From: Tycho Andersen <tycho@tycho.pizza>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@redhat.com>,
"Serge E. Hallyn" <serge@hallyn.com>,
Miklos Szeredi <miklos@szeredi.hu>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched: __fatal_signal_pending() should also check PF_EXITING
Date: Fri, 29 Jul 2022 07:50:34 -0600 [thread overview]
Message-ID: <YuPlqp0jSvVu4WBK@tycho.pizza> (raw)
In-Reply-To: <87pmhofr1q.fsf@email.froward.int.ebiederm.org>
On Fri, Jul 29, 2022 at 12:04:17AM -0500, Eric W. Biederman wrote:
> Tycho Andersen <tycho@tycho.pizza> writes:
>
> > On Thu, Jul 28, 2022 at 11:12:20AM +0200, Oleg Nesterov wrote:
> >> This is clear, but it seems you do not understand me. Let me try again
> >> to explain and please correct me if I am wrong.
> >>
> >> To simplify, lets suppose we have a single-thread task T which simply
> >> does
> >> __set_current_state(TASK_KILLABLE);
> >> schedule();
> >>
> >> in the do_exit() paths after exit_signals() which sets PF_EXITING. Btw,
> >> note that it even documents that this thread is not "visible" for the
> >> group-wide signals, see below.
> >>
> >> Now, suppose that this task is running and you send SIGKILL. T will
> >> dequeue SIGKILL from T->penging and call do_exit(). However, it won't
> >> remove SIGKILL from T->signal.shared_pending(), and this means that
> >> signal_pending(T) is still true.
> >>
> >> Now. If we add a PF_EXITING or sigismember(shared_pending, SIGKILL) check
> >> into __fatal_signal_pending(), then yes, T won't block in schedule(),
> >> schedule()->signal_pending_state() will return true.
> >>
> >> But what if T exits on its own? It will block in schedule() forever.
> >> schedule()->signal_pending_state() will not even check __fatal_signal_pending(),
> >> signal_pending() == F.
> >>
> >> Now if you send SIGKILL to this task, SIGKILL won't wake it up or even
> >> set TIF_SIGPENDING, complete_signal() will do nothing.
> >>
> >> See?
> >>
> >> I agree, we should probably cleanup this logic and define how exactly
> >> the exiting task should react to signals (not only fatal signals). But
> >> your patch certainly doesn't look good to me and it is not enough.
> >> May be we can change get_signal() to not remove SIGKILL from t->pending
> >> for the start... not sure, this needs another discussion.
> >
> > Thank you for this! Between that and Eric's line about:
> >
> >> Frankly that there are some left over SIGKILL bits in the pending mask
> >> is a misfeature, and it is definitely not something you should count on.
> >
> > I think I finally maybe understand the objections.
> >
> > Is it fair to say that a task with PF_EXITING should never wait? I'm
> > wondering if a solution would be to patch the wait code to look for
> > PF_EXITING, in addition to checking the signal state.
>
> That will at a minimum change zap_pid_ns_processes to busy wait
> instead of sleeping while it waits for children to die.
>
> So we would need to survey the waits that can happen when closing file
> descriptors and any other place on the exit path to see how much impact
> a such a change would do.
Oh, yes, of course.
> It might be possible to allow an extra SIGKILL to terminate such waits.
> We do something like that for coredumps. But that is incredibly subtle
> and a pain to maintain so I want to avoid that if we can.
Yeah, it feels better to clean up these waits. If we thought we got
them all we could maybe even stick a WARN() in the wait code.
> >> Finally. if fuse_flush() wants __fatal_signal_pending() == T when the
> >> caller exits, perhaps it can do it itself? Something like
> >>
> >> if (current->flags & PF_EXITING) {
> >> spin_lock_irq(siglock);
> >> set_thread_flag(TIF_SIGPENDING);
> >> sigaddset(¤t->pending.signal, SIGKILL);
> >> spin_unlock_irq(siglock);
> >> }
> >>
> >> Sure, this is ugly as hell. But perhaps this can serve as a workaround?
> >
> > or even just
> >
> > if (current->flags & PF_EXITING)
> > return;
> >
> > since we don't have anyone to send the result of the flush to anyway.
> > If we don't end up converging on a fix here, I'll just send that
> > patch. Thanks for the suggestion.
>
> If that was limited to the case you care about that would be reasonable.
>
> That will have an effect on any time a process that opens files on a
> fuse filesystem exits and depends upon the exit path to close it's file
> descriptors to the fuse filesystem.
