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To: cygwin@cygwin.com, Takashi Yano <takashi.yano@nifty.ne.jp>
Subject: Re: multithreading broken in Cygwin 3.5.3
Date: Wed, 29 May 2024 12:26:31 +0200
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 <20240529190619.53c5820ecafcf31909c4320c@nifty.ne.jp>
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From: Bruno Haible via Cygwin <cygwin@cygwin.com>
Reply-To: Bruno Haible <bruno@clisp.org>
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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.

Bruno


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