Fwd: local equivalent for pthread_once() in glibc?

[Adhemerval Zanella]

On 26/06/2017 09:51, Florian Weimer wrote:
> On 06/26/2017 02:13 PM, Adhemerval Zanella wrote:
>>
>>
>> On 25/06/2017 12:46, Florian Weimer wrote:
>>> On 05/17/2017 04:51 PM, Adhemerval Zanella wrote:
>>>> Right, but this is not seem the case for tunable where malloc_consolidate is
>>>> called from ptmalloc_init (at least for main_arena).  In any case, I still
>>>> think that for adequate __malloc_initialized access using C11 atomic since
>>>> its access is still done concurrently (that why I asked if using __libc_once
>>>> would be simpler).
>>>
>>> I don't understand.  The concurrent access solely consists of reads.  We
>>> do not use atomics in that case.
>>
>> My understanding and my point is even for these cases we should aim for 
>> C11 atomic accesses, even for relaxed loads which on most architectures
>> will map to normal loads.
> 
> I don't think this is true.  If the last write happen before all the
> concurrent read accesses, we don't need atomics.  To me, this is quite
> clear because this is what happens with locks, where we usually don't
> use atomics within the critical section, either.

I do agree with your rationale, but from Torvald comment on BZ #20822 [1]
fix my understanding is to still use atomic relaxed MO for such cases
simply for consistency (and I will add to add more readability to state
the variable is indeed read concurrently and relaxed MO is suffice).

[1] https://sourceware.org/ml/libc-alpha/2016-12/msg00820.html