Is this pthread_cond_wait() race possible

[Sebastian Andrzej Siewior]

Hi Torvald,

I've been staring at the new pthread_cond_* code. Could you please tell
if this race possible (two wait threads looking at the same __g_signals
value while a third thread signals a wake before going to futex_wait()):

T1                               T2                       T3

__pthread_cond_wait_common()                            
 __condvar_fetch_add_wseq_acquire (cond, 2);
 __pthread_mutex_unlock_usercnt();                       

                                __pthread_cond_wait_common()
                                 __condvar_fetch_add_wseq_acquire (cond, 2);
                                 __pthread_mutex_unlock_usercnt();

                                                        __pthread_cond_signal()
                                                          if ((cond->__data.__g_size[g1] != 0) …

                                                           /* Add a signal. */
                                                           atomic_fetch_add_relaxed (cond->__data.__g_signals + g1, 2);

 signals = atomic_load_acquire (cond->__data.__g_signals + g);
                                 signals = atomic_load_acquire (cond->__data.__g_signals + g);

  signals = 2                     signals = 2

          /* If there is an available signal, don't block.  */
  if (signals != 0)               if (signals != 0)
      break;                         break;

  while (!atomic_compare_exchange_weak_acquire (cond->__data.__g_signals + g,
                                                 &signals, signals - 2));
  signals = 0
                                  while (!atomic_compare_exchange_weak_acquire (cond->__data.__g_signals + g,
                                                &signals, signals - 2));
                                   signals = (int) -2

   …                               …                          …

Or is there maybe a synchronisation I missed?

Sebastian