open file descriptors remain after threaded exit() in 2.5.44

In linux 2.5.44, there seems to be a race between process exit and pthread creation that can leave an open file descriptor that has no task associated with it.

two test programs are included at the end of this mail. Run the first program on a SMP system with at least two processors. The system I'm using has 8 pentium 4 processors and 16 gigabytes of memory. After the first program exits, run the second test program to demonstrate that there is still state remaining from the first program.

To run the tests, you must first create a file that will be used to place record locks. You can edit the program sources to put that file wherever you like.

The first test program begins by opening that file and using fcntl(F_SETLK) to put a write lock on the file. The main thread then creates 8 child threads and then exits immediately. Each child thread sleeps for 60 seconds. But, when the parent thread exits, the child threads are forced to exit also.

The second program tries to get a lock on that same
file. Since the first program has exited, its lock on that file should nolonger be present, and the second program should successfully get its lock. But instead, the second program fails. A ps -eaf shows there are no threads from the first process still present.

If you modify the first program so that the main thread sleeps 30 seconds before exiting, then this problem is no longer seen. The main thread exiting still forces the child threads to exit. But those child threads seem to now be in a state where the forced exit doesn't expose this apparent race.

Here are the test programs

-------------threaded test program. Run this first--------------------------- ------------ compile with -lpthread -lm flags --------------------------

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <math.h>
#include <pthread.h>
#include <getopt.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/fcntl.h>
#include <sys/time.h>
#include <sys/utsname.h>

void *worker_thread(void *arg)

{
	sleep(60);
}

static pthread_attr_t thread_attr;
#define NTHREADS 8

main()

{
	int i;
	int fd;
	struct flock lock;

	fd = open("/home/dmo/PTH/l", O_RDWR);
	if (fd == -1) {
		perror("open failed");
		exit(0);
	}
	lock.l_whence = SEEK_SET;
	lock.l_type = F_WRLCK;
	lock.l_start = 0;
	lock.l_len = 1;

	if (fcntl(fd, F_SETLK, &lock) == -1) {
		perror("F_SETLK failed\n");
		exit(0);
	}

	pthread_attr_init(&thread_attr);
	pthread_attr_setdetachstate(&thread_attr, PTHREAD_CREATE_DETACHED);

	for (i = 0; i < NTHREADS; i++) {
		pthread_t worker_tid;

		if (pthread_create(&worker_tid, &thread_attr, worker_thread,
					(void *)NULL) != 0) {
			perror("thread create failed");
			exit(1);
		}
	}
	/*sleep(30);*/
}

---------------------- The second test program ------------------------------ ---------------------- This tests for the failure of the first --------------

#include <unistd.h>
#include <fcntl.h>

main()

{
	int fd;
	struct flock lock;

	fd = open("/home/dmo/PTH/l", O_RDWR);
	if (fd == -1) {
		perror("open failed");
		exit(0);
	}
	lock.l_whence = SEEK_SET;
	lock.l_type = F_WRLCK;
	lock.l_start = 0;
	lock.l_len = 1;

	if (fcntl(fd, F_SETLK, &lock) == -1) {
		perror("F_SETLK failed\n");
		exit(0);
	}
	printf("lock succeeded\n");
	/*sleep(30);*/
}

Dave,

I've verified the problem still exists for SAP DB on 2.5.49 and your test case still fails. Then I installed NGPT 2.0.4 and found that I still cannot start, stop and start SAP DB, but your test case now passes.

Does that offer any clues?

Mark

Mark Wong - - [email protected]
Open Source Development Lab Inc - A non-profit corporation
15275 SW Koll Parkway - Suite H - Beaverton OR, 97006
(503)-626-2455 x 32 (office)
(503)-626-2436      (fax)
http://www.osdl.org/archive/markw/