Open Radar

 
The pthread library behaves unexpectedly when mixing recursive read locks with write locks. Unbalanced unlock calls will unlock a different threads' lock.

Steps to Reproduce:

-Compile the below c file with gcc -Wall -pthread t2.c
-Run a.out

Expected Results:

The third unbalanced unlock call from the read thread should return EINVAL or an equivalent error. The unlock of the write thread should succeed.

Actual Results:

The third unbalanced unlock call from the read thread unlocks the write lock that the write thread currently holds. The first unlock call to the write thread fails because the unbalanced unlock from the read thread has already unlocked the write thread.

Code:

t2.c
//-----------------

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>

//gcc -Wall -pthread t2.c

pthread_t t;
pthread_t t2;
pthread_rwlock_t m;

void * fn(void * arg) {
	int res;
	res = pthread_rwlock_rdlock(&m);
	fprintf(stderr,"read lock acquire 1 res: %i\n",res);
	res = pthread_rwlock_rdlock(&m);
	fprintf(stderr,"read lock acquire 2 res: %i\n",res);
	res = pthread_rwlock_unlock(&m);
	fprintf(stderr,"read lock release 1 res: %i\n",res);
	usleep( 10000);
	res = pthread_rwlock_unlock(&m);
	fprintf(stderr,"read lock release 2 res: %i\n",res);
	
	/** UNLOCKS THE WRITE LOCK (from a read thread) **/
	res = pthread_rwlock_unlock(&m);
	fprintf( stderr, "read lock release 3 res: %i\n", res);
	
	return NULL;
}

void * fn2(void * arg) {
	int res;
	res = pthread_rwlock_wrlock(&m);
	fprintf(stderr,"write lock acquire res: %i\n",res);
	usleep( 10000);
	res = pthread_rwlock_unlock(&m);
	fprintf(stderr,"write lock release res: %i\n",res);
	return NULL;
}

int main(int argc, char ** argv)
{
	pthread_rwlock_init( &m, NULL);
	pthread_create( &t, NULL, fn, NULL);
	usleep( 2000);
	pthread_create( &t2, NULL, fn2, NULL);
	pthread_join( t, NULL);
	pthread_join( t2, NULL);
	return 0;
}

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