +static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
> > +{
> > +	long curr;
> > +	long curr_nmi;
> > +	long snap;
> > +	long snap_nmi;
> > +
> > +	curr = rdp->dynticks->dynticks;
> > +	snap = rdp->dynticks_snap;
> > +	curr_nmi = rdp->dynticks->dynticks_nmi;
> > +	snap_nmi = rdp->dynticks_nmi_snap;
> > +	smp_mb(); /* force ordering with cpu entering/leaving dynticks. */
> > +
> > +	/*
> > +	 * If the CPU passed through or entered a dynticks idle phase with
> > +	 * no active irq/NMI handlers, then we can safely pretend that the CPU
> > +	 * already acknowledged the request to pass through a quiescent
> > +	 * state.  Either way, that CPU cannot possibly be in an RCU
> > +	 * read-side critical section that started before the beginning
> > +	 * of the current RCU grace period.
> > +	 */
> > +	if ((curr != snap || (curr & 0x1) == 0) &&
> > +	    (curr_nmi != snap_nmi || (curr_nmi & 0x1) == 0)) {
> > +		rdp->dynticks_fqs++;
> > +		return 1;
> > +	}
> > +
> > +	/* Go check for the CPU being offline. */
> > +	return rcu_implicit_offline_qs(rdp);
> > +}
> > +
> > +#endif /* #ifdef CONFIG_SMP */
> > +
> > +#else /* #ifdef CONFIG_NO_HZ */
> > +
> > +static void dyntick_record_completed(struct rcu_state *rsp, int comp)
> > +{
> > +}
> > +
> > +#ifdef CONFIG_SMP
> > +
> > +/*
> > + * If there are no dynticks, then the only way that a CPU can passively
> > + * be in a quiescent state is to be offline.  Unlike dynticks idle, which
> > + * is a point in time during the prior (already finished) grace period,
> > + * an offline CPU is always in a quiescent state, and thus can be
> > + * unconditionally applied.  So just return the current value of completed.
> > + */
> > +static long dyntick_recall_completed(struct rcu_state *rsp)
> > +{
> > +	return rsp->completed;
> > +}
> > +
> > +static int dyntick_save_progress_counter(struct rcu_data *rdp)
> > +{
> > +	return 0;
> > +}
> > +
> > +static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
> > +{
> > +	return rcu_implicit_offline_qs(rdp);
> > +}
> > +
> > +#endif /* #ifdef CONFIG_SMP */
> > +
> > +#endif /* #else #ifdef CONFIG_NO_HZ */
> > +
> > +#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
> > +
> > +static void record_gp_stall_check_time(struct rcu_state *rsp)
> > +{
> > +	rsp->gp_start = jiffies;
> > +	rsp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_CHECK;
> > +}
> > +
> > +static void print_other_cpu_stall(struct rcu_state *rsp)
> > +{
> > +	int cpu;
> > +	long delta;
> > +	unsigned long flags;
> > +	struct rcu_node *rnp = rcu_get_root(rsp);
> > +	struct rcu_node *rnp_cur = rsp->level[NUM_RCU_LVLS - 1];
> > +	struct rcu_node *rnp_end = &rsp->node[NUM_RCU_NODES];
> > +
> > +	/* Only let one CPU complain about others per time interval. */
> > +
> > +	spin_lock_irqsave(&rnp->lock, flags);
> > +	delta = jiffies - rsp->jiffies_stall;
> > +	if (delta < RCU_STALL_RAT_DELAY || rsp->gpnum != rsp->completed) {
> =>	----------------> [1]
> 	See comment in check_cpu_stall()
> > +		spin_unlock_irqrestore(&rnp->lock, flags);
> > +		return;
> > +	}
> > +	rsp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
> > +	spin_unlock_irqrestore(&rnp->lock, flags);
> > +
> > +	/* OK, time to rat on our buddy... */
> > +
> > +	printk(KERN_ERR "RCU detected CPU stalls:");
> > +	for (; rnp_cur < rnp_end; rnp_cur++) {
> > +		if (rnp_cur->qsmask == 0)
> > +			continue;
> > +		for (cpu = 0; cpu <= rnp_cur->grphi - rnp_cur->grplo; cpu++)
> > +			if (rnp_cur->qsmask & (1UL << cpu))
> > +				printk(" %d", rnp_cur->grplo + cpu);
> > +	}
> > +	printk(" (detected by %d, t=%ld jiffies)\n",
> > +	       smp_processor_id(), (long)(jiffies - rsp->gp_start));
> > +	force_quiescent_state(rsp, 0);  /* Kick them all. */
> > +}
> > +
> > +static void print_cpu_stall(struct rcu_state *rsp)
> > +{
> > +	unsigned long flags;
> > +	struct rcu_node *rnp = rcu_get_root(rsp);
> > +
> > +	printk(KERN_ERR "RCU detected CPU %d stall (t=%lu jiffies)\n",
> > +			smp_processor_id(), jiffies - rsp->gp_start);
> > +	dump_stack();
> > +	spin_lock_irqsave(&rnp->lock, flags);
> > +	if ((long)(jiffies - rsp->jiffies_stall) >= 0)
> > +		rsp->jiffies_stall =
> > +			jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
> > +	spin_unlock_irqrestore(&rnp->lock, flags);
> > +	set_need_resched();  /* kick ourselves to get things going. */
> > +}
> > +
> > +static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
> > +{
> > +	long delta;
> > +	struct rcu_node *rnp;
> > +
> > +	delta = jiffies - rsp->jiffies_stall;
> > +	rnp = rdp->mynode;
> > +	if ((rnp->qsmask & rdp->grpmask) && delta >= 0) {
> > +
> > +		/* We haven't checked in, so go dump stack. */
> > +		print_cpu_stall(rsp);
> > +
> > +	} else if (rsp->gpnum != rsp->completed &&
> > +		   delta >= RCU_STALL_RAT_DELAY) {
> 
> =>		If this condition is true, then,
> 		rsp->gpnum != rsp->completed. Hence, we will always enter
> 		the if() condition in print_other_cpu_stall() at
> 		[1] (See above), and return without ratting our buddy.
> 
> 		That defeats the purpose of the stall check or I am
> 		missing the obvious, which is quite possible :-)

Let's see...  The goal of this code is as follows:

o	If possible, we want the stalled CPU to dump its own stack,
	since self-stack-tracing is more reliable.  (In fact, the
	code simply declines to do stack traces on other CPUs.)

o	But if the stalled CPU doesn't dump its own stack, we do
	want some other CPU to at least call attention to the stalled
	CPU.

o	If 4095 CPUs all note that a given CPU is stalled, we really
	don't want 4096 concurrent intermixed complaints on the console.

So the idea is that print_other_cpu_stall() acquires rnp->lock, and
rechecks the jiffies ("<" in print_other_cpu_stall() vs. ">=" in
check_cpu_stall()).  Only the first guy in will complain.

Make sense, or did I mess up something?

> > +
> > +		/* They had two time units to dump stack, so complain. */
> > +		print_other_cpu_stall(rsp);
> > +	}
> > +}
> > +
> > +#else /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
> > +
> > +static void record_gp_stall_check_time(struct rcu_state *rsp)
> > +{
> > +}
> > +
> > +static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
> > +{
> > +}
> > +
> > +#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
> > +
> > +/*
> > + * Update CPU-local rcu_data state to record the newly noticed grace period.
> > + * This is used both when we started the grace period and when we notice
> > + * that someone else started the grace period.
> > + */
> > +static void note_new_gpnum(struct rcu_state *rsp, struct rcu_data *rdp)
> > +{
> > +	rdp->qs_pending = 1;
> > +	rdp->passed_quiesc = 0;
> > +	rdp->gpnum = rsp->gpnum;
> > +}
> > +
> > +/*
> > + * Did someone else start a new RCU grace period start since we last
> > + * checked?  Update local state appropriately if so.  Must be called
> > + * on the CPU corresponding to rdp.
