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Date:   Thu, 18 May 2023 19:04:00 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Mike Christie <michael.christie@oracle.com>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>, linux@leemhuis.info,
        nicolas.dichtel@6wind.com, axboe@kernel.dk,
        torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, mst@redhat.com,
        sgarzare@redhat.com, jasowang@redhat.com, stefanha@redhat.com,
        brauner@kernel.org
Subject: Re: [RFC PATCH 1/8] signal: Dequeue SIGKILL even if
 SIGNAL_GROUP_EXIT/group_exec_task is set
Message-ID: <20230518170359.GC20779@redhat.com>
References: <20230518000920.191583-1-michael.christie@oracle.com>
 <20230518000920.191583-2-michael.christie@oracle.com>
 <87ednei9is.fsf@email.froward.int.ebiederm.org>
 <ab7d07ba-5dc3-95c0-aa7c-c2575d03f429@oracle.com>
 <20230518162508.GB20779@redhat.com>
 <05236dee-59b7-f394-db3d-cbb4d4163ce8@oracle.com>
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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.