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: <20151001161916.GK24077@dhcp22.suse.cz>
Date:	Thu, 1 Oct 2015 18:19:16 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
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 Thu 01-10-15 17:41:15, Oleg Nesterov wrote:
> On 10/01, Michal Hocko wrote:
> >
> > On Thu 01-10-15 17:00:10, Oleg Nesterov wrote:
> > > 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.
> >
> > I am still utterly confused :( Where do we kill T if it is not in the
> > same thread group with P?
> 
> But it is in the same thread group?

The whole loop is about sending sigkill to a process from a different
thread group though. And this is what confused me completely. But I got
the point finally. zap_process will add SIGKILL to all threads but the
current which will go on without being killed and if this is not a
thread group leader then we would miss it.

Thanks for the clarification and feel free to add
Acked-by: Michal Hocko <mhocko@...e.com>

-- 
Michal Hocko
SUSE Labs
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