Re: [PATCH -v4 5/7] locking, arch: Update spin_unlock_wait()

[Peter Zijlstra]

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?