linux-kernel - Re: [PATCH v2 1/6] locking/pvqspinlock: Unconditional PV kick with _Q_SLOW

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Date:	Thu, 16 Jul 2015 07:42:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>,
	Davidlohr Bueso <dave@...olabs.net>, will.deacon@....com
Subject: Re: [PATCH v2 1/6] locking/pvqspinlock: Unconditional PV kick with
 _Q_SLOW_VAL

On Wed, Jul 15, 2015 at 08:18:23PM -0400, Waiman Long wrote:
> On 07/15/2015 05:10 AM, Peter Zijlstra wrote:
> >  	/*
> >+	 * A failed cmpxchg doesn't provide any memory-ordering guarantees,
> >+	 * so we need a barrier to order the read of the node data in
> >+	 * pv_unhash *after* we've read the lock being _Q_SLOW_VAL.
> >+	 *
> >+	 * Matches the cmpxchg() in pv_wait_head() setting _Q_SLOW_VAL.
> >+	 */
> >+	smp_rmb();
> 
> According to memory_barriers.txt, cmpxchg() is a full memory barrier. It
> didn't say a failed cmpxchg will lose its memory guarantee. So is the
> documentation right? 

The documentation is not entirely clear on this; but there are hints
that this is so.

> Or is that true for some architectures? I think it is
> not true for x86.

On x86 LOCK CMPXCHG is always a sync point, but yes there are archs for
which a failed cmpxchg does _NOT_ provide any barrier semantics.

The reason I started looking was because Will made Argh64 one of those.
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