At 14:54 +0100 on 23 Apr (1429800874), Jan Beulich wrote:
> >>> On 23.04.15 at 14:03, <t...@xen.org> 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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