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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     linux-kernel@vger.kernel.org
References: <YsyFhfAunVfVQKci@zx2c4.com>
        <20220711202136.64458-1-Jason@zx2c4.com>
Date:   Mon, 11 Jul 2022 17:05:32 -0500
In-Reply-To: <20220711202136.64458-1-Jason@zx2c4.com> (Jason A. Donenfeld's
        message of "Mon, 11 Jul 2022 22:21:36 +0200")
Message-ID: <87h73n9ufn.fsf@email.froward.int.ebiederm.org>
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Subject: Re: [PATCH v4] signal: break out of wait loops on kthread_stop()
Precedence: bulk

"Jason A. Donenfeld" <Jason@zx2c4.com> writes:

> 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;

Minor it.  Unless I have missed something that should just be
set_tsk_thread_flag.  You aren't using the return value so I don't
think there is any point in testing the previous state of
TIF_NOTIFY_SIGNAL.

Eric