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

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> David Vrabel <david.vrabel@xxxxxxxxxx> 04/29/15 5:39 PM >>>
>On 23/04/15 15:58, Jan Beulich wrote:
>>>>> On 23.04.15 at 16:43, <tim@xxxxxxx> wrote:
>>> At 14:54 +0100 on 23 Apr (1429800874), Jan Beulich wrote:
>>> 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.
>
>lock->tickets.head++;
>
  >7b:   66 83 07 01             addw   $0x1,(%rdi)
>
>write_atomic(&lock->tickets.head, lock->tickets.head + 1);
>
  >7b:   0f b7 07                movzwl (%rdi),%eax
  >7e:   83 c0 01                add    $0x1,%eax
  >81:   66 89 07                mov    %ax,(%rdi)
>
>Do you want a new add_atomic() operation? e.g.,
>
>#define add_atomic(ptr, inc) \
        >asm volatile ("addw %1,%w" \
            >: "+m" (*(ptr)) : "ri" (inc) : "memory")
<
>(but obviously handling all the different sizes.)

I think that would be desirable.

Jan


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