Messages in this thread | | | From | "Jason A. Donenfeld" <> | Date | Mon, 11 Jul 2022 19:53:58 +0200 | Subject | Re: [PATCH v3] signal: break out of wait loops on kthread_stop() |
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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 = 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 > > > > 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.ebiederm.org/
Paging again...
Jason
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