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

From: Peter Zijlstra
Date: Thu Jul 31 2014 - 09:13:48 EST

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

> 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

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);
+ }
spin_lock_mutex(&lock->wait_lock, flags);

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