> > + */
> > +static int
> > +check_for_new_grace_period(struct rcu_state *rsp, struct rcu_data *rdp)
> > +{
> > +	unsigned long flags;
> > +	int ret = 0;
> > +
> > +	local_irq_save(flags);
> > +	if (rdp->gpnum != rsp->gpnum) {
> > +		note_new_gpnum(rsp, rdp);
> > +		ret = 1;
> > +	}
> > +	local_irq_restore(flags);
> > +	return ret;
> > +}
> > +
> > +/*
> > + * Start a new RCU grace period if warranted, re-initializing the hierarchy
> > + * in preparation for detecting the next grace period.  The caller must hold
> > + * the root node's ->lock, which is released before return.  Hard irqs must
> > + * be disabled.
> > + */
> > +static void
> > +rcu_start_gp(struct rcu_state *rsp, unsigned long iflg)
> > +	__releases(rsp->rda[smp_processor_id()]->lock)
> > +{
> > +	unsigned long flags = iflg;
> > +	struct rcu_data *rdp = rsp->rda[smp_processor_id()];
> > +	struct rcu_node *rnp = rcu_get_root(rsp);
> > +	struct rcu_node *rnp_cur;
> > +	struct rcu_node *rnp_end;
> > +
> > +	if (!cpu_needs_another_gp(rsp, rdp)) {
> > +		spin_unlock_irqrestore(&rnp->lock, flags);
> > +		return;
> > +	}
> > +
> > +	/* Advance to a new grace period and initialize state. */
> > +	rsp->gpnum++;
> > +	rsp->signaled = RCU_SIGNAL_INIT;
> > +	rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
> > +	record_gp_stall_check_time(rsp);
> > +	dyntick_record_completed(rsp, rsp->completed - 1);
> > +	note_new_gpnum(rsp, rdp);
> > +
> > +	/*
> > +	 * Because we are first, we know that all our callbacks will
> > +	 * be covered by this upcoming grace period, even the ones
> > +	 * that were registered arbitrarily recently.
> > +	 */
> > +	rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
> > +	rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
> > +
> > +	/* Special-case the common single-level case. */
> > +	if (NUM_RCU_NODES == 1) {
> > +		rnp->qsmask = rnp->qsmaskinit;
> > +		spin_unlock_irqrestore(&rnp->lock, flags);
> > +		return;
> > +	}
> > +
> > +	spin_unlock_irqrestore(&rnp->lock, flags);
> > +
> > +
> > +	/* Exclude any concurrent CPU-hotplug operations. */
> > +	spin_lock_irqsave(&rsp->onofflock, flags);
> > +
> > +	/*
> > +	 * Set the quiescent-state-needed bits in all the non-leaf RCU
> > +	 * nodes for all currently online CPUs.  This operation relies
> > +	 * on the layout of the hierarchy within the rsp->node[] array.
> > +	 * Note that other CPUs will access only the leaves of the
> > +	 * hierarchy, which still indicate that no grace period is in
> > +	 * progress.  In addition, we have excluded CPU-hotplug operations.
> > +	 *
> > +	 * We therefore do not need to hold any locks.  Any required
> > +	 * memory barriers will be supplied by the locks guarding the
> > +	 * leaf rcu_nodes in the hierarchy.
> > +	 */
> > +
> > +	rnp_end = rsp->level[NUM_RCU_LVLS - 1];
> > +	for (rnp_cur = &rsp->node[0]; rnp_cur < rnp_end; rnp_cur++)
> > +		rnp_cur->qsmask = rnp_cur->qsmaskinit;
> > +
> > +	/*
> > +	 * Now set up the leaf nodes.  Here we must be careful.  First,
> > +	 * we need to hold the lock in order to exclude other CPUs, which
> > +	 * might be contending for the leaf nodes' locks.  Second, as
> > +	 * soon as we initialize a given leaf node, its CPUs might run
> > +	 * up the rest of the hierarchy.  We must therefore acquire locks
> > +	 * for each node that we touch during this stage.  (But we still
> > +	 * are excluding CPU-hotplug operations.)
> > +	 *
> > +	 * Note that the grace period cannot complete until we finish
> > +	 * the initialization process, as there will be at least one
> > +	 * qsmask bit set in the root node until that time, namely the
> > +	 * one corresponding to this CPU.
> > +	 */
> > +	rnp_end = &rsp->node[NUM_RCU_NODES];
> > +	rnp_cur = rsp->level[NUM_RCU_LVLS - 1];
> > +	for (; rnp_cur < rnp_end; rnp_cur++) {
> > +		spin_lock(&rnp_cur->lock);	/* irqs already disabled. */
> > +		rnp_cur->qsmask = rnp_cur->qsmaskinit;
> > +		spin_unlock(&rnp_cur->lock);	/* irqs already disabled. */
> > +	}
> > +
> > +	spin_unlock_irqrestore(&rsp->onofflock, flags);
> > +}
> > +
> > +/*
> > + * Advance this CPU's callbacks, but only if the current grace period
> > + * has ended.  This may be called only from the CPU to whom the rdp
> > + * belongs.
> > + */
> > +static void
> > +rcu_process_gp_end(struct rcu_state *rsp, struct rcu_data *rdp)
> > +{
> > +	long completed_snap;
> > +	unsigned long flags;
> > +
> > +	local_irq_save(flags);
> > +	completed_snap = ACCESS_ONCE(rsp->completed);  /* outside of lock. */
> > +
> > +	/* Did another grace period end? */
> > +	if (rdp->completed != completed_snap) {
> > +
> > +		/* Advance callbacks.  No harm if list empty. */
> > +		rdp->nxttail[RCU_DONE_TAIL] = rdp->nxttail[RCU_WAIT_TAIL];
> > +		rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_READY_TAIL];
> > +		rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
> > +
> > +		/* Remember that we saw this grace-period completion. */
> > +		rdp->completed = completed_snap;
> > +	}
> > +	local_irq_restore(flags);
> > +}
> > +
> > +/*
> > + * Similar to cpu_quiet(), for which it is a helper function.  Allows
> > + * a group of CPUs to be quieted at one go, though all the CPUs in the
> > + * group must be represented by the same leaf rcu_node structure.
> > + * That structure's lock must be held upon entry, and it is released
> > + * before return.
> > + */
> > +static void
> > +cpu_quiet_msk(unsigned long mask, struct rcu_state *rsp, struct rcu_node *rnp,
> > +	      unsigned long flags)
> > +	__releases(rnp->lock)
> > +{
> > +	/* Walk up the rcu_node hierarchy. */
> > +	for (;;) {
> > +		if (!(rnp->qsmask & mask)) {
> > +
> > +			/* Our bit has already been cleared, so done. */
> > +			spin_unlock_irqrestore(&rnp->lock, flags);
> > +			return;
> > +		}
> > +		rnp->qsmask &= ~mask;
> > +		if (rnp->qsmask != 0) {
> > +
> > +			/* Other bits still set at this level, so done. */
> > +			spin_unlock_irqrestore(&rnp->lock, flags);
> > +			return;
> > +		}
> > +		mask = rnp->grpmask;
> > +		if (rnp->parent == NULL) {
> > +
> > +			/* No more levels.  Exit loop holding root lock. */
> > +
> > +			break;
> > +		}
> > +		spin_unlock_irqrestore(&rnp->lock, flags);
> > +		rnp = rnp->parent;
> > +		spin_lock_irqsave(&rnp->lock, flags);
> > +	}
> > +
> > +	/*
> > +	 * Get here if we are the last CPU to pass through a quiescent
> > +	 * state for this grace period.  Clean up and let rcu_start_gp()
> > +	 * start up the next grace period if one is needed.  Note that
> > +	 * we still hold rnp->lock, as required by rcu_start_gp(), which
> > +	 * will release it.
