[Devel] Re: [PATCH] cgroup: Fix root_count when mount fails due to busy subsystem

[Paul Menage]

On Fri, Jan 23, 2009 at 10:10 AM, Ingo Molnar <mingo at elte.hu> wrote:
>
> * Paul Menage <menage at google.com> wrote:
>
>> cgroup: Fix root_count when mount fails due to busy subsystem
>>
>> root_count was being incremented in cgroup_get_sb() after all error
>> checking was complete, but decremented in cgroup_kill_sb(), which can be
>> called on a superblock that we gave up on due to an error.  This patch
>> changes cgroup_kill_sb() to only decrement root_count if the root was
>> previously linked into the list of roots.
>
> i'm wondering, what happens in the buggy case: does cgroup_kill_sb() get
> called twice (if yes, why?),

No.

> or do we call cgroup_kill_sb() on a not yet
> added sb and hence root_count has not been elevated yet?

Right.

> (if yes, which
> codepath does this?)

It's via the call to deactivate_super().

The code could be restructured such that:

- we don't set sb->s_fs_info until we've linked the new root into the root_list
- do any necessary cleanup for a failed root in cgroup_get_sb()
- have cgroup_kill_sb() handle either no root or a fully-initialized root

But then you're replacing "only decrement root_count if root was
linked in to list" with "only do root cleanup if root was atached to
sb" in cgroup_kill_sb(). I don't see that one is much cleaner than the
other.

For 2.6.29, we should fix this by reverting the broken part of the
patch that made it into 2.6.29-rcX

Paul
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