linux-kernel - Re: [PATCH] Fix race between oom kill and task exit

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Date:	Fri, 6 Dec 2013 16:19:44 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	William Dauchy <wdauchy@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Ma, Xindong" <xindong.ma@...el.com>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	gregkh@...uxfoundation.org, "Tu, Xiaobing" <xiaobing.tu@...el.com>,
	azurIt <azurit@...ox.sk>, Sameer Nanda <snanda@...omium.org>
Subject: Re: [PATCH] Fix race between oom kill and task exit

On 12/05, David Rientjes wrote:
>
> On Thu, 5 Dec 2013, Oleg Nesterov wrote:
>
> > > Your v2 series looks good and I suspect anybody trying them doesn't have
> > > additional reports of the infinite loop?  Should they be marked for
> > > stable?
> >
> > Unlikely...
> >
> > I think the patch from Sameer makes more sense for stable as a temporary
> > (and obviously incomplete) fix.
>
> There's a problem because none of this is currently even in linux-next.  I
> think we could make a case for getting Sameer's patch at
> http://marc.info/?l=linux-kernel&m=138436313021133 to be merged for
> stable,

Probably.

Ah, I just noticed that this change

	-	if (p->flags & PF_EXITING) {
	+	if (p->flags & PF_EXITING || !pid_alive(p)) {

is not needed. !pid_alive(p) obviously implies PF_EXITING.

> but then we'd have to revert it in linux-next

Or perhaps Sameer can just send his fix to stable/gregkh.

Just the changelog should clearly explain that this is the minimal
workaround for stable. Once again it doesn't (and can't) fix all
problems even in oom_kill_process() paths, but it helps anyway to
avoid the easy-to-trigger hang.

> before merging your
> series at http://marc.info/?l=linux-kernel&m=138616217925981.

Just in case, I won't mind to rediff my patches on top of Sameer's
patch and then add git-revert patch.

> All of the
> issues you present in that series seem to be stable material, so why not
> just go ahead with your series and mark it for stable for 3.13?

OK... I can do this too.

I do not really like this because it adds thread_head/node but doesn't
remove the old ->thread_group. We will do this later, but obviously
this is not the stable material.

IOW, if we send this to stable, thread_head/node/for_each_thread will
be only used by oom_kill.c.

And this is risky. For example, 1/4 depends on (at least) another patch
I sent in preparation for this change, commit 81907739851
"kernel/fork.c:copy_process(): don't add the uninitialized
child to thread/task/pid lists", perhaps on something else.

So personally I'd prefer to simply send the workaround for stable.

Oleg.

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