> > +	 */
> > +	rsp->completed = rsp->gpnum;
> > +	rcu_process_gp_end(rsp, rsp->rda[smp_processor_id()]);
> > +	rcu_start_gp(rsp, flags);  /* releases rnp->lock. */
> > +}
> > +
> > +/*
> > + * Record a quiescent state for the specified CPU, which must either be
> > + * the current CPU or an offline CPU.  When invoking this on one's own
> > + * behalf, lastcomp is used to make sure we are still in the grace period
> > + * of interest.  We don't want to end the current grace period based on
> > + * quiescent states detected in an earlier grace period!  On the other hand,
> > + * it the CPU being quieted is offline, we can safely pass in lastcomp==NULL,
> > + * since an offline CPU is in a quiescent state with respect to any grace
> > + * period, unlike pesky online CPUs, which can go non-quiescent with
> > + * absolutely no warning.
> > + */
> > +static void
> > +cpu_quiet(int cpu, struct rcu_state *rsp, struct rcu_data *rdp, long *lastcomp)
> > +{
> > +	unsigned long flags;
> > +	unsigned long mask;
> > +	struct rcu_node *rnp;
> > +
> > +	rnp = rdp->mynode;
> > +	spin_lock_irqsave(&rnp->lock, flags);
> > +	if (lastcomp != NULL &&
> > +	    *lastcomp != ACCESS_ONCE(rsp->completed)) {
> > +
> > +		/*
> > +		 * Someone beat us to it for this grace period, so leave.
> > +		 * The race with GP start is resolved by the fact that we
> > +		 * hold the leaf rcu_node lock, so that the per-CPU bits
> > +		 * cannot yet be initialized -- so we would simply find our
> > +		 * CPU's bit already cleared in cpu_quiet_msk() if this race
> > +		 * occurred.
> > +		 */
> > +		rdp->passed_quiesc = 0;	/* try again later! */
> > +		spin_unlock_irqrestore(&rnp->lock, flags);
> > +		return;
> > +	}
> > +	mask = rdp->grpmask;
> > +	if ((rnp->qsmask & mask) == 0) {
> > +		spin_unlock_irqrestore(&rnp->lock, flags);
> > +	} else {
> > +		rdp->qs_pending = 0;
> > +
> > +		/*
> > +		 * This GP can't end until cpu checks in, so all of our
> > +		 * callbacks can be processed during the next GP.
> > +		 */
> > +		rdp = rsp->rda[smp_processor_id()];
> > +		rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
> > +
> > +		cpu_quiet_msk(mask, rsp, rnp, flags); /* releases rnp->lock */
> > +	}
> > +}
> > +
> > +/*
> > + * Check to see if there is a new grace period of which this CPU
> > + * is not yet aware, and if so, set up local rcu_data state for it.
> > + * Otherwise, see if this CPU has just passed through its first
> > + * quiescent state for this grace period, and record that fact if so.
> > + */
> > +static void
> > +rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
> > +{
> > +	/* If there is now a new grace period, record and return. */
> > +	if (check_for_new_grace_period(rsp, rdp))
> > +		return;
> > +
> > +	/*
> > +	 * Does this CPU still need to do its part for current grace period?
> > +	 * If no, return and let the other CPUs do their part as well.
> > +	 */
> > +	if (!rdp->qs_pending)
> > +		return;
> > +
> > +	/*
> > +	 * Was there a quiescent state since the beginning of the grace
> > +	 * period? If no, then exit and wait for the next call.
> > +	 */
> > +	if (!rdp->passed_quiesc)
> > +		return;
> > +
> > +	/* Tell RCU we are done (but cpu_quiet() will be the judge of that). */
> > +	cpu_quiet(rdp->cpu, rsp, rdp, &rdp->passed_quiesc_completed);
> > +}
> > +
> > +#ifdef CONFIG_HOTPLUG_CPU
> > +
> > +/*
> > + * Remove the outgoing CPU from the bitmasks in the rcu_node hierarchy
> > + * and move all callbacks from the outgoing CPU to the current one.
> > + */
> > +static void __rcu_offline_cpu(int cpu, struct rcu_state *rsp)
> > +{
> > +	int i;
> > +	unsigned long flags;
> > +	unsigned long mask;
> > +	struct rcu_data *rdp = rsp->rda[cpu];
> > +	struct rcu_data *rdp_me;
> > +	struct rcu_node *rnp;
> > +
> > +	/* Exclude any attempts to start a new grace period. */
> > +	spin_lock_irqsave(&rsp->onofflock, flags);
> > +
> > +	/* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */
> > +	rnp = rdp->mynode;
> > +	mask = rdp->grpmask;	/* rnp->grplo is constant. */
> > +	do {
> > +		spin_lock(&rnp->lock);		/* irqs already disabled. */
> > +		rnp->qsmaskinit &= ~mask;
> > +		if (rnp->qsmaskinit != 0) {
> > +			spin_unlock(&rnp->lock); /* irqs already disabled. */
> > +			break;
> > +		}
> > +		mask = rnp->grpmask;
> > +		spin_unlock(&rnp->lock);	/* irqs already disabled. */
> > +		rnp = rnp->parent;
> > +	} while (rnp != NULL);
> > +
> > +	spin_unlock(&rsp->onofflock);		/* irqs remain disabled. */
> > +
> > +	/* Being offline is a quiescent state, so go record it. */
> > +	cpu_quiet(cpu, rsp, rdp, NULL);
> > +
> > +	/*
> > +	 * Move callbacks from the outgoing CPU to the running CPU.
> > +	 * Note that the outgoing CPU is now quiscent, so it is now
> > +	 * (uncharacteristically) safe to access it rcu_data structure.
> > +	 * Note also that we must carefully retain the order of the
> > +	 * outgoing CPU's callbacks in order for rcu_barrier() to work
> > +	 * correctly.  Finally, note that we start all the callbacks
> > +	 * afresh, even those that have passed through a grace period
> > +	 * and are therefore ready to invoke.  The theory is that hotplug
> > +	 * events are rare, and that if they are frequent enough to
> > +	 * indefinitely delay callbacks, you have far worse things to
> > +	 * be worrying about.
> > +	 */
> > +	rdp_me = rsp->rda[smp_processor_id()];
> > +	if (rdp->nxtlist != NULL) {
> > +		*rdp_me->nxttail[RCU_NEXT_TAIL] = rdp->nxtlist;
> > +		rdp_me->nxttail[RCU_NEXT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
> > +		rdp->nxtlist = NULL;
> > +		for (i = 0; i < RCU_NEXT_SIZE; i++)
> > +			rdp->nxttail[i] = &rdp->nxtlist;
> > +		rdp_me->qlen += rdp->qlen;
> > +		rdp->qlen = 0;
> > +	}
> > +	local_irq_restore(flags);
> > +}
> > +
> > +/*
> > + * Remove the specified CPU from the RCU hierarchy and move any pending
> > + * callbacks that it might have to the current CPU.  This code assumes
> > + * that at least one CPU in the system will remain running at all times.
> > + * Any attempt to offline -all- CPUs is likely to strand RCU callbacks.
> > + */
> > +static void rcu_offline_cpu(int cpu)
> > +{
> > +	__rcu_offline_cpu(cpu, &rcu_state);
> > +	__rcu_offline_cpu(cpu, &rcu_bh_state);
> > +}
> > +
> > +#else /* #ifdef CONFIG_HOTPLUG_CPU */
> > +
> > +static void rcu_offline_cpu(int cpu)
> > +{
> > +}
> > +
> > +#endif /* #else #ifdef CONFIG_HOTPLUG_CPU */
> > +
> > +/*
> > + * Invoke any RCU callbacks that have made it to the end of their grace
> > + * period.  Thottle as specified by rdp->blimit.
> > + */
> > +static void rcu_do_batch(struct rcu_data *rdp)
> > +{
> > +	unsigned long flags;
> > +	struct rcu_head *next, *list, **tail;
> > +	int count;
> > +
> > +	/* If no callbacks are ready, just return.*/
> > +	if (!cpu_has_callbacks_ready_to_invoke(rdp))
> > +		return;
> > +
> > +	/*
> > +	 * Extract the list of ready callbacks, disabling to prevent
> > +	 * races with call_rcu() from interrupt handlers.
