Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier

Waiman Long
2016-06-15T19:08:48Z

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.

Cheers,
Longman