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