> > +	 */
> > +	local_irq_save(flags);
> > +	list = rdp->nxtlist;
> > +	rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];
> > +	*rdp->nxttail[RCU_DONE_TAIL] = NULL;
> > +	tail = rdp->nxttail[RCU_DONE_TAIL];
> > +	for (count = RCU_NEXT_SIZE - 1; count >= 0; count--)
> > +		if (rdp->nxttail[count] == rdp->nxttail[RCU_DONE_TAIL])
> > +			rdp->nxttail[count] = &rdp->nxtlist;
> > +	local_irq_restore(flags);
> > +
> > +	/* Invoke callbacks. */
> > +	count = 0;
> > +	while (list) {
> > +		next = list->next;
> > +		prefetch(next);
> > +		list->func(list);
> > +		list = next;
> > +		if (++count >= rdp->blimit)
> > +			break;
> > +	}
> > +
> > +	/* Update count, and requeue any remaining callbacks. */
> > +	local_irq_save(flags);
> > +	rdp->qlen -= count;
> > +	if (list != NULL) {
> > +		*tail = rdp->nxtlist;
> > +		rdp->nxtlist = list;
> > +		for (count = 0; count < RCU_NEXT_SIZE; count++)
> > +			if (&rdp->nxtlist == rdp->nxttail[count])
> > +				rdp->nxttail[count] = tail;
> > +			else
> > +				break;
> > +	}
> > +	local_irq_restore(flags);
> > +
> > +	/* Reinstate batch limit if we have worked down the excess. */
> > +	if (rdp->blimit == INT_MAX && rdp->qlen <= qlowmark)
> > +		rdp->blimit = blimit;
> > +
> > +	/* Re-raise the RCU softirq if there are callbacks remaining. */
> > +	if (cpu_has_callbacks_ready_to_invoke(rdp))
> > +		raise_softirq(RCU_SOFTIRQ);
> > +}
> > +
> > +/*
> > + * Check to see if this CPU is in a non-context-switch quiescent state
> > + * (user mode or idle loop for rcu, non-softirq execution for rcu_bh).
> > + * Also schedule the RCU softirq handler.
> > + *
> > + * This function must be called with hardirqs disabled.  It is normally
> > + * invoked from the scheduling-clock interrupt.  If rcu_pending returns
> > + * false, there is no point in invoking rcu_check_callbacks().
> > + */
> > +void rcu_check_callbacks(int cpu, int user)
> > +{
> > +	if (user ||
> > +	    (idle_cpu(cpu) && !in_softirq() &&
> > +				hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
> > +
> > +		/*
> > +		 * Get here if this CPU took its interrupt from user
> > +		 * mode or from the idle loop, and if this is not a
> > +		 * nested interrupt.  In this case, the CPU is in
> > +		 * a quiescent state, so count it.
> > +		 *
> > +		 * Also do a memory barrier.  This is needed to handle
> > +		 * the case where writes from a preempt-disable section
> > +		 * of code get reordered into schedule() by this CPU's
> > +		 * write buffer.  The memory barrier makes sure that
> > +		 * the rcu_qsctr_inc() and rcu_bh_qsctr_inc() are see
> > +		 * by other CPUs to happen after any such write.
> > +		 */
> > +
> > +		smp_mb();  /* See above block comment. */
> > +		rcu_qsctr_inc(cpu);
> > +		rcu_bh_qsctr_inc(cpu);
> > +
> > +	} else if (!in_softirq()) {
> > +
> > +		/*
> > +		 * Get here if this CPU did not take its interrupt from
> > +		 * softirq, in other words, if it is not interrupting
> > +		 * a rcu_bh read-side critical section.  This is an _bh
> > +		 * critical section, so count it.  The memory barrier
> > +		 * is needed for the same reason as is the above one.
> > +		 */
> > +
> > +		smp_mb();  /* See above block comment. */
> > +		rcu_bh_qsctr_inc(cpu);
> > +	}
> > +	raise_softirq(RCU_SOFTIRQ);
> > +}
> > +
> > +#ifdef CONFIG_SMP
> > +
> > +/*
> > + * Scan the leaf rcu_node structures, processing dyntick state for any that
> > + * have not yet encountered a quiescent state, using the function specified.
> > + * Returns 1 if the current grace period ends while scanning (possibly
> > + * because we made it end).
> > + */
> > +static int rcu_process_dyntick(struct rcu_state *rsp, long lastcomp,
> > +			       int (*f)(struct rcu_data *))
> > +{
> > +	unsigned long bit;
> > +	int cpu;
> > +	unsigned long flags;
> > +	unsigned long mask;
> > +	struct rcu_node *rnp_cur = rsp->level[NUM_RCU_LVLS - 1];
> > +	struct rcu_node *rnp_end = &rsp->node[NUM_RCU_NODES];
> > +
> > +	for (; rnp_cur < rnp_end; rnp_cur++) {
> > +		mask = 0;
> > +		spin_lock_irqsave(&rnp_cur->lock, flags);
> > +		if (rsp->completed != lastcomp) {
> > +			spin_unlock_irqrestore(&rnp_cur->lock, flags);
> > +			return 1;
> > +		}
> > +		if (rnp_cur->qsmask == 0) {
> > +			spin_unlock_irqrestore(&rnp_cur->lock, flags);
> > +			continue;
> > +		}
> > +		cpu = rnp_cur->grplo;
> > +		bit = 1;
> > +		mask = 0;
> > +		for (; cpu <= rnp_cur->grphi; cpu++, bit <<= 1) {
> > +			if ((rnp_cur->qsmask & bit) != 0 && f(rsp->rda[cpu]))
> > +				mask |= bit;
> > +		}
> > +		if (mask != 0 && rsp->completed == lastcomp) {
> > +
> > +			/* cpu_quiet_msk() releases rnp_cur->lock. */
> > +			cpu_quiet_msk(mask, rsp, rnp_cur, flags);
> > +			continue;
> > +		}
> > +		spin_unlock_irqrestore(&rnp_cur->lock, flags);
> > +	}
> > +	return 0;
> > +}
> > +
> > +/*
> > + * Force quiescent states on reluctant CPUs, and also detect which
> > + * CPUs are in dyntick-idle mode.
> > + */
> > +static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
> > +{
> > +	unsigned long flags;
> > +	long lastcomp;
> > +	struct rcu_node *rnp = rcu_get_root(rsp);
> > +	u8 signaled;
> > +
> > +	if (ACCESS_ONCE(rsp->completed) == ACCESS_ONCE(rsp->gpnum))
> > +		return;  /* No grace period in progress, nothing to force. */
> > +	if (!spin_trylock_irqsave(&rsp->fqslock, flags))
> > +		return;	/* Someone else is already on the job. */
> > +	if (relaxed && (long)(rsp->jiffies_force_qs - jiffies) >= 0)
> > +		goto unlock_ret; /* no emergency and done recently. */
> > +	rsp->n_force_qs++;
> > +	spin_lock(&rnp->lock);
> > +	lastcomp = rsp->completed;
> > +	signaled = rsp->signaled;
> > +	rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
> > +	if (rsp->completed == rsp->gpnum) {
> > +		rsp->n_force_qs_ngp++;
> > +		spin_unlock(&rnp->lock);
> > +		goto unlock_ret;  /* no GP in progress, time updated. */
> > +	}
> > +	spin_unlock(&rnp->lock);
> > +	switch (signaled) {
> > +	case RCU_SAVE_DYNTICK:
> > +
> > +		if (RCU_SIGNAL_INIT != RCU_SAVE_DYNTICK)
> > +			break; /* So gcc recognizes the dead code. */
> > +
> > +		/* Record dyntick-idle state. */
> > +		if (rcu_process_dyntick(rsp, lastcomp,
> > +					dyntick_save_progress_counter))
> > +			goto unlock_ret;
> > +
> > +		/* Update state, record completion counter. */
> > +		spin_lock(&rnp->lock);
> > +		if (lastcomp == rsp->completed) {
> > +			rsp->signaled = RCU_FORCE_QS;
> > +			dyntick_record_completed(rsp, lastcomp);
> > +		}
> > +		spin_unlock(&rnp->lock);
> > +		break;
> > +
> > +	case RCU_FORCE_QS:
> > +
> > +		/* Check dyntick-idle state, send IPI to laggarts. */
> > +		if (rcu_process_dyntick(rsp, dyntick_recall_completed(rsp),
> > +					rcu_implicit_dynticks_qs))
> > +			goto unlock_ret;
> > +
> > +		/* Leave state in case more forcing is required. */
> > +
> > +		break;
> > +	}
> > +unlock_ret:
> > +	spin_unlock_irqrestore(&rsp->fqslock, flags);
> > +}
> > +
> > +#else /* #ifdef CONFIG_SMP */
> > +
> > +static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
> > +{
> > +	set_need_resched();
> > +}
> > +
> > +#endif /* #else #ifdef CONFIG_SMP */
> > +
> > +/*
> > + * This does the RCU processing work from softirq context for the
> > + * specified rcu_state and rcu_data structures.  This may be called
> > + * only from the CPU to whom the rdp belongs.
