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>

include/linux/sched.h

@@ -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

@@ -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;
 	}