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SubjectRe: RFC: mutex: hung tasks on SMP platforms with asm-generic/mutex-xchg.h
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On Tue, 2012-08-07 at 12:56 +0100, Will Deacon wrote:
> Hello,
>
> ARM recently moved to asm-generic/mutex-xchg.h for its mutex implementation
> after our previous implementation was found to be missing some crucial
> memory barriers.


This is a76d7bd96d ("ARM: 7467/1: mutex: use generic xchg-based
implementation for ARMv6+"), right? Why do you use xchg and not dec
based? The changelog mumbles something about shorter critical sections,
but me not knowing anything about ARM wonders about the why of that.

> However, I'm seeing some problems running hackbench on
> SMP platforms due to the way in which the MUTEX_SPIN_ON_OWNER code operates.
>
> The symptoms are that a bunch of hackbench tasks are left waiting on an
> unlocked mutex and therefore never get woken up to claim it. I think this
> boils down to the following sequence:
>
>
> Task A Task B Task C Lock value
> 0 1
> 1 lock() 0
> 2 lock() 0
> 3 spin(A) 0
> 4 unlock() 1
> 5 lock() 0
> 6 cmpxchg(1,0) 0
> 7 contended() -1
> 8 lock() 0
> 9 spin(C) 0
> 10 unlock() 1
> 11 cmpxchg(1,0) 0
> 12 unlock() 1
>
>
> At this point, the lock is unlocked, but Task B is in an uninterruptible
> sleep with nobody to wake it up.
>
> The following patch fixes the problem by ensuring we put the lock into
> the contended state if we acquire it from the spin loop on the slowpath
> but I'd like to be sure that this won't cause problems with other mutex
> implementations:
>
>
> diff --git a/kernel/mutex.c b/kernel/mutex.c
> index a307cc9..27b7887 100644
> --- a/kernel/mutex.c
> +++ b/kernel/mutex.c
> @@ -170,7 +170,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> if (owner && !mutex_spin_on_owner(lock, owner))
> break;
>
> - if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
> + if (atomic_cmpxchg(&lock->count, 1, -1) == 1) {
> lock_acquired(&lock->dep_map, ip);
> mutex_set_owner(lock);
> preempt_enable();
>

But in this case, either B is still spinning in our spin-loop, or it has
already passed the atomic_xchg(&lock->count, -1) when we fell out.

Since you say B is in UNINTERRUPTIBLE state, we'll assume it fell
through and so the lock count should be -1 (or less) to mark it
contended.




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