> > + */
> > +static void
> > +__rcu_process_callbacks(struct rcu_state *rsp, struct rcu_data *rdp)
> > +{
> > +	unsigned long flags;
> > +
> > +	/*
> > +	 * If an RCU GP has gone long enough, go check for dyntick
> > +	 * idle CPUs and, if needed, send resched IPIs.
> > +	 */
> > +	if ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0)
> > +		force_quiescent_state(rsp, 1);
> > +
> > +	/*
> > +	 * Advance callbacks in response to end of earlier grace
> > +	 * period that some other CPU ended.
> > +	 */
> > +	rcu_process_gp_end(rsp, rdp);
> > +
> > +	/* Update RCU state based on any recent quiescent states. */
> > +	rcu_check_quiescent_state(rsp, rdp);
> > +
> > +	/* Does this CPU require a not-yet-started grace period? */
> > +	if (cpu_needs_another_gp(rsp, rdp)) {
> > +		spin_lock_irqsave(&rcu_get_root(rsp)->lock, flags);
> > +		rcu_start_gp(rsp, flags);  /* releases above lock */
> > +	}
> > +
> > +	/* If there are callbacks ready, invoke them. */
> > +	rcu_do_batch(rdp);
> > +}
> > +
> > +/*
> > + * Do softirq processing for the current CPU.
> > + */
> > +static void rcu_process_callbacks(struct softirq_action *unused)
> > +{
> > +	/*
> > +	 * Memory references from any prior RCU read-side critical sections
> > +	 * executed by the interrupted code must be seen before any RCU
> > +	 * grace-period manupulations below.
> > +	 */
> > +	smp_mb(); /* See above block comment. */
> > +
> > +	__rcu_process_callbacks(&rcu_state, &__get_cpu_var(rcu_data));
> > +	__rcu_process_callbacks(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
> > +
> > +	/*
> > +	 * Memory references from any later RCU read-side critical sections
> > +	 * executed by the interrupted code must be seen after any RCU
> > +	 * grace-period manupulations above.
> > +	 */
> > +	smp_mb(); /* See above block comment. */
> > +}
> > +
> > +static void
> > +__call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
> > +	   struct rcu_state *rsp)
> > +{
> > +	unsigned long flags;
> > +	struct rcu_data *rdp;
> > +
> > +	head->func = func;
> > +	head->next = NULL;
> > +
> > +	smp_mb(); /* Ensure RCU update seen before callback registry. */
> > +
> > +	/*
> > +	 * Opportunistically note grace-period endings and beginnings.
> > +	 * Note that we might see a beginning right after we see an
> > +	 * end, but never vice versa, since this CPU has to pass through
> > +	 * a quiescent state betweentimes.
> > +	 */
> > +	local_irq_save(flags);
> > +	rdp = rsp->rda[smp_processor_id()];
> > +	rcu_process_gp_end(rsp, rdp);
> > +	check_for_new_grace_period(rsp, rdp);
> > +
> > +	/* Add the callback to our list. */
> > +	*rdp->nxttail[RCU_NEXT_TAIL] = head;
> > +	rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
> > +
> > +	/* Start a new grace period if one not already started. */
> > +	if (ACCESS_ONCE(rsp->completed) == ACCESS_ONCE(rsp->gpnum)) {
> > +		unsigned long nestflag;
> > +		struct rcu_node *rnp_root = rcu_get_root(rsp);
> > +
> > +		spin_lock_irqsave(&rnp_root->lock, nestflag);
> > +		rcu_start_gp(rsp, nestflag);  /* releases rnp_root->lock. */
> > +	}
> > +
> > +	/* Force the grace period if too many callbacks or too long waiting. */
> > +	if (unlikely(++rdp->qlen > qhimark)) {
> > +		rdp->blimit = INT_MAX;
> > +		force_quiescent_state(rsp, 0);
> > +	} else if ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0)
> > +		force_quiescent_state(rsp, 1);
> > +	local_irq_restore(flags);
> > +}
> > +
> > +/*
> > + * Queue an RCU callback for invocation after a grace period.
> > + */
> > +void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
> > +{
> > +	__call_rcu(head, func, &rcu_state);
> > +}
> > +EXPORT_SYMBOL_GPL(call_rcu);
> > +
> > +/*
> > + * Queue an RCU for invocation after a quicker grace period.
> > + */
> > +void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
> > +{
> > +	__call_rcu(head, func, &rcu_bh_state);
> > +}
> > +EXPORT_SYMBOL_GPL(call_rcu_bh);
> > +
> > +/*
> > + * Check to see if there is any immediate RCU-related work to be done
> > + * by the current CPU, for the specified type of RCU, returning 1 if so.
> > + * The checks are in order of increasing expense: checks that can be
> > + * carried out against CPU-local state are performed first.  However,
> > + * we must check for CPU stalls first, else we might not get a chance.
> > + */
> > +static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
> > +{
> > +	/* Check for CPU stalls, if enabled. */
> > +	check_cpu_stall(rsp, rdp);
> > +
> > +	/* Is the RCU core waiting for a quiescent state from this CPU? */
> > +	if (rdp->qs_pending)
> > +		return 1;
> > +
> > +	/* Does this CPU have callbacks ready to invoke? */
> > +	if (cpu_has_callbacks_ready_to_invoke(rdp))
> > +		return 1;
> > +
> > +	/* Has RCU gone idle with this CPU needing another grace period? */
> > +	if (cpu_needs_another_gp(rsp, rdp))
> > +		return 1;
> > +
> > +	/* Has another RCU grace period completed?  */
> > +	if (ACCESS_ONCE(rsp->completed) != rdp->completed) /* outside of lock */
> > +		return 1;
> > +
> > +	/* Has a new RCU grace period started? */
> > +	if (ACCESS_ONCE(rsp->gpnum) != rdp->gpnum) /* outside of lock */
> > +		return 1;
> > +
> > +	/* Has an RCU GP gone long enough to send resched IPIs &c? */
> > +	if (ACCESS_ONCE(rsp->completed) != ACCESS_ONCE(rsp->gpnum) &&
> > +	    (long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0)
> > +		return 1;
> > +
> > +	/* nothing to do */
> > +	return 0;
> > +}
> > +
> > +/*
> > + * Check to see if there is any immediate RCU-related work to be done
> > + * by the current CPU, returning 1 if so.  This function is part of the
> > + * RCU implementation; it is -not- an exported member of the RCU API.
> > + */
> > +int rcu_pending(int cpu)
> > +{
> > +	return __rcu_pending(&rcu_state, &per_cpu(rcu_data, cpu)) ||
> > +	       __rcu_pending(&rcu_bh_state, &per_cpu(rcu_bh_data, cpu));
> > +}
> > +
> > +/*
> > + * Check to see if any future RCU-related work will need to be done
> > + * by the current CPU, even if none need be done immediately, returning
> > + * 1 if so.  This function is part of the RCU implementation; it is -not-
> > + * an exported member of the RCU API.
