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

Ingo Molnar
2014-07-31T13:44:23Z

* Peter Zijlstra <peterz@infradead.org> 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