Deadlock interaction between pthread_cond_check_need_init.c and pthread_cond_destroy.c

[Michael Johnson <michaelj@maine.rr.com>]

When two different threads exist, and one is attempting to destroy a 
condition variable while the other is attempting to initialize a 
condition variable that was created with PTHREAD_COND_INITIALIZER, a 
deadlock can occur:

Thread A Thread B
Enters ptw32_cond_check_need_init


Enters pthread_cond_destroy
EnterCriticalSection(&ptw32_cond_test_init_lock)

(now holds ptw32_cond_test_init_lock)
EnterCriticalSection(&ptw32_cond_list_lock)
Enters pthread_cond_init
Determines that condvar is static initialized
EnterCriticalSection(&ptw32_cond_list_lock)
EnterCriticalSection(&ptw32_cond_test_init_lock)
(now waiting to enter ptw32_cond_list_lock)
(now waiting to enter ptw32_cond_test_init_lock)
deadlocked


It appears from reading the code and from a patch that I made that the 
following change to pthread_cond_destroy appears to fix this:
|
|

    |{|
    |  pthread_cond_t cv;|
    |  int result = 0, result1 = 0, result2 = 0;|
    ||
    |  /*|
    |   * Assuming any race condition here is harmless.|
    |   */|
    |  if (cond == NULL|
    |      || *cond == NULL)|
    |    {|
    |      return EINVAL;|
    |    }|
    ||
    |  if (*cond != PTHREAD_COND_INITIALIZER)|
    |    {|
    |      EnterCriticalSection(&ptw32_cond_list_lock);|
    ||
    |      cv = *cond;|
    ||
    |      /*|
    |       * Close the gate; this will synchronize this thread with|
    |       * all already signaled waiters to let them retract their|
    |       * waiter status - SEE NOTE 1 ABOVE!!!|
    |       */|
    |      if (sem_wait(&(cv->semBlockLock)) != 0)|
    |        {|
    |          return errno;|
    |        }|
    ||
    |      /*|
    |       * !TRY! lock mtxUnblockLock; try will detect busy condition|
    |       * and will not cause a deadlock with respect to concurrent|
    |       * signal/broadcast.|
    |       */|
    |      if ((result = pthread_mutex_trylock(&(cv->mtxUnblockLock)))
    != 0)|
    |        {|
    |          (void) sem_post(&(cv->semBlockLock));|
    |          return result;|
    |        }|
    ||
    |      /*|
    |       * Check whether cv is still busy (still has waiters)|
    |       */|
    |      if (cv->nWaitersBlocked > cv->nWaitersGone)|
    |        {|
    |          if (sem_post(&(cv->semBlockLock)) != 0)|
    |            {|
    |              result = errno;|
    |            }|
    |          result1 = pthread_mutex_unlock(&(cv->mtxUnblockLock));|
    |          result2 = EBUSY;|
    |        }|
    |      else|
    |        {|
    |          /*|
    |           * Now it is safe to destroy|
    |           */|
    |          *cond = NULL;|
    ||
    |          if (sem_destroy(&(cv->semBlockLock)) != 0)|
    |            {|
    |              result = errno;|
    |            }|
    |          if (sem_destroy(&(cv->semBlockQueue)) != 0)|
    |            {|
    |              result1 = errno;|
    |            }|
    |          if ((result2 =
    pthread_mutex_unlock(&(cv->mtxUnblockLock))) == 0)|
    |            {|
    |              result2 = pthread_mutex_destroy(&(cv->mtxUnblockLock));|
    |            }|
    ||
    |          /* Unlink the CV from the list */|
    ||
    |          if (ptw32_cond_list_head == cv)|
    |            {|
    |              ptw32_cond_list_head = cv->next;|
    |            }|
    |          else|
    |            {|
    |              cv->prev->next = cv->next;|
    |            }|
    ||
    |          if (ptw32_cond_list_tail == cv)|
    |            {|
    |              ptw32_cond_list_tail = cv->prev;|
    |            }|
    |          else|
    |            {|
    |              cv->next->prev = cv->prev;|
    |            }|
    ||
    |          (void) free(cv);|
    |        }|
    ||
    |      LeaveCriticalSection(&ptw32_cond_list_lock);|
    |    }|
    |  else|
    |    {|
    |      /*|
    |       * See notes in ptw32_cond_check_need_init() above also.|
    |       */|
    |      EnterCriticalSection(&ptw32_cond_test_init_lock);|
    ||
    |      /*|
    |       * Check again.|
    |       */|
    |      if (*cond == PTHREAD_COND_INITIALIZER)|
    |        {|
    |          /*|
    |           * This is all we need to do to destroy a statically|
    |           * initialised cond that has not yet been used
    (initialised).|
    |           * If we get to here, another thread waiting to initialise|
    |           * this cond will get an EINVAL. That's OK.|
    |           */|
    |          *cond = NULL;|
    |        }|
    |      else|
    |        {|
    |          /*|
    |           * The cv has been initialised while we were waiting|
    |           * so assume it's in use.|
    |           */|
    |          result = EBUSY;|
    |        }|
    ||
    |      LeaveCriticalSection(&ptw32_cond_test_init_lock);|
    |    }|
    ||
    |  return ((result != 0) ? result : ((result1 != 0) ? result1 :
    result2));|
    |}|