> > + */
> > +int rcu_needs_cpu(int cpu)
> > +{
> > +	/* RCU callbacks either ready or pending? */
> > +	return per_cpu(rcu_data, cpu).nxtlist ||
> > +	       per_cpu(rcu_bh_data, cpu).nxtlist;
> > +}
> > +
> > +/*
> > + * Initialize a CPU's per-CPU RCU data.  We take this "scorched earth"
> > + * approach so that we don't have to worry about how long the CPU has
> > + * been gone, or whether it ever was online previously.  We do trust the
> > + * ->mynode field, as it is constant for a given struct rcu_data and
> > + * initialized during early boot.
> > + *
> > + * Note that only one online or offline event can be happening at a given
> > + * time.  Note also that we can accept some slop in the rsp->completed
> > + * access due to the fact that this CPU cannot possibly have any RCU
> > + * callbacks in flight yet.
> > + */
> > +static void
> > +rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
> > +{
> > +	unsigned long flags;
> > +	int i;
> > +	unsigned long mask;
> > +	struct rcu_data *rdp = rsp->rda[cpu];
> > +	struct rcu_node *rnp = rcu_get_root(rsp);
> > +
> > +	/* Set up local state, ensuring consistent view of global state. */
> > +	spin_lock_irqsave(&rnp->lock, flags);
> > +	rdp->completed = rsp->completed;
> > +	rdp->gpnum = rsp->completed;
> > +	rdp->passed_quiesc = 0;  /* We could be racing with new GP, */
> > +	rdp->qs_pending = 1;	 /*  so set up to respond to current GP. */
> > +	rdp->beenonline = 1;	 /* We have now been online. */
> > +	rdp->passed_quiesc_completed = rsp->completed - 1;
> > +	rdp->grpmask = 1UL << (cpu - rdp->mynode->grplo);
> > +	rdp->nxtlist = NULL;
> > +	for (i = 0; i < RCU_NEXT_SIZE; i++)
> > +		rdp->nxttail[i] = &rdp->nxtlist;
> > +	rdp->qlen = 0;
> > +	rdp->blimit = blimit;
> > +#ifdef CONFIG_NO_HZ
> > +	rdp->dynticks = &per_cpu(rcu_dynticks, cpu);
> > +#endif /* #ifdef CONFIG_NO_HZ */
> > +	rdp->cpu = cpu;
> > +	spin_unlock(&rnp->lock);		/* irqs remain disabled. */
> > +
> > +	/*
> > +	 * A new grace period might start here.  If so, we won't be part
> > +	 * of it, but that is OK, as we are currently in a quiescent state.
> > +	 */
> > +
> > +	/* Exclude any attempts to start a new GP on large systems. */
> > +	spin_lock(&rsp->onofflock);		/* irqs already disabled. */
> > +
> > +	/* Add CPU to rcu_node bitmasks. */
> > +	rnp = rdp->mynode;
> > +	mask = rdp->grpmask;
> > +	do {
> > +		/* Exclude any attempts to start a new GP on small systems. */
> > +		spin_lock(&rnp->lock);	/* irqs already disabled. */
> > +		rnp->qsmaskinit |= mask;
> > +		mask = rnp->grpmask;
> > +		spin_unlock(&rnp->lock); /* irqs already disabled. */
> > +		rnp = rnp->parent;
> > +	} while (rnp != NULL && !(rnp->qsmaskinit & mask));
> > +
> > +	spin_unlock(&rsp->onofflock);		/* irqs remain disabled. */
> > +
> > +	/*
> > +	 * A new grace period might start here.  If so, we will be part of
> > +	 * it, and its gpnum will be greater than ours, so we will
> > +	 * participate.  It is also possible for the gpnum to have been
> > +	 * incremented before this function was called, and the bitmasks
> > +	 * to not be filled out until now, in which case we will also
> > +	 * participate due to our gpnum being behind.
> > +	 */
> > +
> > +	/* Since it is coming online, the CPU is in a quiescent state. */
> > +	cpu_quiet(cpu, rsp, rdp, NULL);
> > +	local_irq_restore(flags);
> > +}
> > +
> > +static void __cpuinit rcu_online_cpu(int cpu)
> > +{
> > +#ifdef CONFIG_NO_HZ
> > +	struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
> > +
> > +	rdtp->dynticks_nesting = 1;
> > +	rdtp->dynticks |= 1; 	/* need consecutive #s even for hotplug. */
> > +	rdtp->dynticks_nmi = (rdtp->dynticks + 1) & ~0x1;
> =>	rdtp->dynticks is odd. Hence rdtp->dynticks + 1 should be even.
> 	Why is the additional & ~0x1 ?

Overly extreme paranoia?

> > +#endif /* #ifdef CONFIG_NO_HZ */
> > +	rcu_init_percpu_data(cpu, &rcu_state);
> > +	rcu_init_percpu_data(cpu, &rcu_bh_state);
> > +	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
> > +}
> > +
> > +/*
> > + * Handle CPU online/offline notifcation events.
> > + */
> > +static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
> > +				unsigned long action, void *hcpu)
> > +{
> > +	long cpu = (long)hcpu;
> > +
> > +	switch (action) {
> > +	case CPU_UP_PREPARE:
> > +	case CPU_UP_PREPARE_FROZEN:
> > +		rcu_online_cpu(cpu);
> > +		break;
> > +	case CPU_DEAD:
> > +	case CPU_DEAD_FROZEN:
> > +	case CPU_UP_CANCELED:
> > +	case CPU_UP_CANCELED_FROZEN:
> > +		rcu_offline_cpu(cpu);
> > +		break;
> > +	default:
> > +		break;
> > +	}
> > +	return NOTIFY_OK;
> > +}
> > +
> > +/*
> > + * Compute the per-level fanout, either using the exact fanout specified
> > + * or balancing the tree, depending on CONFIG_RCU_FANOUT_EXACT.
> > + */
> > +#ifdef CONFIG_RCU_FANOUT_EXACT
> > +static void __init rcu_init_levelspread(struct rcu_state *rsp)
> > +{
> > +	int i;
> > +
> > +	for (i = NUM_RCU_LVLS - 1; i >= 0; i--)
> > +		rsp->levelspread[i] = CONFIG_RCU_FANOUT;
> > +}
> > +#else /* #ifdef CONFIG_RCU_FANOUT_EXACT */
> > +static void __init rcu_init_levelspread(struct rcu_state *rsp)
> > +{
> > +	int ccur;
> > +	int cprv;
> > +	int i;
> > +
> > +	cprv = NR_CPUS;
> > +	for (i = NUM_RCU_LVLS - 1; i >= 0; i--) {
> > +		ccur = rsp->levelcnt[i];
> > +		rsp->levelspread[i] = (cprv + ccur - 1) / ccur;
> > +		cprv = ccur;
> > +	}
> > +}
> > +#endif /* #else #ifdef CONFIG_RCU_FANOUT_EXACT */
> > +
> > +/*
> > + * Helper function for rcu_init() that initializes one rcu_state structure.
> > + */
> > +static void __init rcu_init_one(struct rcu_state *rsp)
> > +{
> > +	int cpustride = 1;
> > +	int i;
> > +	int j;
> > +	struct rcu_node *rnp;
> > +
> > +	/* Initialize the level-tracking arrays. */
> > +
> > +	for (i = 1; i < NUM_RCU_LVLS; i++)
> > +		rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
> > +	rcu_init_levelspread(rsp);
> > +
> > +	/* Initialize the elements themselves, starting from the leaves. */
> > +
> > +	for (i = NUM_RCU_LVLS - 1; i >= 0; i--) {
> > +		cpustride *= rsp->levelspread[i];
> > +		rnp = rsp->level[i];
> > +		for (j = 0; j < rsp->levelcnt[i]; j++, rnp++) {
> > +			spin_lock_init(&rnp->lock);
> > +			rnp->qsmask = 0;
> > +			rnp->qsmaskinit = 0;
> > +			rnp->grplo = j * cpustride;
> > +			rnp->grphi = (j + 1) * cpustride - 1;
> > +			if (rnp->grphi >= NR_CPUS)
> > +				rnp->grphi = NR_CPUS - 1;
> > +			if (i == 0) {
> > +				rnp->grpnum = 0;
> > +				rnp->grpmask = 0;
> > +				rnp->parent = NULL;
> > +			} else {
> > +				rnp->grpnum = j % rsp->levelspread[i - 1];
> > +				rnp->grpmask = 1UL << rnp->grpnum;
> > +				rnp->parent = rsp->level[i - 1] +
> > +					      j / rsp->levelspread[i - 1];
> > +			}
> > +			rnp->level = i;
> > +		}
> > +	}
> > +}
> > +
> > +/*
> > + * Helper macro for __rcu_init().  To be used nowhere else!
