From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: ebiederm@xmission.com, linux-kernel@vger.kernel.org
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Subject: [PATCH v5] signal: break out of wait loops on kthread_stop()
Date: Tue, 12 Jul 2022 01:21:23 +0200 [thread overview]
Message-ID: <20220711232123.136330-1-Jason@zx2c4.com> (raw)
In-Reply-To: <87h73n9ufn.fsf@email.froward.int.ebiederm.org>
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 v4->v5:
- Use set_tsk_thread_flag instead of test_and_set_tsk_thread_flag. Must
have been a copy and paste mistarget.
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..7243a010f433 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);
+ set_tsk_thread_flag(k, TIF_NOTIFY_SIGNAL);
wake_up_process(k);
wait_for_completion(&kthread->exited);
ret = kthread->result;
--
2.35.1
next prev parent reply other threads:[~2022-07-11 23:21 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-27 12:00 [PATCH] signal: break out of wait loops on kthread_stop() Jason A. Donenfeld
2022-06-27 13:27 ` Peter Zijlstra
2022-06-27 14:54 ` Jason A. Donenfeld
2022-06-27 14:57 ` [PATCH v2] " Jason A. Donenfeld
2022-06-27 19:16 ` Eric W. Biederman
2022-06-28 15:59 ` Jason A. Donenfeld
2022-06-28 16:14 ` [PATCH v3] " Jason A. Donenfeld
2022-07-04 12:22 ` Jason A. Donenfeld
2022-07-11 17:53 ` Jason A. Donenfeld
2022-07-11 18:57 ` Eric W. Biederman
2022-07-11 20:18 ` Jason A. Donenfeld
2022-07-11 20:21 ` [PATCH v4] " Jason A. Donenfeld
2022-07-11 22:05 ` Eric W. Biederman
2022-07-11 23:21 ` Jason A. Donenfeld [this message]
2022-07-12 0:00 ` [PATCH v5] " Eric W. Biederman
2022-07-12 0:18 ` Jason A. Donenfeld
2022-07-11 22:04 ` [PATCH v3] " Eric W. Biederman
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