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    SubjectRe: [RFC][PATCH 1/4] New css->refcnt implementation.
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    On Fri, Dec 5, 2008 at 12:28 AM, KAMEZAWA Hiroyuki
    <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    >
    > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujisu.com>
    >
    > Now, the last check of refcnt is done after pre_destroy(), so rmdir() can fail
    > after pre_destroy(). But memcg set mem->obsolete to be 1 at pre_destroy.
    > This is a bug. So, removing memcg->obsolete flag is sane.
    >
    > But there is no interface to confirm "css" is oboslete or not. I.e. there is
    > no flag to check whether we can increase css_refcnt or not!

    The basic rule is that you're only supposed to increment the css
    refcount if you have:

    - a reference to a task in the cgroup (that is pinned via task_lock()
    so it can't be moved away)
    or
    - an existing reference to the css

    >
    > This patch changes this css->refcnt rule as following
    > - css->refcnt is no longer private counter, just point to
    > css->cgroup->css_refcnt.

    The reason I didn't do this is that I'd like to keep the ref counts
    separate to make it possible to add/remove subsystems from a hiearchy
    - if they're all mingled into a single refcount, it's impossible to
    tell if a particular subsystem has refcounts.

    >
    > - css_put() is changed not to call notify_on_release().
    >
    > From documentation, notify_on_release() is called when there is no
    > tasks/children in cgroup. On implementation, notify_on_release is
    > not called if css->refcnt > 0.

    The documentation is a little inaccurate - it's called when the cgroup
    is removable. In the original cpusets this implied that there were no
    tasks or children; in cgroups, a refcount can keep the group alive
    too, so it's right to not call notify_on_release if there are
    remaining refcounts.

    > This is problem. Memcg has css->refcnt by each page even when
    > there are no tasks. Release handler will be never called.

    Right, because it can't remove the dir if there are still refcounts.

    Early in the development of cgroups I did have a css refcount scheme
    similar to what you have, with tryget, etc, but still with separate
    refcounts for each subsystem; I got rid of it since it seemed more
    complicated than we needed at the time. But I'll see if I can dig it
    up.

    Paul


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