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From: Oleg Nesterov <oleg@redhat.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "Jürg Billeter" <j@bitron.ch>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Kees Cook" <keescook@chromium.org>,
	"Andy Lutomirski" <luto@kernel.org>,
	linux-api@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] prctl: add PR_{GET,SET}_KILL_DESCENDANTS_ON_EXIT
Date: Thu, 29 Nov 2018 13:34:09 +0100	[thread overview]
Message-ID: <20181129123409.GA10645@redhat.com> (raw)
In-Reply-To: <87r2f5gr9g.fsf@xmission.com>

On 11/28, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@redhat.com> writes:
>
> > On 11/27, Jürg Billeter wrote:
> >>
> >> @@ -704,6 +713,9 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
> >>  	struct task_struct *p, *n;
> >>  	LIST_HEAD(dead);
> >>
> >> +	if (group_dead && tsk->signal->kill_descendants_on_exit)
> >> +		walk_process_tree(tsk, kill_descendant_visitor, NULL);
> >
> > Well, this is not exactly right, at least this is suboptimal in that
> > other sub-threads can too call walk_process_tree(kill_descendant_visitor)
> > later for no reason.
>
> Oleg I think I am missing something.

No, it is stupid me who can't read,

> Reading kernel/exit.c I see "group_dead = atomic_dec_and_test(&tsk->signal->live)".
> Which seems like enough to ensure exactly one task/thread calls walk_process_tree.

Of course you right, sorry for confusion.

To me it would be more clean to call walk_process_tree(kill_descendant_visitor)
unconditionally in find_new_reaper() right before "if (has_child_subreaper)", but
then we will need to shift read_lock(tasklist) from walk_process_tree().

So I think the patch is mostly fine, the only problem I can see is that
PR_SET_KILL_DESCENDANTS_ON_EXIT can race with PR_SET_CHILD_SUBREAPER, they both
need to update the bits in the same word.

Oleg.


  reply	other threads:[~2018-11-29 12:34 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-27 22:54 [PATCH 0/1] Add prctl to kill descendants on exit Jürg Billeter
2018-11-27 22:54 ` [PATCH] prctl: add PR_{GET,SET}_KILL_DESCENDANTS_ON_EXIT Jürg Billeter
2018-11-28 14:42   ` Oleg Nesterov
2018-11-28 15:23     ` Eric W. Biederman
2018-11-29 12:34       ` Oleg Nesterov [this message]
2018-11-29 15:41         ` Jürg Billeter
2018-11-30 10:33           ` Oleg Nesterov
2018-12-01  4:28             ` Kees Cook
2018-11-30  8:00   ` [PATCH v2 0/1] Add prctl to kill descendants on exit Jürg Billeter
2018-11-30  8:00     ` [PATCH v2 1/1] prctl: add PR_{GET,SET}_KILL_DESCENDANTS_ON_EXIT Jürg Billeter
2018-11-30 11:22       ` Oleg Nesterov
2018-11-30 13:40       ` Florian Weimer
2018-12-01 10:39         ` Jürg Billeter
2018-12-01 12:28           ` Florian Weimer
2018-12-01 13:57             ` Jürg Billeter
2018-12-06 15:54     ` [PATCH v2 0/1] Add prctl to kill descendants on exit Jürg Billeter
2019-01-18 13:11 ` [RESEND PATCH " Jürg Billeter
2019-01-18 13:11   ` [RESEND PATCH v2 1/1] prctl: add PR_{GET,SET}_KILL_DESCENDANTS_ON_EXIT Jürg Billeter
2019-01-29  1:23     ` Andrew Morton

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