Messages in this thread | | | Date | Thu, 25 Aug 2016 18:00:19 -0400 | From | Waiman Long <> | Subject | Re: [RFC][PATCH -v2 4/4] locking/mutex: Add lock handoff to avoid starvation |
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On 08/25/2016 02:37 PM, Peter Zijlstra wrote: > @@ -468,9 +496,12 @@ void __sched mutex_unlock(struct mutex * > DEBUG_LOCKS_WARN_ON(__mutex_owner(lock) != current); > #endif > > - owner = atomic_long_fetch_and(MUTEX_FLAGS,&lock->owner); > + owner = atomic_long_read(&lock->owner); > + if (!(owner& MUTEX_FLAG_HANDOFF)) > + owner = atomic_long_fetch_and(MUTEX_FLAGS,&lock->owner); > + > if (__owner_flags(owner)) > - __mutex_unlock_slowpath(lock); > + __mutex_unlock_slowpath(lock, owner); > } > EXPORT_SYMBOL(mutex_unlock);
I don't think the race condition is fixed when we don't make sure that lock handoff only happens from current=>new. The problem is due to the fact that the MUTEX_FLAG_HANDOFF check in the unlock fastpath isn't serialized by the wait_lock. As a result, it is possible that the owner is NULL while the HANDOFF bit is set. Or an optimistic spinner may have stolen the lock in the interim.
Cheers, Longman
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