linux-kernel - Re: [PATCH] posix-timers: Target group sigqueue to current task only if not exiting

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Message-ID: <20241125111254.GB20402@redhat.com>
Date: Mon, 25 Nov 2024 12:12:55 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Frederic Weisbecker <frederic@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Anthony Mallet <anthony.mallet@...s.fr>
Subject: Re: [PATCH] posix-timers: Target group sigqueue to current task only
 if not exiting

On 11/25, Dmitry Vyukov wrote:
>
> On Sat, 23 Nov 2024 at 11:13, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > On 11/23, Frederic Weisbecker wrote:
> > >
> > > - * the same thread group as the target process, which avoids
> > > - * unnecessarily waking up a potentially idle task.
> > > + * the same thread group as the target process and its sighand is
> > > + * stable, which avoids unnecessarily waking up a potentially idle task.
> > >   */
> > >  static inline struct task_struct *posixtimer_get_target(struct k_itimer *tmr)
> > >  {
> > >       struct task_struct *t = pid_task(tmr->it_pid, tmr->it_pid_type);
> > >
> > > -     if (t && tmr->it_pid_type != PIDTYPE_PID && same_thread_group(t, current))
> > > +     if (t && tmr->it_pid_type != PIDTYPE_PID &&
> > > +         same_thread_group(t, current) && !current->exit_state)
> > >               t = current;
> >
> > Thanks!
> >
> > Acked-by: Oleg Nesterov <oleg@...hat.com>
>
> Can't the group leader be exiting as well?

It can. It is even possible that the group leader is already a zombie.

But this is fine. release_task(zombie-or-exiting-leader) (which does __exit_signal()
and clears ->sighand) won't be called until all the sub-threads have exited.

And. If all the sub-threads (and the group leader) have exited, then send_sigqueue()
makes no sense, the whole process is dead. so we do not care if lock_task_sighand()
fails in this case.

> Though, that's still an
> improvements. People usually don't do that (exiting from main w/o
> killing the process).

See above. Nothing to improve, AFAICS.

> So thanks for the fix.

Yes, thank you Anthony and Frederic!

Oleg.