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    SubjectRe: [PATCH 4/8] cgroup: deactivate CSS's and mark cgroup dead before invoking ->pre_destroy()
    (2012/10/31 13:22), Tejun Heo wrote:
    > Because ->pre_destroy() could fail and can't be called under
    > cgroup_mutex, cgroup destruction did something very ugly.
    >
    > 1. Grab cgroup_mutex and verify it can be destroyed; fail otherwise.
    >
    > 2. Release cgroup_mutex and call ->pre_destroy().
    >
    > 3. Re-grab cgroup_mutex and verify it can still be destroyed; fail
    > otherwise.
    >
    > 4. Continue destroying.
    >
    > In addition to being ugly, it has been always broken in various ways.
    > For example, memcg ->pre_destroy() expects the cgroup to be inactive
    > after it's done but tasks can be attached and detached between #2 and
    > #3 and the conditions that memcg verified in ->pre_destroy() might no
    > longer hold by the time control reaches #3.
    >
    > Now that ->pre_destroy() is no longer allowed to fail. We can switch
    > to the following.
    >
    > 1. Grab cgroup_mutex and fail if it can't be destroyed; fail
    > otherwise.
    >
    > 2. Deactivate CSS's and mark the cgroup removed thus preventing any
    > further operations which can invalidate the verification from #1.
    >
    > 3. Release cgroup_mutex and call ->pre_destroy().
    >
    > 4. Re-grab cgroup_mutex and continue destroying.
    >
    > After this change, controllers can safely assume that ->pre_destroy()
    > will only be called only once for a given cgroup and, once
    > ->pre_destroy() is called, the cgroup will stay dormant till it's
    > destroyed.
    >
    > Signed-off-by: Tejun Heo <tj@kernel.org>


    Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

    Ok, cgroup_lock_live_group() will work synchronously against attach_task().
    I welcome this.




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