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 16:45:36 +0100
From:	Will Deacon <will.deacon@....com>
To:	Waiman Long <waiman.long@....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 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