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Date: Mon, 30 Jan 2017 15:01:15 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Cc: Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Lennart Poettering <lennart@poettering.net>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        Kay Sievers <kay.sievers@vrfy.org>
Subject: Re: [PATCH v2 2/2] prctl: propagate has_child_subreaper flag to
 every descendant
Message-ID: <20170130140114.GB24718@redhat.com>
References: <20170127100543.18390-1-ptikhomirov@virtuozzo.com>
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On 01/30, Pavel Tikhomirov wrote:
>
> On 01/30/2017 03:51 PM, Oleg Nesterov wrote:
> >>+			/*
> >>+			 * Inherit has_child_subreaper flag under the same
> >>+			 * tasklist_lock with adding child to the process tree
> >>+			 * for propagate_has_child_subreaper optimization.
> >>+			 */
> >>+			p->signal->has_child_subreaper = current->signal->has_child_subreaper ||
> >>+							 current->signal->is_child_subreaper;
> >
> >Ah yes, we need this change too...
> >
> >And perhaps it would be more correct to do
> >
> >			p->signal->has_child_subreaper =
> >				p->real_parent->signal->has_child_subreaper ||
> >				p->real_parent->signal->is_child_subreaper;
> >
> >the current code is not exactly right if CLONE_PARENT...
>
> I'm fine with inheriting 'has' flag from real_parent, because if real_parent
> does not have 'has' flag set but parent has 'has' set, we inherited the flag
> in vain.
>
> But I don't actually think that inheritance from parent not real_parent
> breaks my optimization: if real_parent has the flag, so does the parent.

Not sure I understand... I meant, the usage of p->real_parent instead of "current"
_looks_ more correct and clear, imo.

Say, a is_child_subreaper task does clone(CLONE_PARENT), in this case (with or
without your change) we set p->signal->has_child_subreaper = 1 and this is not
really correct even if we do not really care.

Oleg.