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

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Message-ID: <20151211134803.GP6356@twins.programming.kicks-ass.net>
Date:	Fri, 11 Dec 2015 14:48:03 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
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 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.


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