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To: Takashi Yano <takashi.yano@nifty.ne.jp>, cygwin@cygwin.com,
        Noel Grandin <noelgrandin@gmail.com>
Subject: Re: SRW locks
Date: Thu, 30 May 2024 15:53:51 +0200
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From: Bruno Haible via Cygwin <cygwin@cygwin.com>
Reply-To: Bruno Haible <bruno@clisp.org>
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Noel Grandin wrote:
> > Still: Does ReleaseSRWLockExclusive notify other threads?
> > 
> 
> Of course? How else would a lock work, it must release other waiters?
> 
> It might not be a fair lock though, which is not a problem for this situation, which does not require fair locking.
> 
> 
> > Functionally, the INIT_ONCE looks interesting. But, like Takashi Yano mentioned,
> > how would you make it fit into a pthread_once_t that is defined as
> >    struct { pthread_mutex_t mutex; int state; }
> > ?
> 
> Something like:
> 
> struct once {
>     union {
>        pthread_mutex_t old_mutex_field_for_size_compatibility;
>        SRWLOCK lock = SRWLOCK_INIT;
>      };
>      int state;
> };

Reading [1], it seems that this might indeed work, and be faster than code
that uses CRITICAL_SECTIONs.

Still, something to watch out is this bug [2].

All in all, I would say that every such change ought to be tested with a
nontrivial test program that performs several thousands of rounds.

Bruno

[1] https://marabos.nl/atomics/os-primitives.html
[2] https://old.reddit.com/r/cpp/comments/1b55686/maybe_possible_bug_in_stdshared_mutex_on_windows/


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