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

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Date:	Fri, 17 Jun 2016 14:17:27 -0400
From:	Waiman Long <waiman.long@....com>
To:	Will Deacon <will.deacon@....com>
CC:	Boqun Feng <boqun.feng@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	<x86@...nel.org>, <linux-alpha@...r.kernel.org>,
	<linux-ia64@...r.kernel.org>, <linux-s390@...r.kernel.org>,
	<linux-arch@...r.kernel.org>, Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release
 barrier

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