Pthread_cond_wait if cancelled leaves mutex locked deadlocking other threads?!

This is a discussion on Pthread_cond_wait if cancelled leaves mutex locked deadlocking other threads?! within the C Programming forums, part of the General Programming Boards category; From the following source code Code: int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex) { volatile pthread_descr self = thread_self(); __pthread_lock(&cond->__c_lock, self); enqueue(&cond->__c_waiting, ...

  1. #1
    Alessio Stella
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    Pthread_cond_wait if cancelled leaves mutex locked deadlocking other threads?!

    From the following source code
    Code:
    int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex)
    {
    volatile pthread_descr self = thread_self();
    
    __pthread_lock(&cond->__c_lock, self);
    enqueue(&cond->__c_waiting, self);
    __pthread_unlock(&cond->__c_lock);
    pthread_mutex_unlock(mutex);
    suspend_with_cancellation(self);
    pthread_mutex_lock(mutex);
    /* This is a cancellation point */
    if (THREAD_GETMEM(self, p_canceled)
    && THREAD_GETMEM(self, p_cancelstate) == PTHREAD_CANCEL_ENABLE) {
    /* Remove ourselves from the waiting queue if we're still on it */
    __pthread_lock(&cond->__c_lock, self);
    remove_from_queue(&cond->__c_waiting, self);
    __pthread_unlock(&cond->__c_lock);
    pthread_exit(PTHREAD_CANCELED);
    }
    return 0;
    }
    it seems that when a thread gets canceled while waiting for cond it performs
    pthread_mutex_lock(mutex);
    ...
    pthread_exit(PTHREAD_CANCELED);

    and leaves mutex locked?!
    so anothe thread is potentially deadlocked?!
    I tried to look in the source for pthread_exit() if and when the mutex is released but I didn't figure out!


    So may be to prevent this the correct usage is

    Code:
      pthread_mutex_lock(mutex);
      pthread_cleanup_push(pthread_mutex_unlock,mutex);
      while (...) pthread_cond_wait(cond,mutex); 
    ......
      pthread_cleanup_pop();
      pthread_mutex_unlock(mutex);
    what you think?
    Last edited by mynickmynick; 05-08-2008 at 07:56 AM.

  2. #2
    Captain Crash brewbuck's Avatar
    Join Date
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    Portland, OR
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    Quote Originally Posted by mynickmynick View Post
    So may be to prevent this the correct usage is

    Code:
      pthread_mutex_lock(mutex);
      pthread_cleanup_push(pthread_mutex_unlock,mutex);
      while (...) pthread_cond_wait(cond,mutex); 
    ......
      pthread_cleanup_pop();
      pthread_mutex_unlock(mutex);
    what you think?
    Almost right, except the call to pthread_cleanup_pop should look like:

    Code:
    pthread_cleanup_pop(0);
    pthread_mutex_unlock(mutex);
    Or, just allow the cleanup handler to run itself when you pop it, unlocking the mutex for you:

    Code:
    pthread_cleanup_pop(1);

  3. #3
    Alessio Stella
    Join Date
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    Location
    Italy, Bologna
    Posts
    251
    ok great
    thank you very much

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