Problem with exiting threads under NPTL

Petr Vandrovec
2003-12-14T05:25:21Z

Hi,
several times one of our threads ended up as ZOMBIE and
nobody wants to pick him up - even init ignores it. Inspection
of kernel structures revealed that task's exit code is 0,
exit_signal is -1, ptrace is 0 and state is 8 (ZOMBIE).

Quick look at exit_notify reveals that it should not happen:
if task has exit_signal == -1 and is not ptraced, we should immediate
move to TASK_DEAD and get rid of task as fast as possible.

But problem is that in do_notify_parent we have code which
sets exit_signal to -1 if parent ignores SIGCHLD, and we call
do_notify_parent() for group leader from release_task() when
last member of thread group exits, without taking any care
about eventual changes in exit_signal field.

So if some process ignores SIGCHLD, and spawns child process
which creates additional thread, and main thread in child exits
before child it created, you'll end up with immortal zombie.

Best regards,
Petr Vandrovec
vandrove@vc.cvut.cz

F UID PID PPID PRI NI VSZ RSS WCHAN STAT TTY TIME COMMAND
5 0 3709 1 15 0 0 0 exit Z tty3 0:00 [test] <defunct>

Creator of immortal zombies:

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

static void* thread(void* dummy) {
/* Make sure that we exit as second thread */
sleep(1);
return NULL;
}

void main(void) {
int pid;

signal(SIGCHLD, SIG_IGN);

pid = fork();
if (pid == 0) {
pthread_t p;

pthread_create(&p, NULL, thread, NULL);
} else {
/* Sleep some time so we know that child threads exit before us */
sleep(2);
printf("Look for task %d...\n", pid);
}
}