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Date:      Sat, 12 Aug 2000 18:14:55 +0200 (CEST)
From:      "Bradley T. Hughes" <bhughes@trolltech.com>
To:        stable@freebsd.org
Subject:   bug in pthread implementation
Message-ID:  <Pine.BSF.4.21.0008121809030.27068-200000@reticent.troll.no>
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[-- Attachment #1 --]
It seems that the mutex implementation for recursive mutexes has a bug.

a single lock/unlock pair works as expected, but locking 3 times only
requires 2 unlocks before the mutex is released.  i've read through
src/lib/libc_r/uthread/uthread_mutex.c but can't quite follow the logic...

i plan on debugging this more and hope to generate a patch... but i
figured i'd send off an email about it to see if anyone else can find the
solution quicker than i can :)

to see the bug, take the attached blah.c and compile it:

gcc -pthread -o blah blah.c

and run it :

./blah

--
Bradley T. Hughes <bhughes@trolltech.com>
Waldemar Thranes gt. 98B N-0175 Oslo, Norway
Office: +47 21 60 48 92
Mobile: +47 92 01 97 81

[-- Attachment #2 --]
#include <pthread.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

pthread_cond_t cond;

void *thread1(void *arg) {
    pthread_mutex_t *mutex = (pthread_mutex_t *) arg;
    
    // lock/unlock once works fine
    printf("thread 1 locking mutex (%s)\n",
           strerror(pthread_mutex_lock(mutex)));
    
    // make sure thread1 is first
    pthread_cond_wait(&cond, mutex);
    pthread_cond_signal(&cond);
    
    printf("thread 1 unlocking mutex (%s)\n",
           strerror(pthread_mutex_unlock(mutex)));
    
    // lock 6 times and then unlock 6 times
    printf("thread 1 locking mutex 1 (%s)\n",
           strerror(pthread_mutex_lock(mutex)));
    printf("thread 1 locking mutex 2 (%s)\n",
           strerror(pthread_mutex_lock(mutex)));
    printf("thread 1 locking mutex 3 (%s)\n",
           strerror(pthread_mutex_lock(mutex)));
    printf("thread 1 locking mutex 4 (%s)\n",
           strerror(pthread_mutex_lock(mutex)));
    printf("thread 1 locking mutex 5 (%s)\n",
           strerror(pthread_mutex_lock(mutex)));
    printf("thread 1 locking mutex 6 (%s)\n",
           strerror(pthread_mutex_lock(mutex)));

    printf("thread 1 unlocking mutex 6 (%s)\n",
           strerror(pthread_mutex_unlock(mutex)));
    printf("thread 1 unlocking mutex 5 (%s)\n",
           strerror(pthread_mutex_unlock(mutex)));
    printf("thread 1 unlocking mutex 4 (%s)\n",
           strerror(pthread_mutex_unlock(mutex)));
    printf("thread 1 unlocking mutex 3 (%s)\n",
           strerror(pthread_mutex_unlock(mutex)));
    printf("thread 1 unlocking mutex 2 (%s)\n",
           strerror(pthread_mutex_unlock(mutex)));
    
    printf("!!! should still be locked, but the other thread will be able to run now\n");
    sleep(1);
    printf("thread 1 unlocking mutex 1 (%s)\n",
           strerror(pthread_mutex_unlock(mutex)));
    printf("!!! notice the errors\n");
    
    return NULL;
}


void *thread2(void *arg) {
    pthread_mutex_t *mutex = (pthread_mutex_t *) arg;
    
    printf("thread 2 locking mutex (%s)\n",
           strerror(pthread_mutex_lock(mutex)));

    pthread_cond_signal(&cond);
    pthread_cond_wait(&cond, mutex);

    printf("thread 2 unlocking mutex (%s)\n",
           strerror(pthread_mutex_unlock(mutex)));

    printf("thread 2 locking mutex 1 (%s)\n",
	   strerror(pthread_mutex_lock(mutex)));
    printf("thread 2 locking mutex 2 (%s)\n",
           strerror(pthread_mutex_lock(mutex)));
    printf("thread 2 locking mutex 3 (%s)\n",
           strerror(pthread_mutex_lock(mutex)));
    printf("thread 2 locking mutex 4 (%s)\n",
           strerror(pthread_mutex_lock(mutex)));
    printf("thread 2 locking mutex 5 (%s)\n",
           strerror(pthread_mutex_lock(mutex)));
    printf("thread 2 locking mutex 6 (%s)\n",
           strerror(pthread_mutex_lock(mutex)));

    printf("thread 2 unlocking mutex 6 (%s)\n",
           strerror(pthread_mutex_unlock(mutex)));
    printf("thread 2 unlocking mutex 5 (%s)\n",
           strerror(pthread_mutex_unlock(mutex)));
    printf("thread 2 unlocking mutex 4 (%s)\n",
           strerror(pthread_mutex_unlock(mutex)));
    printf("thread 2 unlocking mutex 3 (%s)\n",
           strerror(pthread_mutex_unlock(mutex)));
    printf("thread 2 unlocking mutex 2 (%s)\n",
           strerror(pthread_mutex_unlock(mutex)));

    // should be locked, but it is not
    printf("thread 2 unlocking mutex 1 (%s)\n",
           strerror(pthread_mutex_unlock(mutex)));

    return NULL;
}


int main() {
    pthread_t thr1, thr2;
    pthread_mutex_t mutex;
    void *junk;

    pthread_mutexattr_t mattr;
    pthread_mutexattr_init(&mattr);
    pthread_mutexattr_settype(&mattr, PTHREAD_MUTEX_RECURSIVE);
    pthread_mutex_init(&mutex, &mattr);

    pthread_cond_init(&cond, NULL);

    pthread_create( &thr1, NULL, thread1, (void *) &mutex );
    pthread_create( &thr2, NULL, thread2, (void *) &mutex );

    pthread_join(thr1, &junk);
    pthread_join(thr2, &junk);

    return 0;
}
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