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Re: Fwd: local equivalent for pthread_once() in glibc?
- From: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- To: Florian Weimer <fweimer at redhat dot com>, libc-alpha at sourceware dot org
- Date: Mon, 26 Jun 2017 10:12:33 -0300
- Subject: Re: Fwd: local equivalent for pthread_once() in glibc?
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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 
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).