From: Tycho Andersen <tycho@tycho.pizza>
To: Oleg Nesterov <oleg@redhat.com>
Cc: "Serge E. Hallyn" <serge@hallyn.com>,
"Eric W . Biederman" <ebiederm@xmission.com>,
Miklos Szeredi <miklos@szeredi.hu>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched: __fatal_signal_pending() should also check PF_EXITING
Date: Thu, 28 Jul 2022 15:20:57 -0600 [thread overview]
Message-ID: <YuL9uc8WfiYlb2Hw@tycho.pizza> (raw)
In-Reply-To: <20220728091220.GA11207@redhat.com>
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.
> 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.
Tycho
next prev parent reply other threads:[~2022-07-28 21:21 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 [this message]
2022-07-29 5:04 ` Eric W. Biederman
2022-07-29 13:50 ` Tycho Andersen
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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