Messages in this thread Patch in this message | | | From | "Jason A. Donenfeld" <> | Subject | [PATCH v4] signal: break out of wait loops on kthread_stop() | Date | Mon, 11 Jul 2022 22:21:36 +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 probably applies to the similar kthread_park() functionality, but that can be addressed later, as its semantics are slightly different.
Cc: Eric W. Biederman <ebiederm@xmission.com> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> --- Changes v3->v4: - Don't address park() for now. - Don't bother clearing the flag, since the task is about to be freed anyway.
kernel/kthread.c | 1 + 1 file changed, 1 insertion(+)
diff --git a/kernel/kthread.c b/kernel/kthread.c index 3c677918d8f2..8888987f2b25 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -704,6 +704,7 @@ 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); ret = kthread->result; -- 2.35.1
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