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

[Kamil Rytarowski]

On 27.03.2018 17:49, Rémi Denis-Courmont wrote:
> Le tiistaina 27. maaliskuuta 2018, 18.48.59 EEST Kamil Rytarowski a écrit :
>> 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();
>>     }
>>   }
> 
> No. The memory release needs to occur *after* the callback, so this is still 
> nto working in non-racy case. And it fails to wait for callback completion in 
> racy case.
> 

OK, if there is a requirement that vlc_once() must execute the callback
function, before any other thread will go over it. We might want to go
for a sleep-free (spinlock-like) implementation with atomic operations
or a mutex-like one if we can sleep.

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