Re: [PATCH cgroup/for-4.4-fixes] cgroup: fix cftype->file_offset handling

[Johannes Weiner]

On Thu, Nov 05, 2015 at 12:12:24AM -0500, Tejun Heo wrote:
> 6f60eade2433 ("cgroup: generalize obtaining the handles of and
> notifying cgroup files") introduced cftype->file_offset so that the
> handles for per-css file instances can be recorded.  These handles
> then can be used, for example, to generate file modified
> notifications.
> 
> Unfortunately, it made the wrong assumption that files are created
> once for a given css and removed on its destruction.  Due to the
> dependencies among subsystems, a css may be hidden from userland and
> then later shown again.  This is implemented by removing and
> re-creating the affected files, so the associated kernfs_node for a
> given cgroup file may change over time.  This incorrect assumption led
> to the corruption of css->files lists.
>
> Reimplement cftype->file_offset handling so that cgroup_file->kn is
> protected by a lock and updated as files are created and destroyed.
> This also makes keeping them on per-cgroup list unnecessary.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Reported-by: James Sedgwick <jsedgwick@xxxxxx>
> Fixes: 6f60eade2433 ("cgroup: generalize obtaining the handles of and notifying cgroup files")

Kudos to whoever debugged this.

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
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