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

From: Will Deacon <will.deacon@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Pinski <andrew.pinski@caviumnetworks.com>,
	Davidlohr Bueso <dbueso@suse.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Ingo Molnar <mingo@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	david.daney@cavium.com
Subject: Re: FW: Commit 81a43adae3b9 (locking/mutex: Use acquire/release semantics) causing failures on arm64 (ThunderX)
Date: Fri, 11 Dec 2015 14:06:49 +0000	[thread overview]
Message-ID: <20151211140649.GI18828@arm.com> (raw)
In-Reply-To: <20151211134803.GP6356@twins.programming.kicks-ass.net>

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