[Devel] Re: [RFC] [PATCH 2/2] cgroups: make procs file writable

[Oleg Nesterov]

On 06/03, Ben Blum wrote:
>
> On Wed, Jun 02, 2010 at 10:58:55PM +0200, Oleg Nesterov wrote:
> > Hmm. The usage of ->thread_group in ->can_attach() methods doesn't
> > look safe to me... but currently bool threadgroup is always false.
>
> I recall putting a rcu_read_lock() around that part and being assured
> that made it safe. But I don't remember the details. Maybe taking
> tasklist_lock is necessary?

rcu_read_lock() is not enough, see another email I sent.

Once again.

	rcu_read_lock()
	list_for_each_rcu(tsk->thread_group)

assumes that at least tsk->thread_group->next can't point to nowhere,
this is not true. This memory can go away _before_ we take rcu lock.

Oleg.

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