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

Davidlohr Bueso
2016-06-17T16:46:59Z

On Fri, 17 Jun 2016, Davidlohr Bueso wrote:

>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.

nm this last part, you're right, x86 smp_store_release is a nop.