Force unlock a mutex that was locked by a different thread

Question

Consider the following test program:

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <strings.h>
#include <unistd.h>
#include <signal.h>
#include <pthread.h>


pthread_mutex_t mutex;
pthread_mutexattr_t mattr;
pthread_t thread1;
pthread_t thread2;
pthread_t thread3;


void mutex_force_unlock(pthread_mutex_t *mutex, pthread_mutexattr_t *mattr)
  {
    int e;
    e = pthread_mutex_destroy(mutex);
    printf("mfu: %s\n", strerror(e));
    e = pthread_mutex_init(mutex, mattr);
    printf("mfu: %s\n", strerror(e));
  }

void *thread(void *d)
  {
    int e;

    e = pthread_mutex_trylock(&mutex);
    if (e != 0)
      {
        printf("thr: %s\n", strerror(e));
        mutex_force_unlock(&mutex, &mattr);
        e = pthread_mutex_unlock(&mutex);
        printf("thr: %s\n", strerror(e));
        if (e != 0) pthread_exit(NULL);
        e = pthread_mutex_lock(&mutex);
        printf("thr: %s\n", strerror(e));
      }
    pthread_exit(NULL);
  }


void * thread_deadtest(void *d)
  {
    int e;
    e = pthread_mutex_lock(&mutex);
    printf("thr2: %s\n", strerror(e));
    e = pthread_mutex_lock(&mutex);
    printf("thr2: %s\n", strerror(e));
    pthread_exit(NULL);
  }


int main(void)
  {
    /* Setup */
    pthread_mutexattr_init(&mattr);
    pthread_mutexattr_settype(&mattr, PTHREAD_MUTEX_ERRORCHECK);
    //pthread_mutexattr_settype(&mattr, PTHREAD_MUTEX_NORMAL);
    pthread_mutex_init(&mutex, &mattr);

    /* Test */
    pthread_create(&thread1, NULL, &thread, NULL);
    pthread_join(thread1, NULL);
    if (pthread_kill(thread1, 0) != 0) printf("Thread 1 has died.\n");
    pthread_create(&thread2, NULL, &thread, NULL);
    pthread_join(thread2, NULL);
    pthread_create(&thread3, NULL, &thread_deadtest, NULL);
    pthread_join(thread3, NULL);
    return(0);
  }

Now when this program runs, I get the following output:

Thread 1 has died.
thr: Device busy
mfu: Device busy
mfu: No error: 0
thr: Operation not permitted
thr2: No error: 0
thr2: Resource deadlock avoided

Now I know this has been asked a number of times before, but is there any way to forcefully unlock a mutex? It seems the implementation will only allow the mutex to be unlocked by the thread that locked it as it seems to actively check, even with a normal mutex type.

Why am I doing this? It has to do with coding a bullet-proof network server that has the ability to recover from most errors, including ones where the thread terminates unexpectedly. At this point, I can see no way of unlocking a mutex from a thread that is different than the one that locked it. So the way that I see it is that I have a few options:

1. Abandon the mutex and create a new one. This is the undesirable option as it creates a memory leak.
2. Close all network ports and restart the server.
3. Go into the kernel internals and release the mutex there bypassing the error checking.

I have asked this before but, the powers that be absolutely want this functionality and they will not take no for an answer (I've already tried), so I'm kinda stuck with this. I didn't design it this way, and I would really like to shoot the person who did, but that's not an option either.

And before someone says anything, my usage of pthread_kill is legal under POSIX...I checked.

I forgot to mention, this is FreeBSD 9.3 that we are working with.

Answer 1

Use a robust mutex, and if the locking thread dies, fix the mutex with pthread_mutex_consistent().

If mutex is a robust mutex in an inconsistent state, the pthread_mutex_consistent() function can be used to mark the state protected by the mutex referenced by mutex as consistent again.

If an owner of a robust mutex terminates while holding the mutex, the mutex becomes inconsistent and the next thread that acquires the mutex lock shall be notified of the state by the return value [EOWNERDEAD]. In this case, the mutex does not become normally usable again until the state is marked consistent.

If the thread which acquired the mutex lock with the return value [EOWNERDEAD] terminates before calling either pthread_mutex_consistent() or pthread_mutex_unlock(), the next thread that acquires the mutex lock shall be notified about the state of the mutex by the return value [EOWNERDEAD].