linux-kernel - Re: FW: Commit 81a43adae3b9 (locking/mutex: Use acquire/release semantics) causing failures on arm64 (ThunderX)

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Message-ID: <20151211140649.GI18828@arm.com>
Date:	Fri, 11 Dec 2015 14:06:49 +0000
From:	Will Deacon <will.deacon@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrew Pinski <andrew.pinski@...iumnetworks.com>,
	Davidlohr Bueso <dbueso@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, david.daney@...ium.com
Subject: Re: FW: Commit 81a43adae3b9 (locking/mutex: Use acquire/release
 semantics) causing failures on arm64 (ThunderX)

On Fri, Dec 11, 2015 at 02:48:03PM +0100, Peter Zijlstra wrote:
> On Fri, Dec 11, 2015 at 01:33:14PM +0000, Will Deacon wrote:
> > On Fri, Dec 11, 2015 at 01:26:47PM +0100, Peter Zijlstra wrote:
> 
> > > While we're there, the acquire in osq_wait_next() seems somewhat ill
> > > documented too.
> > > 
> > > I _think_ we need ACQUIRE semantics there because we want to strictly
> > > order the lock-unqueue A,B,C steps and we get that with:
> > > 
> > >  A: SC
> > >  B: ACQ
> > >  C: Relaxed
> > > 
> > > Similarly for unlock we want the WRITE_ONCE to happen after
> > > osq_wait_next, but in that case we can even rely on the control
> > > dependency there.
> > 
> > Even for the lock-unqueue case, isn't B->C ordered by a control dependency
> > because C consists only of stores?
> 
> Hmm, indeed. So we could go fully relaxed on it I suppose, since the
> same is true for the unlock site.

In which case, we should be able to relax the xchg in there (osq_wait_next)
too, right?

Will
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