why is it hanged in pthread_cond_signal

Question

I implement a "thread_pool" which causes hang in pthread_cond_signal. I'm curious why it happens?

This is the gdb stack:

(gdb) bt

#0  0x00007fc1ac92ee24 in __lll_lock_wait () from /opt/compiler/gcc-       4.8.2/lib/libpthread.so.0
#1  0x00007fc1ac92ccb1 in pthread_cond_signal@@GLIBC_2.3.2 () from /opt/compiler/gcc-4.8.2/lib/libpthread.so.0
#2  0x00000000004ac736 in ThreadPool::dispatch (this=0x415e0a0, method=0x40d1080) at ../../common/thread_pool.cc:76

Here is the code snippet:

int ThreadPool::dispatch(Method* method) {
    pthread_mutex_lock(&_method_lock);
    _method_list.push_back(method);
    pthread_cond_signal(&_method_cond);
    pthread_mutex_unlock(&_method_lock);

    return 0;
}

ThreadPool::Method* ThreadPool::get() {
    Method* method = NULL;

    pthread_mutex_lock(&_method_lock);

    if (_method_list.empty()) {
        struct timeval now;
        struct timespec timeout;
        gettimeofday(&now, 0);
        timeout.tv_sec = now.tv_sec + 1;
        timeout.tv_nsec = now.tv_usec * 1000;

        pthread_cond_timedwait(&_method_cond, &_method_lock, &timeout);
    }

    if (!_method_list.empty()) {
        method = _method_list.front();
        _method_list.pop_front();
    }
    pthread_mutex_unlock(&_method_lock);

    return method;
}

Possible duplicate of [pthread_cond_signal causing deadlock](http://stackoverflow.com/questions/8248458/pthread-cond-signal-causing-deadlock) — Mohamad Elghawi, Nov 20 '15 at 13:42
@Let_Me_Be It is not waiting. It is hanging when calling `pthread_cond_signal` — user3619782, Nov 20 '15 at 14:00
@Mohamad Elghawi I did not destroy the mutex and condvar in a method other than destructor. So I do not think it is a duplicate. — user3619782, Nov 20 '15 at 14:06
@user3619782 Could you then show more of your code. How are you using this from the outside? — Mohamad Elghawi, Nov 20 '15 at 14:10
@Mohamad Elghawi Here is the complete code of thread pool impl. https://gist.github.com/liyinhgqw/def6099ee3122c14f655 — user3619782, Nov 20 '15 at 14:16
@Mohamad Elghawi I used it in a timer service. gist.github.com/liyinhgqw/b3b57e502289bb5f0840 — user3619782, Nov 20 '15 at 14:30
@Mohamad Elghawi I checked my code. I did destroy the cond and lock then thread pool is destructed, while timer trying to dispatch some task to the thread pool. I forgot to delete timer before thread pool was destructed. — user3619782, Nov 21 '15 at 01:51

score 0 · Answer 1 · answered Nov 21 '15 at 01:59

0

I did destroy the cond and lock then thread pool was destructed, while timer trying to dispatch some task to the thread pool. I forgot to delete timer before thread pool was destructed.

answered Nov 21 '15 at 01:59

user3619782

13
3

why is it hanged in pthread_cond_signal

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