From mboxrd@z Thu Jan  1 00:00:00 1970
From: Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH -v4 5/7] locking, arch: Update spin_unlock_wait()
Date: Fri, 3 Jun 2016 18:35:37 +0100
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Cc: Boqun Feng <boqun.feng@gmail.com>, linux-kernel@vger.kernel.org,
	torvalds@linux-foundation.org, manfred@colorfullife.com,
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To: Peter Zijlstra <peterz@infradead.org>
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On Fri, Jun 03, 2016 at 03:42:49PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 03, 2016 at 01:47:34PM +0100, Will Deacon wrote:
> > Even on x86, I think you need a fence here:
> > 
> > X86 lock
> > {
> > }
> >  P0                | P1                ;
> >  MOV EAX,$1        | MOV EAX,$1        ;
> >  LOCK XCHG [x],EAX | LOCK XCHG [y],EAX ;
> >  MOV EBX,[y]       | MOV EBX,[x]       ;
> > exists
> > (0:EAX=0 /\ 0:EBX=0 /\ 1:EAX=0 /\ 1:EBX=0)
> > 
> > is permitted by herd.
> 
> I am puzzled.. this should not be. You say adding MFENCE after LOCK XCHG
> makes it 'work', but we assume LOCK <op> is a full fence all over the
> place.
> 
> I'm thinking herd is busted.

FWIW -- I spoke to the Herd authors and they confirmed that this is a
regresion in herd (fixed in the bleeding edge version).

Will