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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Mon, 11 Jul 2022 19:53:58 +0200
Message-ID: <CAHmME9qt9hUTgJQSdv3_Yzkc3e7H8OkhPSv_pUaXV_2AOA1S=g@mail.gmail.com>
Subject: Re: [PATCH v3] signal: break out of wait loops on kthread_stop()
To:     "Eric W. Biederman" <ebiederm@xmission.com>,
        LKML <linux-kernel@vger.kernel.org>
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Hi Eric,

On Mon, Jul 4, 2022 at 2:22 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> Hey Eric,
>
> On Tue, Jun 28, 2022 at 06:14:41PM +0200, Jason A. Donenfeld wrote:
> > 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 =3D 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 H=C3=B8iland-J=C3=B8rgensen <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 !=3D 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 =3D 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 =3D kthread->result;
> >       put_task_struct(k);
> >
> > --
> > 2.35.1
> >
>
> Is this more to the tune of what you had in mind in your message [1]?
>
> Jason
>
> [1] https://lore.kernel.org/lkml/877d51udc7.fsf@email.froward.int.ebieder=
m.org/

Paging again...

Jason