linux-kernel - Re: [PATCH 1/2] cgroup: Set CGRP_RELEASABLE when adding to a cgroup

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Date:	Thu, 16 Dec 2010 16:54:09 -0800
From:	Paul Menage <menage@...gle.com>
To:	Colin Cross <ccross@...roid.com>
Cc:	Li Zefan <lizf@...fujitsu.com>,
	containers@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] cgroup: Set CGRP_RELEASABLE when adding to a cgroup

On Thu, Dec 2, 2010 at 7:07 PM, Colin Cross <ccross@...roid.com> wrote:
>> Not in one case - if we create a new cgroup and try to move a thread
>> into it, but the thread is exiting as we move it, we'll call
>> put_css_set() on the new css_set, which will drop the refcount on the
>> target cgroup back to 0. We wouldn't want the auto-release
>> notification to kick in in that situation, I think.
>
> Clearing the CGRP_RELEASABLE bit any time after the tests in
> check_for_release introduces a race if __css_get is called between the
> check and clearing the bit - the cgroup will have an entry, but the
> bit will not be set.  Without additional locking in __css_get, I don't
> see any way to safely clear CGRP_RELEASABLE.

I don't quite follow your argument here. Are you saying that the
problem is that you could end up spawning a release agent for a cgroup
that was no longer releasable since it now had a process in it again?
If so, then I don't think that's a problem - spurious release agent
invocations for non-empty cgroups will always happen occasionally due
to races between the kernel and userspace. But a failed move of a task
into a previously-empty cgroup shouldn't trigger the agent.

Paul
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