Re: [Xen-devel] [PATCHv3 3/4] xen: use ticket locks for spin locks

[Tim Deegan <tim@xxxxxxx>]

At 14:54 +0100 on 23 Apr (1429800874), Jan Beulich wrote:
> >>> On 23.04.15 at 14:03, <tim@xxxxxxx> wrote:
> > At 11:11 +0100 on 21 Apr (1429614687), David Vrabel wrote:
> >>  void _spin_unlock(spinlock_t *lock)
> >>  {
> >> +    smp_mb();
> >>      preempt_enable();
> >>      LOCK_PROFILE_REL;
> >> -    _raw_spin_unlock(&lock->raw);
> >> +    lock->tickets.head++;
> > 
> > This needs to be done with an explicit atomic (though not locked)
> > write; otherwise the compiler might use some unsuitable operation that
> > clobbers .tail as well.
> 
> How do you imagine that to happen? An increment of one
> structure member surely won't modify any others.

AIUI, the '++' could end up as a word-size read, modify, and word-size
write.  If another CPU updates .tail parallel, that update could get
lost.

Tim.

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