linux-kernel - Re: Possible race between cgroup_attach_proc and de_thread, and questionable code in de

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Date:	Mon, 15 Aug 2011 21:09:35 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	NeilBrown <neilb@...e.de>
Cc:	Paul Menage <menage@...gle.com>, Ben Blum <bblum@...rew.cmu.edu>,
	Li Zefan <lizf@...fujitsu.com>,
	containers@...ts.linux-foundation.org,
	"Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Possible race between cgroup_attach_proc and de_thread, and
	questionable code in de_thread.

On 08/15, NeilBrown wrote:
>
> de_thread can change the group_leader of a thread_group, and release_task can
> remove a non-leader while leaving the rest of the thread_group intact.  So
> any while_each_thread() loop needs some extra care to ensure that it doesn't
> loop infinitely, because the "head" that it is looking for might not be there
> any more.
> Maybe there are other rules that ensure this can never happen, but they sure
> aren't obvious to me (i.e. if you know them - please tell ;-)

No, I don't know ;)

And note also that if g != leader, then while_each_thread(g, t) can hang
simply because g exits. I am still trying to invent something simple to
fix while_each_thread-under-rcu.

This looks possible, but I am starting to think that, say, zap_threads()
needs locking anyway. With any fix I can imagine, it can miss a thread
we should care about.

Oleg.

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