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

Waiman Long
2016-06-15T20:04:07Z

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