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

From: Tejun Heo
Date: Mon Nov 16 2015 - 10:59:20 EST

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.

Applied to cgroup/for-4.4-fixes.

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