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Commit 6db840fa authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
Browse files

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: default avatarOleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 356d6d50

fs/exec.c

+15 −13
Original line number Diff line number Diff line
@@ -801,16 +801,15 @@ static int de_thread(struct task_struct *tsk) 
			hrtimer_restart(&sig->real_timer);

		spin_lock_irq(lock);

	}

	while (atomic_read(&sig->count) > count) {

		sig->group_exit_task = tsk;



	sig->notify_count = count;

	sig->group_exit_task = tsk;

	while (atomic_read(&sig->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 @@ static int de_thread(struct task_struct *tsk) 
	 * 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 @@ static int de_thread(struct task_struct *tsk) 
		 */

		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 @@ static int de_thread(struct task_struct *tsk) 
		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.

kernel/exit.c

+7 −3
Original line number Diff line number Diff line
@@ -92,10 +92,9 @@ static void __exit_signal(struct task_struct *tsk) 
		 * If there is any task waiting for the group exit

		 * then notify it:

		 */

		if (sig->group_exit_task && atomic_read(&sig->count) == sig->notify_count) {

		if (sig->group_exit_task && atomic_read(&sig->count) == sig->notify_count)

			wake_up_process(sig->group_exit_task);

			sig->group_exit_task = NULL;

		}



		if (tsk == sig->curr_target)

			sig->curr_target = next_thread(tsk);

		/*
@@ -827,6 +826,11 @@ static void exit_notify(struct task_struct *tsk) 
		state = EXIT_DEAD;

	tsk->exit_state = state;



	if (thread_group_leader(tsk) &&

	    tsk->signal->notify_count < 0 &&

	    tsk->signal->group_exit_task)

		wake_up_process(tsk->signal->group_exit_task);



	write_unlock_irq(&tasklist_lock);



	list_for_each_safe(_p, _n, &ptrace_dead) {