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

Davidlohr Bueso
2016-06-17T16:29:26Z

On Fri, 17 Jun 2016, Waiman Long wrote:

>On 06/16/2016 09:11 PM, Davidlohr Bueso wrote:
>>On Wed, 15 Jun 2016, Peter Zijlstra wrote:
>>
>>>Yeah, see a few patches further in this series, where he guards a
>>>variables with the osq_lock.
>>
>>So one problem I have with all this is that if we are hardening
>>osq_lock/unlock()
>>because of some future use that is specific to rwsems, then we will
>>immediately
>>be hurting mutexes for no good reason.
>>
>
>I am going to change it to use smp_acquire__after_ctrl_dep() as
>suggested by PeterZ. Is that a good enough compromise? I have also
>changed the xchg in the unlock side to xchg_release which could help
>performance in some archs. The thing is when developers see the name
>osq_lock/osq_unlock, they will naturally assume the proper barrriers
>are provided which is not currently the case.

Oh, from your discussions with Boqun, I was under the impression that ->locked
was now going to be properly ordered in all cases now, which is why
I worry about mutexes.

>Anyway, the change won't affect x86, it is probably ARM or PPC that
>may have an impact.

Yes, that xchg() won't affect x86, but adding an smp_store_release(node->locked, 1)
or such will obviously.

Thanks,
Davidlohr