Is this pthread_cond_wait() race possible

[Torvald Riegel]

On Wed, 2017-06-07 at 18:48 +0200, Sebastian Andrzej Siewior wrote:
> 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

The second CAS would expect a value of 2 in *(cond->__data.__g_signals +
g).  Because the first CAS succeeded in your example, the second CAS
will fail, and T2 will update it's view of __g_signals and store that in
signals.