Re: [RFC][PATCH] Fix hang in posix_locks_deadlock()

George G. Davis
2007-10-17T23:41:30Z

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On Wed, Oct 17, 2007 at 02:51:57PM -0400, George G. Davis wrote:
> Not sure if this is the correct fix but it does resolve the hangs we're
> observing in posix_locks_deadlock(). Comments greatly appreciated...

Attached is a test case which exhibits the hang on an F7 host with all
updates applied. Also occurs on linux-2.6.git latest.

--
Regards,
George

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// gcc -o fcntltest -pthread posix_locks_deadlock-hang.c

#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>

#define __USE_GNU 1
#include <fcntl.h>
#include <pthread.h>
#include <sys/types.h>
#include <unistd.h>

#define NUM_THREADS 100
#define NUM_PROCS 5
#define NUM_FILES 64

pthread_mutex_t locks[NUM_FILES];
pthread_mutex_t gross_lock = PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP;

static void *thread_routine(int number)
{
const char *fname = NULL;
int f, rc;
struct flock lock_it;
pthread_mutex_t *lock = NULL;

// get the gross lock
pthread_mutex_lock(&gross_lock);

lock_it.l_whence = SEEK_SET; /* from current point */
lock_it.l_start = 0; /* -"- */
lock_it.l_len = 0; /* until end of file */
lock_it.l_type = F_WRLCK; /* set exclusive/write lock */
lock_it.l_pid = 0; /* pid not actually interesting */

fname = calloc(1, 500);

snprintf(fname, 499, "%i.txt", number);
lock = &locks[number];

if (pthread_mutex_trylock(lock) != 0) {
pthread_mutex_unlock(&gross_lock);
pthread_mutex_lock(lock);
pthread_mutex_unlock(&gross_lock);
}

int randm = (rand() % 32768) * 10;
printf("Opening %s...\n", fname);
f = open(fname, O_TRUNC | O_CREAT | O_WRONLY, 0644);
if (f < 0) {
perror(fname);
exit(1);
}
printf("Locking %s...\n", fname);
// release the gross lock before attempting the fcntl
pthread_mutex_unlock(&gross_lock);
rc = fcntl(f, F_SETLKW, &lock_it);
sleep(5);
// gain the gross lock again
pthread_mutex_lock(&gross_lock);
if (rc < 0) {
perror(fname);
usleep(randm);
pthread_mutex_unlock(lock);
pthread_mutex_unlock(&gross_lock);
exit(1);
}

usleep(randm);
// unlocks the file
close(rc);
pthread_mutex_unlock(lock);
pthread_mutex_unlock(&gross_lock);
return NULL;
}

int main(int argc, char **argv)
{
int i, rc;
pthread_t tids[NUM_THREADS];
memset(tids, 0, sizeof(tids));
memset(locks, 0, sizeof(locks));

for (i = 0; i < NUM_PROCS; i++) {
pid_t mypid = fork();
if (mypid == 0)
break;
}

for (i = 0; i < NUM_THREADS; i++) {
rc = pthread_create(&tids[i], NULL, thread_routine,
(rand() % NUM_FILES));
if (rc != 0) {
tids[i] = 0;
}
}

for (i = 0; i < 500; i++) {
if (tids[i] != 0) {
pthread_join(tids[i], NULL);
}
}

return 0;
}