Messages in this thread | | | Date | Wed, 15 Jun 2016 16:04:07 -0400 | From | Waiman Long <> | Subject | Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier |
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On 06/15/2016 03:08 PM, Waiman Long wrote: > On 06/15/2016 01:12 PM, Peter Zijlstra wrote: >> On Wed, Jun 15, 2016 at 09:56:59AM -0700, Davidlohr Bueso wrote: >>> On Tue, 14 Jun 2016, Waiman Long wrote: >>>> +++ b/kernel/locking/osq_lock.c >>>> @@ -115,7 +115,7 @@ bool osq_lock(struct optimistic_spin_queue *lock) >>>> * cmpxchg in an attempt to undo our queueing. >>>> */ >>>> >>>> - while (!READ_ONCE(node->locked)) { >>>> + while (!smp_load_acquire(&node->locked)) { >>> Hmm this being a polling path, that barrier can get pretty expensive >>> and >>> last I checked it was unnecessary: >> I think he'll go rely on it later on. >> >> In any case, its fairly simple to cure, just add >> smp_acquire__after_ctrl_dep() at the end. If we bail because >> need_resched() we don't need the acquire I think. > > Yes, I only need the acquire barrier when the locking is successful. > Thanks for the suggestion. I will make the change accordingly. >
BTW, when will the smp_acquire__after_ctrl_dep() patch goes into the tip tree? My patch will have a dependency on that when I make the change.
Cheers, Longman
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