linux-kernel - Re: [PATCH -mm] proc: don't take ->siglock for /proc/pid/oom

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Message-ID: <20100401153756.GD14603@redhat.com>
Date:	Thu, 1 Apr 2010 17:37:56 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	anfei <anfei.zhou@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	nishimura@....nes.nec.co.jp,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm] proc: don't take ->siglock for /proc/pid/oom_adj

On 04/01, David Rientjes wrote:
>
> On Thu, 1 Apr 2010, Oleg Nesterov wrote:
>
> > > That doesn't work for depraceted_mode (sic), you'd need to test for
> > > OOM_ADJUST_MIN and OOM_ADJUST_MAX in that case.
> >
> > Yes, probably "if (depraceted_mode)" should do more checks, I didn't try
> > to verify that MIN/MAX are correctly converted. I showed this code to explain
> > what I mean.
> >
>
> Ok, please cc me on the patch, it will be good to get rid of the duplicate 
> code and remove oom_adj from struct signal_struct.

OK, great, will do tomorrow.

> Do we need ->siglock?  Why can't we just do
>
> 	struct sighand_struct *sighand;
> 	struct signal_struct *sig;
>
> 	rcu_read_lock();
> 	sighand = rcu_dereference(task->sighand);
> 	if (!sighand) {
> 		rcu_read_unlock();
> 		return;
> 	}
> 	sig = task->signal;
>
> 	... load/store to sig ...
>
> 	rcu_read_unlock();

No.

Before signals-make-task_struct-signal-immutable-refcountable.patch (actually,
series of patches), this can't work. ->signal is not protected by rcu, and
->sighand != NULL doesn't mean ->signal != NULL.

(yes, thread_group_cputime() is wrong too, but currently it is never called
 lockless).

After signals-make-task_struct-signal-immutable-refcountable.patch, we do not
need any checks at all, it is always safe to use ->signal.


But. Unless we kill signal->oom_adj, we have another reason for ->siglock,
we can't update both oom_adj and oom_score_adj atomically, and if we race
with another thread they can be inconsistent wrt each other. Yes, oom_adj
is not actually used, except we report it back to user-space, but still.

So, I am going to send 2 patches. The first one factors out the code
in base.c and kills signal->oom_adj, the next one removes ->siglock.

Oleg.

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