linux-kernel - Re: [PATCH -v4 5/7] locking, arch: Update spin_unlock

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Date:	Fri, 3 Jun 2016 15:42:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
Cc:	Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, manfred@...orfullife.com,
	dave@...olabs.net, paulmck@...ux.vnet.ibm.com, Waiman.Long@....com,
	tj@...nel.org, pablo@...filter.org, kaber@...sh.net,
	davem@...emloft.net, oleg@...hat.com,
	netfilter-devel@...r.kernel.org, sasha.levin@...cle.com,
	hofrat@...dl.org, jejb@...isc-linux.org, chris@...kel.net,
	rth@...ddle.net, dhowells@...hat.com, schwidefsky@...ibm.com,
	mpe@...erman.id.au, ralf@...ux-mips.org, linux@...linux.org.uk,
	rkuo@...eaurora.org, vgupta@...opsys.com, james.hogan@...tec.com,
	realmz6@...il.com, ysato@...rs.sourceforge.jp, tony.luck@...el.com,
	cmetcalf@...lanox.com, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH -v4 5/7] locking, arch: Update spin_unlock_wait()

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.

Anybody? hpa, Linus?