This is the mail archive of the
pthreads-win32@sources.redhat.com
mailing list for the pthreas-win32 project.
Re: Deadlock interaction between pthread_cond_check_need_init.c andpthread_cond_destroy.c
- From: Ross Johnson <rpj at ise dot canberra dot edu dot au>
- To: Michael Johnson <michaelj at maine dot rr dot com>
- Cc: pthreads-win32 discussion list <pthreads-win32 at sources dot redhat dot com>
- Date: Fri, 20 Sep 2002 12:40:08 +1000
- Subject: Re: Deadlock interaction between pthread_cond_check_need_init.c andpthread_cond_destroy.c
- Organization: University of Canberra, Information Sciences and Engineering
- References: <3D89F90A.2030805@maine.rr.com>
Michael,
I've applied your patch and committed it to CVS. I'll write a test
for it later.
Many thanks for this.
Ross
Michael Johnson wrote:
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));|
|}|