>
>
> I do see a plausible solution along those lines.
>
> In fuse_flush instead of using fuse_simple_request call an equivalent
> function that when PF_EXITING is true skips calling request_wait_answer.
> Or perhaps when PF_EXITING is set uses schedule_work to call the
> request_wait_answer.
I don't see why this is any different than what I proposed. It changes
the semantics to flush happening out-of-order with task exit, instead
of strictly before, which you point out might be a problem. What am I
missing?
Tycho
next prev parent reply other threads:[~2022-07-29 13:51 UTC|newest]
Thread overview: 61+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-23 17:21 strange interaction between fuse + pidns Tycho Andersen
2022-06-23 21:55 ` Vivek Goyal
2022-06-23 23:41 ` Tycho Andersen
2022-06-24 17:36 ` Vivek Goyal
2022-07-11 10:35 ` Miklos Szeredi
2022-07-11 13:59 ` Miklos Szeredi
2022-07-11 20:25 ` Tycho Andersen
2022-07-11 21:37 ` Eric W. Biederman
2022-07-11 22:53 ` Tycho Andersen
2022-07-11 23:06 ` Eric W. Biederman
2022-07-12 13:43 ` Tycho Andersen
2022-07-12 14:34 ` Eric W. Biederman
2022-07-12 15:14 ` Tycho Andersen
2022-07-13 17:53 ` [PATCH] sched: __fatal_signal_pending() should also check PF_EXITING Tycho Andersen
2022-07-20 15:03 ` Serge E. Hallyn
2022-07-20 20:58 ` Tycho Andersen
2022-07-21 1:54 ` Serge E. Hallyn
2022-07-27 15:44 ` Tycho Andersen
2022-07-27 16:32 ` Eric W. Biederman
2022-07-27 17:55 ` Tycho Andersen
2022-07-28 18:48 ` Eric W. Biederman
2022-07-27 17:55 ` Oleg Nesterov
2022-07-27 18:18 ` Tycho Andersen
2022-07-27 19:19 ` Oleg Nesterov
2022-07-27 19:40 ` Tycho Andersen
2022-07-28 9:12 ` Oleg Nesterov
2022-07-28 21:20 ` Tycho Andersen
2022-07-29 5:04 ` Eric W. Biederman
2022-07-29 13:50 ` Tycho Andersen [this message]
2022-07-29 16:15 ` Eric W. Biederman
2022-07-29 16:48 ` Tycho Andersen
2022-07-29 17:40 ` [RFC][PATCH] fuse: In fuse_flush only wait if someone wants the return code Eric W. Biederman
2022-07-29 20:47 ` Oleg Nesterov
2022-07-30 0:15 ` Al Viro
2022-07-30 5:10 ` [RFC][PATCH v2] " Eric W. Biederman
2022-08-01 15:16 ` Tycho Andersen
2022-08-02 12:50 ` Miklos Szeredi
2022-08-15 13:59 ` Tycho Andersen
2022-08-15 17:55 ` Serge E. Hallyn
2022-09-01 14:06 ` [PATCH] " Tycho Andersen
2022-09-19 15:03 ` Tycho Andersen
2022-09-20 18:02 ` Serge E. Hallyn
2022-09-26 14:17 ` Tycho Andersen
2022-09-27 9:46 ` Miklos Szeredi
2022-09-29 14:05 ` [fuse-devel] " Stef Bon
2022-09-29 16:39 ` [PATCH v2] " Tycho Andersen
2022-09-30 13:35 ` Miklos Szeredi
2022-09-30 14:01 ` Tycho Andersen
2022-09-30 14:41 ` Miklos Szeredi
2022-09-30 16:09 ` Tycho Andersen
2022-10-26 9:01 ` Miklos Szeredi
2022-11-14 16:02 ` [PATCH v3] " Tycho Andersen
2022-11-28 15:00 ` Tycho Andersen
2022-12-08 14:26 ` Miklos Szeredi
2022-12-08 17:49 ` Tycho Andersen
2022-12-19 19:16 ` Tycho Andersen
2023-01-03 14:51 ` Tycho Andersen
2023-01-05 15:15 ` Serge E. Hallyn
2023-01-26 14:12 ` Miklos Szeredi
2022-09-30 19:47 ` [PATCH] " Serge E. Hallyn
2022-09-19 15:46 ` [RFC][PATCH v2] " Eric W. Biederman
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