> > + * Assigns leaf node pointers into each CPU's rcu_data structure.
> > + */
> > +#define RCU_DATA_PTR_INIT(rsp, rcu_data) \
> > +do { \
> > +	rnp = (rsp)->level[NUM_RCU_LVLS - 1]; \
> > +	j = 0; \
> > +	for_each_possible_cpu(i) { \
> > +		if (i > rnp[j].grphi) \
> > +			j++; \
> > +		per_cpu(rcu_data, i).mynode = &rnp[j]; \
> > +		(rsp)->rda[i] = &per_cpu(rcu_data, i); \
> > +	} \
> > +} while (0)
> > +
> > +static struct notifier_block __cpuinitdata rcu_nb = {
> > +	.notifier_call	= rcu_cpu_notify,
> > +};
> > +
> > +void __init __rcu_init(void)
> > +{
> > +	int i;			/* All used by RCU_DATA_PTR_INIT(). */
> > +	int j;
> > +	struct rcu_node *rnp;
> > +
> > +	printk(KERN_WARNING "Experimental hierarchical RCU implementation.\n");
> > +#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
> > +	printk(KERN_INFO "RCU-based detection of stalled CPUs is enabled.\n");
> > +#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
> > +	rcu_init_one(&rcu_state);
> > +	RCU_DATA_PTR_INIT(&rcu_state, rcu_data);
> > +	rcu_init_one(&rcu_bh_state);
> > +	RCU_DATA_PTR_INIT(&rcu_bh_state, rcu_bh_data);
> > +
> > +	for_each_online_cpu(i)
> > +		rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE, (void *)(long)i);
> > +	/* Register notifier for non-boot CPUs */
> > +	register_cpu_notifier(&rcu_nb);
> > +	printk(KERN_WARNING "Experimental hierarchical RCU init done.\n");
> > +}
> > +
> > +module_param(blimit, int, 0);
> > +module_param(qhimark, int, 0);
> > +module_param(qlowmark, int, 0);
> > diff --git a/kernel/rcutree_trace.c b/kernel/rcutree_trace.c
> > new file mode 100644
> > index 0000000..1691327
> > --- /dev/null
> > +++ b/kernel/rcutree_trace.c
> > @@ -0,0 +1,232 @@
> > +/*
> > + * Read-Copy Update tracing for classic implementation
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License as published by
> > + * the Free Software Foundation; either version 2 of the License, or
> > + * (at your option) any later version.
> > + *
> > + * This program is distributed in the hope that it will be useful,
> > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> > + * GNU General Public License for more details.
> > + *
> > + * You should have received a copy of the GNU General Public License
> > + * along with this program; if not, write to the Free Software
> > + * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
> > + *
> > + * Copyright IBM Corporation, 2008
> > + *
> > + * Papers:  http://www.rdrop.com/users/paulmck/RCU
> > + *
> > + * For detailed explanation of Read-Copy Update mechanism see -
> > + * 		Documentation/RCU
> > + *
> > + */
> > +#include <linux/types.h>
> > +#include <linux/kernel.h>
> > +#include <linux/init.h>
> > +#include <linux/spinlock.h>
> > +#include <linux/smp.h>
> > +#include <linux/rcupdate.h>
> > +#include <linux/interrupt.h>
> > +#include <linux/sched.h>
> > +#include <asm/atomic.h>
> > +#include <linux/bitops.h>
> > +#include <linux/module.h>
> > +#include <linux/completion.h>
> > +#include <linux/moduleparam.h>
> > +#include <linux/percpu.h>
> > +#include <linux/notifier.h>
> > +#include <linux/cpu.h>
> > +#include <linux/mutex.h>
> > +#include <linux/debugfs.h>
> > +
> > +static DEFINE_MUTEX(rcuclassic_trace_mutex);
> > +static char *rcuclassic_trace_buf;
> > +#define RCUPREEMPT_TRACE_BUF_SIZE (512*NR_CPUS)
> > +
> > +static int print_one_rcu_data(struct rcu_data *rdp, char *buf, char *ebuf)
> > +{
> > +	int cnt = 0;
> > +
> > +	if (!rdp->beenonline)
> > +		return 0;
> > +	cnt += snprintf(&buf[cnt], ebuf - &buf[cnt],
> > +		"%3d%cc=%ld g=%ld pq=%d pqc=%ld qp=%d",
> > +		rdp->cpu,
> > +		cpu_is_offline(rdp->cpu) ? '!' : ' ',
> > +		rdp->completed, rdp->gpnum,
> > +		rdp->passed_quiesc, rdp->passed_quiesc_completed,
> > +		rdp->qs_pending);
> > +#ifdef CONFIG_NO_HZ
> > +	cnt += snprintf(&buf[cnt], ebuf - &buf[cnt],
> > +		" dt=%d dn=%d df=%lu",
> > +		rdp->dynticks->dynticks, rdp->dynticks->dynticks_nmi,
> > +		rdp->dynticks_fqs);
> > +#endif /* #ifdef CONFIG_NO_HZ */
> > +	cnt += snprintf(&buf[cnt], ebuf - &buf[cnt],
> > +		" of=%lu ri=%lu", rdp->offline_fqs, rdp->resched_ipi);
> > +	cnt += snprintf(&buf[cnt], ebuf - &buf[cnt],
> > +		" ql=%ld b=%ld\n", rdp->qlen, rdp->blimit);
> > +	return cnt;
> > +}
> > +
> > +#define PRINT_RCU_DATA(name, buf, ebuf) \
> > +	do { \
> > +		int _p_r_d_i; \
> > +		\
> > +		for_each_possible_cpu(_p_r_d_i) \
> > +			(buf) += print_one_rcu_data(&per_cpu(name, _p_r_d_i), \
> > +						    buf, ebuf); \
> > +	} while (0)
> > +
> > +static ssize_t rcudata_read(struct file *filp, char __user *buffer,
> > +				size_t count, loff_t *ppos)
> > +{
> > +	ssize_t bcount;
> > +	char *buf = rcuclassic_trace_buf;
> > +	char *ebuf = &rcuclassic_trace_buf[RCUPREEMPT_TRACE_BUF_SIZE];
> > +
> > +	mutex_lock(&rcuclassic_trace_mutex);
> > +	buf += snprintf(buf, ebuf - buf, "rcu:\n");
> > +	PRINT_RCU_DATA(rcu_data, buf, ebuf);
> > +	buf += snprintf(buf, ebuf - buf, "rcu_bh:\n");
> > +	PRINT_RCU_DATA(rcu_bh_data, buf, ebuf);
> > +	bcount = simple_read_from_buffer(buffer, count, ppos,
> > +			rcuclassic_trace_buf, strlen(rcuclassic_trace_buf));
> > +	mutex_unlock(&rcuclassic_trace_mutex);
> > +	return bcount;
> > +}
> > +
> > +static int print_one_rcu_state(struct rcu_state *rsp, char *buf, char *ebuf)
> > +{
> > +	int cnt = 0;
> > +	int level = 0;
> > +	struct rcu_node *rnp;
> > +
> > +	cnt += snprintf(&buf[cnt], ebuf - &buf[cnt],
> > +			"c=%ld g=%ld s=%d jfq=%ld nfqs=%lu/nfqsng=%lu(%lu)\n",
> > +			rsp->completed, rsp->gpnum, rsp->signaled,
> > +			(long)(rsp->jiffies_force_qs - jiffies),
> > +			rsp->n_force_qs, rsp->n_force_qs_ngp,
> > +			rsp->n_force_qs - rsp->n_force_qs_ngp);
