Re: [PATCH tip/core/rcu 06/19] rcu: Add warning to detect half-interrupts

From: Byungchul Park
Date: Fri Mar 15 2019 - 03:45:26 EST

On 03/15/2019 04:31 PM, Byungchul Park wrote:
On Mon, Mar 11, 2019 at 09:39:39AM -0400, Joel Fernandes wrote:
On Wed, Aug 29, 2018 at 03:20:34PM -0700, Paul E. McKenney wrote:
RCU's dyntick-idle code is written to tolerate half-interrupts, that it,
either an interrupt that invokes rcu_irq_enter() but never invokes the
corresponding rcu_irq_exit() on the one hand, or an interrupt that never
invokes rcu_irq_enter() but does invoke the "corresponding" rcu_irq_exit()
on the other. These things really did happen at one time, as evidenced
by this ca-2011 LKML post:

http://lkml.kernel.org/r/20111014170019.GE2428@xxxxxxxxxxxxxxxxxx

The reason why RCU tolerates half-interrupts is that usermode helpers
used exceptions to invoke a system call from within the kernel such that
the system call did a normal return (not a return from exception) to
the calling context. This caused rcu_irq_enter() to be invoked without
a matching rcu_irq_exit(). However, usermode helpers have since been
rewritten to make much more housebroken use of workqueues, kernel threads,
and do_execve(), and therefore should no longer produce half-interrupts.
No one knows of any other source of half-interrupts, but then again,
no one seems insane enough to go audit the entire kernel to verify that
half-interrupts really are a relic of the past.

This commit therefore adds a pair of WARN_ON_ONCE() calls that will
trigger in the presence of half interrupts, which the code will continue
to handle correctly. If neither of these WARN_ON_ONCE() trigger by
mid-2021, then perhaps RCU can stop handling half-interrupts, which
would be a considerable simplification.
Hi Paul and everyone,
I was thinking some more about this patch and whether we can simplify this code
much in 2021. Since 2021 is a bit far away, I thought working on it in again to
keep it fresh in memory is a good idea ;-)

To me it seems we cannot easily combine the counters (dynticks_nesting and
dynticks_nmi_nesting) even if we confirmed that there is no possibility of a
half-interrupt scenario (assuming simplication means counter combining like
Byungchul tried to do in https://goo.gl/X1U77X). The reason is because these
2 counters need to be tracked separately as they are used differently in the
following function:
Hi Joel and Paul,

I always love the way to logically approach problems so I'm a fan of
all your works :) But I'm JUST curious about something here. Why can't
we combine them the way I tried even if we confirm no possibility of
half-interrupt? IMHO, the only thing we want to know through calling
rcu_is_cpu_rrupt_from_idle() is whether the interrupt comes from
RCU-idle or not - of course assuming the caller context always be an
well-defined interrupt context like e.g. the tick handler.

So the function can return true if the caller is within a RCU-idle
region except a well-known single interrupt nested.

Of course, now that we cannot confirm it yet, the crowbar is necessary.
But does it still have a problem even after confirming it? Why? What am
I missing? Could you explain why for me? :(


Did you also want to consider the case the function is called from others than
well-known interrupt contexts? If yes, then I agree with you, there doesn't
seem to be the kind of code and it's not a good idea to let the function be
called generally though.


Thanks,
Byungchul

static int rcu_is_cpu_rrupt_from_idle(void)
{
return __this_cpu_read(rcu_data.dynticks_nesting) <= 0 &&
__this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 1;
}

dynticks_nesting actually tracks if we entered/exited idle or user mode.

dynticks_nmi_nesting tracks if we entered/exited interrupts.

We have to do the "dynticks_nmi_nesting <= 1" check because
rcu_is_cpu_rrupt_from_idle() can possibly be called from an interrupt itself
(like timer) so we discount 1 interrupt, and, the "dynticks_nesting <= 0"
check is because the CPU MUST be in user or idle for the check to return
true. We can't really combine these two into one counter then I think because
they both convey different messages.

The only simplication we can do, is probably the "crowbar" updates to
dynticks_nmi_nesting can be removed from rcu_eqs_enter/exit once we confirm
no more half-interrupts are possible. Which might still be a worthwhile thing
to do (while still keeping both counters separate).

However, I think we could combine the counters and lead to simplying the code
in case we implement rcu_is_cpu_rrupt_from_idle differently such that it does
not need the counters but NOHZ_FULL may take issue with that since it needs
rcu_user_enter->rcu_eqs_enter to convey that the CPU is "RCU"-idle.

Actually, I had another question... rcu_user_enter() is a NOOP in !NOHZ_FULL config.
In this case I was wondering if the the warning Paul added (in the patch I'm replying to)
will really get fired for half-interrupts. The vast majority of the systems I believe are
NOHZ_IDLE not NOHZ_FULL.
This is what a half-interrupt really looks like right? Please correct me if I'm wrong:
rcu_irq_enter() [half interrupt causes an exception and thus rcu_irq_enter]
rcu_user_enter() [due to usermode upcall]
rcu_user_exit()
(no more rcu_irq_exit() - hence half an interrupt)

But the rcu_user_enter()/exit is a NOOP in some configs, so will the warning in
rcu_eqs_e{xit,nter} really do anything?

Or was the idea with adding the new warnings, that they would fire the next
time rcu_idle_enter/exit is called? Like for example:

rcu_irq_enter() [This is due to half-interrupt]
rcu_idle_enter() [Eventually we enter the idle loop at some point
after the half-interrupt and the rcu_eqs_enter()
would "crowbar" the dynticks_nmi_nesting counter to 0].

thanks!

- Joel

Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Reported-by: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Reported-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
kernel/rcu/tree.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index dc041c2afbcc..d2b6ade692c9 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -714,6 +714,7 @@ static void rcu_eqs_enter(bool user)
struct rcu_dynticks *rdtp;
rdtp = this_cpu_ptr(&rcu_dynticks);
+ WARN_ON_ONCE(rdtp->dynticks_nmi_nesting != DYNTICK_IRQ_NONIDLE);
WRITE_ONCE(rdtp->dynticks_nmi_nesting, 0);
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
rdtp->dynticks_nesting == 0);
@@ -895,6 +896,7 @@ static void rcu_eqs_exit(bool user)
trace_rcu_dyntick(TPS("End"), rdtp->dynticks_nesting, 1, rdtp->dynticks);
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
WRITE_ONCE(rdtp->dynticks_nesting, 1);
+ WARN_ON_ONCE(rdtp->dynticks_nmi_nesting);
WRITE_ONCE(rdtp->dynticks_nmi_nesting, DYNTICK_IRQ_NONIDLE);
}
--
2.17.1