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Date:	Wed, 29 May 2024 20:04:48 +0900
To:	Bruno Haible <bruno AT clisp DOT org>
Cc:	cygwin AT cygwin DOT com
Subject:	Re: multithreading broken in Cygwin 3.5.3
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From:	Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
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On Wed, 29 May 2024 12:26:31 +0200
Bruno Haible wrote:
> Takashi Yano wrote:
> > As you mentioned in private mail to me, this seems to be a regression of
> > pthread::once() introduced by
> > commit 2c5433e5da8216aaf7458e50c63683c68fb0d3e8.
> > 
> > I'll submit a patch for that issue shortly.
> 
> My workaround implementation of pthread_once (in gnulib) looks like this:
> 
>   /* This would be the code, for
>        typedef struct
>          {
>            pthread_mutex_t mutex;
>            _Atomic unsigned int num_threads;
>            _Atomic unsigned int done;
>          }
>        pthread_once_t;
>    */
>   if (once_control->done == 0)
>     {
>       once_control->num_threads += 1;
>       pthread_mutex_lock (&once_control->mutex);
>       if (once_control->done == 0)
>         {
>           (*initfunction) ();
>           once_control->done = 1;
>         }
>       pthread_mutex_unlock (&once_control->mutex);
>       if ((once_control->num_threads -= 1) == 0)
>         pthread_mutex_destroy (&once_control->mutex);
>     }
> 
> It makes sure that
>   - The last thread that had been dealing with the mutex deletes
>     the mutex.
>   - Once the mutex is deleted, is it never again accessed. The
>     entry test of the 'done' boolean ensures this.

Thanks for the advice.

The problem is that we cannot change the type of pthread_once_t
for binary compatibility. My patch stops to use
pthread_mutex_t mutex in pthread_once_t however it cannot be
deleted despite it is not used at all.

-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

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