linux-kernel - Re: [RFC PATCH 1/8] signal: Dequeue SIGKILL even if SIGNAL_GROUP_EXIT/group_exec

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Date:   Thu, 18 May 2023 19:04:00 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Mike Christie <michael.christie@...cle.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>, linux@...mhuis.info,
        nicolas.dichtel@...nd.com, axboe@...nel.dk,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, mst@...hat.com,
        sgarzare@...hat.com, jasowang@...hat.com, stefanha@...hat.com,
        brauner@...nel.org
Subject: Re: [RFC PATCH 1/8] signal: Dequeue SIGKILL even if
 SIGNAL_GROUP_EXIT/group_exec_task is set

On 05/18, Mike Christie wrote:
>
> On 5/18/23 11:25 AM, Oleg Nesterov wrote:
> > I too do not understand the 1st change in this patch ...
> >
> > On 05/18, Mike Christie wrote:
> >>
> >> In the other patches we do:
> >>
> >> if (get_signal(ksig))
> >> 	start_exit_cleanup_by_stopping_newIO()
> >> 	flush running IO()
> >> 	exit()
> >>
> >> But to do the flush running IO() part of this I need to wait for it so
> >> that's why I wanted to be able to dequeue the SIGKILL and clear the
> >> TIF_SIGPENDING bit.
> >
> > But get_signal() will do what you need, dequeue SIGKILL and clear SIGPENDING ?
> >
> > 	if ((signal->flags & SIGNAL_GROUP_EXIT) ||
> > 	     signal->group_exec_task) {
> > 		clear_siginfo(&ksig->info);
> > 		ksig->info.si_signo = signr = SIGKILL;
> > 		sigdelset(&current->pending.signal, SIGKILL);
> >
> > this "dequeues" SIGKILL,

OOPS. this doesn't remove SIGKILL from current->signal->shared_pending

> >
> > 		trace_signal_deliver(SIGKILL, SEND_SIG_NOINFO,
> > 			&sighand->action[SIGKILL - 1]);
> > 		recalc_sigpending();
> >
> > this clears TIF_SIGPENDING.

No, I was wrong, recalc_sigpending() won't clear TIF_SIGPENDING if
SIGKILL is in signal->shared_pending

> I see what you guys meant. TIF_SIGPENDING isn't getting cleared.
> I'll dig into why.

See above, sorry for confusion.



And again, there is another problem with SIGSTOP. To simplify, suppose
a PF_IO_WORKER thread does something like

	while (signal_pending(current))
		get_signal(...);

this will loop forever if (SIGNAL_GROUP_EXIT || group_exec_task) and
SIGSTOP is pending.

Oleg.