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

Waiman Long
2016-06-17T18:17:27Z

On 06/17/2016 11:45 AM, Will Deacon wrote:
> On Fri, Jun 17, 2016 at 11:26:41AM -0400, Waiman Long wrote:
>> On 06/16/2016 08:48 PM, Boqun Feng wrote:
>>> On Thu, Jun 16, 2016 at 05:35:54PM -0400, Waiman Long wrote:
>>>> If you look into the actual code:
>>>>
>>>> next = xchg_release(&node->next, NULL);
>>>> if (next) {
>>>> WRITE_ONCE(next->locked, 1);
>>>> return;
>>>> }
>>>>
>>>> There is a control dependency that WRITE_ONCE() won't happen until
>>> But a control dependency only orders LOAD->STORE pairs, right? And here
>>> the control dependency orders the LOAD part of xchg_release() and the
>>> WRITE_ONCE().
>>>
>>> Along with the fact that RELEASE only orders the STORE part of xchg with
>>> the memory operations preceding the STORE part, so for the following
>>> code:
>>>
>>> WRTIE_ONCE(x,1);
>>> next = xchg_release(&node->next, NULL);
>>> if (next)
>>> WRITE_ONCE(next->locked, 1);
>>>
>>> such a reordering is allowed to happen on ARM64v8
>>>
>>> next = ldxr [&node->next] // LOAD part of xchg_release()
>>>
>>> if (next)
>>> WRITE_ONCE(next->locked, 1);
>>>
>>> WRITE_ONCE(x,1);
>>> stlxr NULL [&node->next] // STORE part of xchg_releae()
>>>
>>> Am I missing your point here?
>> My understanding of the release barrier is that both prior LOADs and STOREs
>> can't move after the barrier. If WRITE_ONCE(x, 1) can move to below as shown
>> above, it is not a real release barrier and we may need to change the
>> barrier code.
> You seem to be missing the point.
>
> {READ,WRITE}_ONCE accesses appearing in program order after a release
> are not externally ordered with respect to the release unless they
> access the same location.
>
> This is illustrated by Boqun's example, which shows two WRITE_ONCE
> accesses being reordered before a store-release forming the write
> component of an xchg_release. In both cases, WRITE_ONCE(x, 1) remains
> ordered before the store-release.
>
> Will

I am sorry that I misread the mail. I am not used to treating xchg as
two separate instructions. Yes, it is a problem. In that case, we have
to either keep the xchg() function as it is or use
smp_store_release(&next->locked, 1). So which one is a better
alternative for ARM or PPC?

Cheers,
Longman