Commit 16882c1e authored Jun 08, 2008 by Oleg Nesterov Committed by Ingo Molnar Jun 10, 2008

sched: fix TASK_WAKEKILL vs SIGKILL race



schedule() has the special "TASK_INTERRUPTIBLE && signal_pending()" case,
this allows us to do

	current->state = TASK_INTERRUPTIBLE;
	schedule();

without fear to sleep with pending signal.

However, the code like

	current->state = TASK_KILLABLE;
	schedule();

is not right, schedule() doesn't take TASK_WAKEKILL into account. This means
that mutex_lock_killable(), wait_for_completion_killable(), down_killable(),
schedule_timeout_killable() can miss SIGKILL (and btw the second SIGKILL has
no effect).

Introduce the new helper, signal_pending_state(), and change schedule() to
use it. Hopefully it will have more users, that is why the task's state is
passed separately.

Note this "__TASK_STOPPED | __TASK_TRACED" check in signal_pending_state().
This is needed to preserve the current behaviour (ptrace_notify). I hope
this check will be removed soon, but this (afaics good) change needs the
separate discussion.

The fast path is "(state & (INTERRUPTIBLE | WAKEKILL)) + signal_pending(p)",
basically the same that schedule() does now. However, this patch of course
bloats schedule().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent 39b945a3

include/linux/sched.h

+13 −0

Original line number	Diff line number	Diff line
		@@ -2026,6 +2026,19 @@ static inline int fatal_signal_pending(struct task_struct *p)
		return signal_pending(p) && __fatal_signal_pending(p);
		}

		static inline int signal_pending_state(long state, struct task_struct *p)
		{
		if (!(state & (TASK_INTERRUPTIBLE \| TASK_WAKEKILL)))
		return 0;
		if (!signal_pending(p))
		return 0;

		if (state & (__TASK_STOPPED \| __TASK_TRACED))
		return 0;

		return (state & TASK_INTERRUPTIBLE) \|\| __fatal_signal_pending(p);
		}

		static inline int need_resched(void)
		{
		return unlikely(test_thread_flag(TIF_NEED_RESCHED));

kernel/sched.c

+2 −4

Original line number	Diff line number	Diff line
		@@ -4159,12 +4159,10 @@ asmlinkage void __sched schedule(void)
		clear_tsk_need_resched(prev);

		if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
		if (unlikely((prev->state & TASK_INTERRUPTIBLE) &&
		signal_pending(prev))) {
		if (unlikely(signal_pending_state(prev->state, prev)))
		prev->state = TASK_RUNNING;
		} else {
		else
		deactivate_task(rq, prev, 1);
		}
		switch_count = &prev->nvcsw;
		}