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

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Date:	Wed, 24 Nov 2010 16:21:58 -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 Wed, Nov 24, 2010 at 4:11 PM, Colin Cross <ccross@...roid.com> wrote:
>>
>> We seem to have lost some notify_on_release() checks - maybe move that
>> to check_for_release()?
> check_for_release immediately calls cgroup_is_releasable, which checks
> for the same bit as notify_on_release.  There's no need for
> CGRP_RELEASABLE to depend on notify_on_release, or to check
> notify_on_release before calling check_for_release.

OK.

> I matched the existing behavior, __css_put sets CGRP_RELEASABLE when
> refcnt goes to 0.
>

Ah, we do appear to have had that behaviour for a while. I don't
remember the justification for it at this point :-)

> check_for_release is only called from __css_put, cgroup_rmdir, and
> __put_css_set (or free_css_set_work after my second patch).  Those all
> imply that __css_get, get_css_set, or cgroup_create have been
> previously called, which are the functions that set CGRP_RELEASABLE.

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.

Paul
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