[vlc-devel] [PATCH 1/6] os2: implement vlc_once()

[Kamil Rytarowski]

On 27.03.2018 17:43, Rémi Denis-Courmont wrote:
> Le tiistaina 27. maaliskuuta 2018, 18.36.32 EEST Kamil Rytarowski a écrit :
>> On 26.03.2018 13:16, KO Myung-Hun wrote:
>>> Rémi Denis-Courmont wrote:
>>>> POSIX model is simpler here, but it wouldn't work with C11 either.
>>>
>>> How about this ?
>>
>> There is no need for a mutex.
> 
> How do you handle the case that another thread is doing the initialization 
> without a mutex?
> 
>> An algorithm like follows should work better:
>>
>> static atomic_uint32_t bg_thread = {};
>> if (atomic_load(&bg_thread, memory_order_relaxed) == 0 &&
>>     atomic_exchange(&bg_thread, 1, memory_order_relaxed) == 0) {
>>   cb();
>> }
> 
> That can be equivalent reduced to:
> 
> static atomic_uint32_t bg_thread = {};
> if (atomic_compare_exchange_strong_explicit(&bg_thread, &{0}, 1, 
> memory_order_relaxed, memory_order_relaxed) == 0)
>   cb();
> 
> This obviously lacks memory synchronization in the simple non-racy case, and 
> utterly fails in the racy corner case. In other words, it does not work at 
> all.
> 

If we want to go for CAS:

  static atomic_uint32_t bg_thread = {};
  u32 cond = atomic_load(&bg_thread, memory_order_acquire);
  if (cond == 0) {
    if (atomic_compare_exchange_strong(&bg_thread, &cond, 1,
                                       memory_order_acq_rel)) {
      cb();
    }
  }

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 850 bytes
Desc: OpenPGP digital signature
URL: <http://mailman.videolan.org/pipermail/vlc-devel/attachments/20180327/6d8d9635/attachment.sig>