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    SubjectRe: v2.6.26-rc7/cgroups: circular locking dependency
    On Mon, Jun 23, 2008 at 11:29 PM, Max Krasnyansky <> wrote:
    > Peter Zijlstra wrote:
    >> On Mon, 2008-06-23 at 00:34 +0900, KOSAKI Motohiro wrote:
    >>> CC'ed Paul Jackson
    >>> it seems typical ABBA deadlock.
    >>> I think cpuset use cgrou_lock() by mistake.
    >>> IMHO, cpuset_handle_cpuhp() sholdn't use cgroup_lock() and
    >>> shouldn't call rebuild_sched_domains().
    >> Looks like Max forgot to test with lockdep enabled...
    > Hmm, I don't think I actually changed any lock nesting/dependencies. Did I ?
    > Oh, I see rebuild_sched_domains() is now called from cpuset hotplug handler.
    > I just looked at the comment for rebuild_sched_domains() and it says
    > " * Call with cgroup_mutex held. ..." that's why I thought it's safe and it
    > worked on the test stations.
    > Anyway, we need definitely need to make rebuild_sched_domains() work from the
    > hotplug handler.

    In that case the obvious solution would be to nest inside
    cgroup_lock() inside cpuhotplug.lock. i.e. require that
    update_sched_domains() be called inside get_online_cpus(), and call
    get_online_cpus() prior to calling cgroup_lock() in any code path that
    might call update_sched_domains(). That's basically:


    i.e. almost all the cpuset userspace APIs. A bit ugly, but probably
    not a big deal given how infrequently CPU hotplug/hotunplug occurs?

    Probably simplest with a wrapper function such as:

    static bool cpuset_lock_live_cgroup(struct cgroup *cgrp)
    if (cgroup_lock_live_cgroup())
    return true;
    return false;

    static void cpuset_unlock()

    and use those in the relevant entry points in place of

    Oh, except that cpuset_destroy() is called firmly inside cgroup_mutex,
    and hence can't nest the call to cgroup_lock() inside the call to

    Second idea - can we just punt the call to rebuild_sched_domains() to
    a workqueue thread if it's due to a flag or cpumask change? Does it
    matter if the call doesn't happen synchronously? The work handler
    could easily nest the cgroup_lock() call inside get_online_cpus() and
    then call rebuild_sched_domains()


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