> > +	for (rnp = &rsp->node[0]; rnp - &rsp->node[0] < NUM_RCU_NODES; rnp++) {
> > +		if (rnp->level != level) {
> > +			cnt += snprintf(&buf[cnt], ebuf - &buf[cnt], "\n");
> > +			level = rnp->level;
> > +		}
> > +		cnt += snprintf(&buf[cnt], ebuf - &buf[cnt],
> > +				"%lx/%lx %d:%d ^%d    ",
> > +				rnp->qsmask, rnp->qsmaskinit,
> > +				rnp->grplo, rnp->grphi, rnp->grpnum);
> > +	}
> > +	cnt += snprintf(&buf[cnt], ebuf - &buf[cnt], "\n");
> > +	return cnt;
> > +}
> > +
> > +static ssize_t rcuhier_read(struct file *filp, char __user *buffer,
> > +				size_t count, loff_t *ppos)
> > +{
> > +	ssize_t bcount;
> > +	char *buf = rcuclassic_trace_buf;
> > +	char *ebuf = &rcuclassic_trace_buf[RCUPREEMPT_TRACE_BUF_SIZE];
> > +
> > +	mutex_lock(&rcuclassic_trace_mutex);
> > +	buf += print_one_rcu_state(&rcu_state, buf, ebuf);
> > +	buf += snprintf(buf, ebuf - buf, "rcu_bh:\n");
> > +	buf += print_one_rcu_state(&rcu_bh_state, buf, ebuf);
> > +	bcount = simple_read_from_buffer(buffer, count, ppos,
> > +			rcuclassic_trace_buf, strlen(rcuclassic_trace_buf));
> > +	mutex_unlock(&rcuclassic_trace_mutex);
> > +	return bcount;
> > +}
> > +
> > +static ssize_t rcugp_read(struct file *filp, char __user *buffer,
> > +				size_t count, loff_t *ppos)
> > +{
> > +	ssize_t bcount;
> > +	char *buf = rcuclassic_trace_buf;
> > +	char *ebuf = &rcuclassic_trace_buf[RCUPREEMPT_TRACE_BUF_SIZE];
> > +
> > +	mutex_lock(&rcuclassic_trace_mutex);
> > +	buf += snprintf(buf, ebuf - buf, "rcu: completed=%ld  gpnum=%ld\n",
> > +			rcu_state.completed, rcu_state.gpnum);
> > +	buf += snprintf(buf, ebuf - buf, "rcu_bh: completed=%ld  gpnum=%ld\n",
> > +			rcu_bh_state.completed, rcu_bh_state.gpnum);
> > +	bcount = simple_read_from_buffer(buffer, count, ppos,
> > +			rcuclassic_trace_buf, strlen(rcuclassic_trace_buf));
> > +	mutex_unlock(&rcuclassic_trace_mutex);
> > +	return bcount;
> > +}
> > +
> > +static struct file_operations rcudata_fops = {
> > +	.owner = THIS_MODULE,
> > +	.read = rcudata_read,
> > +};
> > +
> > +static struct file_operations rcuhier_fops = {
> > +	.owner = THIS_MODULE,
> > +	.read = rcuhier_read,
> > +};
> > +
> > +static struct file_operations rcugp_fops = {
> > +	.owner = THIS_MODULE,
> > +	.read = rcugp_read,
> > +};
> > +
> > +static struct dentry *rcudir, *datadir, *hierdir, *gpdir;
> > +static int rcuclassic_debugfs_init(void)
> > +{
> > +	rcudir = debugfs_create_dir("rcu", NULL);
> > +	if (!rcudir)
> > +		goto out;
> > +	datadir = debugfs_create_file("rcudata", 0444, rcudir,
> > +						NULL, &rcudata_fops);
> > +	if (!datadir)
> > +		goto free_out;
> > +
> > +	gpdir = debugfs_create_file("rcugp", 0444, rcudir, NULL, &rcugp_fops);
> > +	if (!gpdir)
> > +		goto free_out;
> > +
> > +	hierdir = debugfs_create_file("rcuhier", 0444, rcudir,
> > +						NULL, &rcuhier_fops);
> > +	if (!hierdir)
> > +		goto free_out;
> > +	return 0;
> > +free_out:
> > +	if (datadir)
> > +		debugfs_remove(datadir);
> > +	if (gpdir)
> > +		debugfs_remove(gpdir);
> > +	debugfs_remove(rcudir);
> > +out:
> > +	return 1;
> > +}
> > +
> > +static int __init rcuclassic_trace_init(void)
> > +{
> > +	int ret;
> > +
> > +	rcuclassic_trace_buf = kmalloc(RCUPREEMPT_TRACE_BUF_SIZE, GFP_KERNEL);
> > +	if (!rcuclassic_trace_buf)
> > +		return 1;
> > +	ret = rcuclassic_debugfs_init();
> > +	if (ret)
> > +		kfree(rcuclassic_trace_buf);
> > +	return ret;
> > +}
> > +
> > +static void __exit rcuclassic_trace_cleanup(void)
> > +{
> > +	debugfs_remove(datadir);
> > +	debugfs_remove(gpdir);
> > +	debugfs_remove(hierdir);
> > +	debugfs_remove(rcudir);
> > +	kfree(rcuclassic_trace_buf);
> > +}
> > +
> > +
> > +module_init(rcuclassic_trace_init);
> > +module_exit(rcuclassic_trace_cleanup);
> > diff --git a/kernel/softirq.c b/kernel/softirq.c
> > index c506f26..ad31780 100644
> > --- a/kernel/softirq.c
> > +++ b/kernel/softirq.c
> > @@ -256,8 +256,11 @@ void irq_enter(void)
> >  {
> >  #ifdef CONFIG_NO_HZ
> >  	int cpu = smp_processor_id();
> > -	if (idle_cpu(cpu) && !in_interrupt())
> > -		tick_nohz_stop_idle(cpu);
> > +	if (idle_cpu(cpu)) {
> > +		if (!in_interrupt())
> > +			tick_nohz_stop_idle(cpu);
> > +		rcu_irq_enter();
> > +	}
> >  #endif
> >  	__irq_enter();
> >  #ifdef CONFIG_NO_HZ
> > @@ -285,9 +288,11 @@ void irq_exit(void)
> > 
> >  #ifdef CONFIG_NO_HZ
> >  	/* Make sure that timer wheel updates are propagated */
> > -	if (!in_interrupt() && idle_cpu(smp_processor_id()) && !need_resched())
> > -		tick_nohz_stop_sched_tick(0);
> > -	rcu_irq_exit();
> > +	if (idle_cpu(smp_processor_id())) {
> > +		rcu_irq_exit();
> > +		if (!in_interrupt() && !need_resched())
> > +			tick_nohz_stop_sched_tick(0);
> > +	}
> >  #endif
> >  	preempt_enable_no_resched();
> >  }
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index 800ac84..804e08c 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -597,6 +597,19 @@ config RCU_TORTURE_TEST_RUNNABLE
> >  	  Say N here if you want the RCU torture tests to start only
> >  	  after being manually enabled via /proc.
> > 
> > +config RCU_CPU_STALL_DETECTOR
> > +	bool "Check for stalled CPUs delaying RCU grace periods"
> > +	depends on CLASSIC_RCU || TREE_RCU
> > +	default n
> > +	help
> > +	  This option causes RCU to printk information on which
> > +	  CPUs are delaying the current grace period, but only when
> > +	  the grace period extends for excessive time periods.
> > +
> > +	  Say Y if you want RCU to perform such checks.
> > +
> > +	  Say N if you are unsure.
> > +
> >  config KPROBES_SANITY_TEST
> >  	bool "Kprobes sanity tests"
> >  	depends on DEBUG_KERNEL
> 
> -- 
> Thanks and Regards
> gautham
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