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

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Message-ID: <20160615171800.GP30921@twins.programming.kicks-ass.net>
Date:	Wed, 15 Jun 2016 19:18:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	Waiman Long <Waiman.Long@....com>, 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 Wed, Jun 15, 2016 at 04:04:46PM +0800, Boqun Feng wrote:
> On Tue, Jun 14, 2016 at 06:48:04PM -0400, Waiman Long wrote:

> > @@ -198,7 +198,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
> >  	 * Second most likely case.
> >  	 */
> >  	node = this_cpu_ptr(&osq_node);
> > -	next = xchg(&node->next, NULL);
> > +	next = xchg_release(&node->next, NULL);
> >  	if (next) {
> >  		WRITE_ONCE(next->locked, 1);
> 
> So we still use WRITE_ONCE() rather than smp_store_release() here?
> 
> Though, IIUC, This is fine for all the archs but ARM64, because there
> will always be a xchg_release()/xchg() before the WRITE_ONCE(), which
> carries a necessary barrier to upgrade WRITE_ONCE() to a RELEASE.

Not sure. On PPC for example, you'll use lwsync() but will that not
attach to the store to &node->next instead?

Still leaving that store and the WRITE_ONCE() unordered.

Also I don't see the control dependency between xchg-load and WRITE_ONCE
helping anything to order the two stores.


So yeah, subtle if not broken, definitely needs more explanation.