linux-kernel - Re: [PATCH 0/2] CGroups: cgroup member list enhancement/fix

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Date:	Tue, 14 Jul 2009 13:38:30 -0700
From:	Paul Menage <menage@...gle.com>
To:	Benjamin Blum <bblum@...gle.com>,
	Matt Helsley <matthltc@...ibm.com>
Cc:	balbir@...ux.vnet.ibm.com, lizf@...fujitzu.com,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	libcg-devel <libcg-devel@...ts.sourceforge.net>,
	akpm@...ux-foundation.org
Subject: Re: [PATCH 0/2] CGroups: cgroup member list enhancement/fix

On Tue, Jul 14, 2009 at 10:43 AM, Paul Menage<menage@...gle.com> wrote:
>
> I've been trying to think of a way to do that. AFAICS the only way to
> do that reliably would be to move the call to cgroup_fork() that hooks
> into the parent's cgroup inside the lock on the group leader's thread
> list, and move the fork callbacks into cgroup_fork(). (Which would
> mean that they'd not be able to sleep/fail, etc).

Currently the only user of the cgroup fork callbacks is the freezer cgroup.

Matt, if this callback was moved inside tasklist_lock, would that
present any problems? Given that in other places you call
freeze_task() from inside other low-level locks like css_set_lock
(within a cgroup iteration) then hopefully it would be OK.

The only question then would be whether anything between the point
where cgroup_fork() is currently called, and the point where the new
thread is added to its thread group list, cares about p->cgroups being
valid. We can probably flush out any such assumptions by clearing
tsk->cgroups in dup_task_struct, so that any attempts to reference it
would immediately oops.

Then (assuming this doesn't expose any such dependencies) we can move
the cgroup_fork() call inside the write_lock_irq(&tasklist_lock) in
do_fork(). The "procs" handler can (once it has done any necessary
allocations) take a read_lock on tasklist_lock, and atomically move
all threads to the new cgroup.

Paul
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