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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.04.15 at 16:43, <tim@xxxxxxx> wrote:
> 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.

Ah, right, compilers are allowed to do that, albeit normally wouldn't
unless the architecture has no suitable loads/stores.

Jan


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