[PATCH v3] signal: break out of wait loops on kthread_stop()

["Jason A. Donenfeld" <Jason@zx2c4.com>]

I was recently surprised to learn that msleep_interruptible(),
wait_for_completion_interruptible_timeout(), and related functions
simply hung when I called kthread_stop() on kthreads using them. The
solution to fixing the case with msleep_interruptible() was more simply
to move to schedule_timeout_interruptible(). Why?

The reason is that msleep_interruptible(), and many functions just like
it, has a loop like this:

        while (timeout && !signal_pending(current))
                timeout = schedule_timeout_interruptible(timeout);

The call to kthread_stop() woke up the thread, so schedule_timeout_
interruptible() returned early, but because signal_pending() returned
true, it went back into another timeout, which was never woken up.

This wait loop pattern is common to various pieces of code, and I
suspect that the subtle misuse in a kthread that caused a deadlock in
the code I looked at last week is also found elsewhere.

So this commit causes signal_pending() to return true when
kthread_stop() is called, by setting TIF_NOTIFY_SIGNAL.

The same also applies to the similar kthread_park() functionality.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Toke Høiland-Jørgensen <toke@redhat.com>
Cc: Kalle Valo <kvalo@kernel.org>
Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 kernel/kthread.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 3c677918d8f2..63d5a1f4cb93 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -661,12 +661,14 @@ int kthread_park(struct task_struct *k)
 
 	set_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
 	if (k != current) {
+		test_and_set_tsk_thread_flag(k, TIF_NOTIFY_SIGNAL);
 		wake_up_process(k);
 		/*
 		 * Wait for __kthread_parkme() to complete(), this means we
 		 * _will_ have TASK_PARKED and are about to call schedule().
 		 */
 		wait_for_completion(&kthread->parked);
+		clear_tsk_thread_flag(k, TIF_NOTIFY_SIGNAL);
 		/*
 		 * Now wait for that schedule() to complete and the task to
 		 * get scheduled out.
@@ -704,8 +706,10 @@ int kthread_stop(struct task_struct *k)
 	kthread = to_kthread(k);
 	set_bit(KTHREAD_SHOULD_STOP, &kthread->flags);
 	kthread_unpark(k);
+	test_and_set_tsk_thread_flag(k, TIF_NOTIFY_SIGNAL);
 	wake_up_process(k);
 	wait_for_completion(&kthread->exited);
+	clear_tsk_thread_flag(k, TIF_NOTIFY_SIGNAL);
 	ret = kthread->result;
 	put_task_struct(k);
 
-- 
2.35.1