linux-kernel - Re: [PATCH -mm v2 1/3] mm/oom_kill: remove the wrong fatal_signal_pending() check in oom_kill

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Message-ID: <20151001150010.GB6781@redhat.com>
Date:	Thu, 1 Oct 2015 17:00:10 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Kyle Walker <kwalker@...hat.com>,
	Stanislav Kozina <skozina@...hat.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm v2 1/3] mm/oom_kill: remove the wrong
	fatal_signal_pending() check in oom_kill_process()

On 10/01, Michal Hocko wrote:
>
> On Wed 30-09-15 20:24:05, Oleg Nesterov wrote:
> [...]
> > It is possible that the group leader
> > has the pending SIGKILL because its sub-thread originated the coredump,
> > in this case we must not skip this process.
>
> I do not understand this. If the group leader has SIGKILL pending it
> will die anyway regardless whether we send another sigkill or not, no?

Yes it will die, but only after the coredump is finished.

Suppose we have a thread group with the group leader P and another
thread T. If T starts the coredump, it sends SIGKILL to P and waits
until it parks in exit_mm(). Then T actually dumps the core which may
need more memory, a lot of time, etc.

We need to kill this process. Yes, P is already killed and it sleeps
in TASK_UNINTERRUPTIBLE so this thread does not need SIGKILL. But
do_send_sig_info(P) will also find T and kill it too to make
dump_interrupted() == T.

Oleg.

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