exec: RT sub-thread can livelock and monopolize CPU on exec
de_thread() yields waiting for ->group_leader to be a zombie. This deadlocks
if an rt-prio execer shares the same cpu with ->group_leader. Change the code
to use ->group_exit_task/notify_count mechanics.
This patch certainly uglifies the code, perhaps someone can suggest something
better.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/exec.c b/fs/exec.c
index ab5a4a3..aa470a9 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -801,16 +801,15 @@
hrtimer_restart(&sig->real_timer);
spin_lock_irq(lock);
}
+
+ sig->notify_count = count;
+ sig->group_exit_task = tsk;
while (atomic_read(&sig->count) > count) {
- sig->group_exit_task = tsk;
- sig->notify_count = count;
__set_current_state(TASK_UNINTERRUPTIBLE);
spin_unlock_irq(lock);
schedule();
spin_lock_irq(lock);
}
- sig->group_exit_task = NULL;
- sig->notify_count = 0;
spin_unlock_irq(lock);
/*
@@ -819,14 +818,17 @@
* and to assume its PID:
*/
if (!thread_group_leader(tsk)) {
- /*
- * Wait for the thread group leader to be a zombie.
- * It should already be zombie at this point, most
- * of the time.
- */
leader = tsk->group_leader;
- while (leader->exit_state != EXIT_ZOMBIE)
- yield();
+
+ sig->notify_count = -1;
+ for (;;) {
+ write_lock_irq(&tasklist_lock);
+ if (likely(leader->exit_state))
+ break;
+ __set_current_state(TASK_UNINTERRUPTIBLE);
+ write_unlock_irq(&tasklist_lock);
+ schedule();
+ }
/*
* The only record we have of the real-time age of a
@@ -840,8 +842,6 @@
*/
tsk->start_time = leader->start_time;
- write_lock_irq(&tasklist_lock);
-
BUG_ON(leader->tgid != tsk->tgid);
BUG_ON(tsk->pid == tsk->tgid);
/*
@@ -874,6 +874,8 @@
write_unlock_irq(&tasklist_lock);
}
+ sig->group_exit_task = NULL;
+ sig->notify_count = 0;
/*
* There may be one thread left which is just exiting,
* but it's safe to stop telling the group to kill themselves.