Re: [PATCH] locking/mutexes: Revert "locking/mutexes: Add extra reschedule point"

[Ingo Molnar]

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Thu, Jul 31, 2014 at 04:37:29PM +0400, Ilya Dryomov wrote:
> 
> > This didn't make sense to me at first too, and I'll be happy to be
> > proven wrong, but we can reproduce this with rbd very reliably under
> > higher than usual load, and the revert makes it go away.  What we are
> > seeing in the rbd scenario is the following.
> 
> This is drivers/block/rbd.c ? I can find but a single mutex_lock() in
> there.
> 
> > Suppose foo needs mutexes A and B, bar needs mutex B.  foo acquires
> > A and then wants to acquire B, but B is held by bar.  foo spins
> > a little and ends up calling schedule_preempt_disabled() on line 484
> > above, but that call never returns, even though a hundred usecs later
> > bar releases B.  foo ends up stuck in mutex_lock() indefinitely, but
> > still holds A and everybody else who needs A gets behind A.  Given that
> > this A happens to be a central libceph mutex all rbd activity halts.
> > Deadlock may not be the best term for this, but never returning from
> > mutex_lock(&B) even though B has been unlocked is *a* problem.
> > 
> > This obviously doesn't happen every time schedule_preempt_disabled() on
> > line 484 is called, so there must be some sort of race here.  I'll send
> > along the actual rbd stack traces shortly.
> 
> Smells like maybe current->state != TASK_RUNNING, does the below
> trigger?
> 
> If so, you've wrecked something in whatever...
> 
> ---
>  kernel/locking/mutex.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index ae712b25e492..3d726fdaa764 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -473,8 +473,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  	 * reschedule now, before we try-lock the mutex. This avoids getting
>  	 * scheduled out right after we obtained the mutex.
>  	 */
> -	if (need_resched())
> +	if (need_resched()) {
> +		if (WARN_ON_ONCE(current->state != TASK_RUNNING))
> +			__set_current_state(TASK_RUNNING);
> +
>  		schedule_preempt_disabled();
> +	}

Might make sense to add that debug check under mutex debugging or so, 
with a sensible kernel message printed.

Thanks,

	Ingo
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