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

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Message-ID: <20151211171128.GC6356@twins.programming.kicks-ass.net>
Date:	Fri, 11 Dec 2015 18:11:28 +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 02:06:49PM +0000, Will Deacon wrote:
> 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?

Can I have second thoughts an confuse matters again? ;-)

A RmW-acq is a load-acquire+store. That means the store is _after_ the
load and thus not required for the completion of the control dependency.

Therefore the store in question can reorder inside the conditional
control block's stores.

Hmm?

Suggesting this acquire is in fact also wrong, since we need full order
ops to guarantee full order both in lock and unlock.
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