Messages in this thread Patch in this message | | | From | "Jason A. Donenfeld" <> | Subject | [PATCH v3] signal: break out of wait loops on kthread_stop() | Date | Tue, 28 Jun 2022 18:14:41 +0200 |
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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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