Re: Quick review of -rt RCU-related patches

[Thomas Gleixner]

On Tue, 4 Oct 2011, Paul E. McKenney wrote:
> On Wed, Oct 05, 2011 at 12:05:47AM +0200, Thomas Gleixner wrote:
> > > 	This means that might_sleep() will never complain about
> > > 	blocking in an RCU read-side critical section.  I guess that
> > > 	this is necessary, though it would be better to have some
> > > 	way to complain for general sleeping (e.g., waiting on
> > > 	network receive) as opposed to blocking on a lock that is
> > > 	subject to priority inheritance.
> > 
> > Well, there's always a remaining problem. We need that stuff fully
> > preemptible on rt. Any ideas ?
> 
> Not yet.  We would have to classify context switches into two groups:
> 
> 1.	Preemptions or blocking waiting for sleeping spinlocks.
> 
> 2.	Everything else.
> 
> Given that classification, it would be straightforward: prohibit
> group #2 context switches while in RCU-preempt read-side critical
> sections.  I know, easy for me to say!  ;-)

Well, you know the preemtible regions of RT, it basically boils down
to #1 - except that it differs a bit from vanilla that locks and bh
stuff does not prevent preemption.

If RCU can deal with that, then #2 is a non issue :)
 
> > > rcu-disable-the-rcu-bh-stuff-for-rt.patch
> > > 
> > > 	This implements RCU-bh in terms of RCU-preempt, but disables
> > > 	BH around the resulting RCU-preempt read-side critical section.
> > > 	May be vulnerable to network-based denial-of-service attacks,
> > > 	which could OOM a system with this patch.
> > > 
> > > 	The implementation of rcu_read_lock_bh_held() is weak, but OK.	In
> > > 	an ideal world, it would also complain if not local_bh_disable().
> > 
> > Well, I dropped that after our IRC conversation, but we still need to
> > have some extra debugging for RT to diagnose situations where we break
> > those rcu_bh assumptions. That _bh rcu stuff should have never been
> > there, we'd rather should drop the softirq processing back to
> > ksoftirqd in such an overload case (in mainline) and voluntary
> > schedule away from ksoftirqd until the situation is resolved.
> > 
> > I consider rcu_bh a bandaid for the nasty implict semantics of BH
> > processing and I'm looking really forward to Peters analysis of the
> > modern cpu local BKL constructs at RTLWS.
> > 
> > The patch stemmed from an earlier discussion about getting rid of
> > those special rcu_bh semantics even in mainline for the sake of not
> > making a special case for a completely over(ab)used construct which
> > originates from the historically grown softirq behaviour. I think that
> > keeping the special cased rcu_bh stuff around is just taking any
> > incentive away from moving that whole softirq processing into a
> > scheduler controllable environment (i.e. threaded interrupts). 
> 
> Between -rt and the guys pushing packets, I can tell that this is going
> to be a fun one.  ;-)

We'll see. At some point they'll find out that a thread context will
make their life easier simply because the locking maze can be
distangled. That's why I'm ranting at the special "foster _bh" rcu
abominations, which keep them thinking that going down that road is
actually a good thing.
 
> I will see if I can come up with a way to make that patch safe to
> apply.  Might not be all that hard.

:)
 
Thanks,

	tglx
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