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

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Message-ID: <AANLkTimwvP2Ey1gJ6AbbFNtDKjGZt4cwqL=08nGBa_PT@mail.gmail.com>
Date:	Wed, 24 Nov 2010 16:11:34 -0800
From:	Colin Cross <ccross@...roid.com>
To:	Paul Menage <menage@...gle.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 3:54 PM, Paul Menage <menage@...gle.com> wrote:
> On Tue, Nov 23, 2010 at 9:37 PM, Colin Cross <ccross@...roid.com> wrote:
>> @@ -364,12 +372,8 @@ static void __put_css_set(struct css_set *cg, int taskexit)
>>                struct cgroup *cgrp = link->cgrp;
>>                list_del(&link->cg_link_list);
>>                list_del(&link->cgrp_link_list);
>> -               if (atomic_dec_and_test(&cgrp->count) &&
>> -                   notify_on_release(cgrp)) {
>> -                       if (taskexit)
>> -                               set_bit(CGRP_RELEASABLE, &cgrp->flags);
>> +               if (atomic_dec_and_test(&cgrp->count))
>>                        check_for_release(cgrp);
>> -               }
>
> 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.

>>  /* Caller must verify that the css is not for root cgroup */
>> +void __css_get(struct cgroup_subsys_state *css, int count)
>> +{
>> +       atomic_add(count, &css->refcnt);
>> +       set_bit(CGRP_RELEASABLE, &css->cgroup->flags);
>> +}
>
> Is css_get() the right place to be putting this? It's not clear to me
> why a subsystem taking a refcount on a cgroup's state should render it
> releasable when it drops that refcount.
I matched the existing behavior, __css_put sets CGRP_RELEASABLE when
refcnt goes to 0.

> Should we maybe clear the CGRP_RELEASABLE flag right before doing the
> userspace callback?
Actually, I think CGRP_RELEASABLE can be dropped entirely.
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.
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