multithreading broken in Cygwin 3.5.3

Wed May 29 10:26:31 GMT 2024

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