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Date: Fri, 3 Jun 2016 21:13:14 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will.deacon@arm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>, linux-kernel@vger.kernel.org,
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Subject: Re: [PATCH -v4 5/7] locking, arch: Update spin_unlock_wait()
Message-ID: <20160603191314.GA3710@twins.programming.kicks-ass.net>
References: <20160602115157.249037373@infradead.org>
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 <20160602142440.GE30064@insomnia>
 <20160602144424.GV3193@twins.programming.kicks-ass.net>
 <20160602163425.GV3205@twins.programming.kicks-ass.net>
 <20160602175659.GB7697@arm.com>
 <20160602215119.GF3190@twins.programming.kicks-ass.net>
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On Fri, Jun 03, 2016 at 06:35:37PM +0100, Will Deacon wrote:
> 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).

Good, means I'm not slowly going crazeh -- or at least, this isn't one
of the signs :-)