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    SubjectRe: [RFC][PATCH] cgroup: fix race between fork and cgroup freezing
    02:26, Tejun Heo wrote:
    > Hello, Li, Frederic.
    >
    > On Thu, Mar 08, 2012 at 04:45:13PM +0800, Li Zefan wrote:
    >> +static void freezer_post_fork(struct cgroup_subsys *ss,
    >> + struct task_struct *task)
    >> +{
    >> + struct freezer *freezer;
    >> +
    >> + cgroup_lock();
    >> +
    >> + freezer = task_freezer(task);
    >> + if (!freezer->css.cgroup->parent)
    >> + goto out;
    >> +
    >> + spin_lock_irq(&freezer->lock);
    >> + if (freezer->state != CGROUP_THAWED)
    >> + freeze_task(task);
    >> + spin_unlock_irq(&freezer->lock);
    >> +out:
    >> + cgroup_unlock();
    >> +}
    >
    > Urgh... this is requiring policy implementations to synchronize with
    > problem caused by cgroup core optimization and it's so very subtle.
    > IMHO, this definitely should be contained in cgroup core and in a very
    > confined form even inside cgroup core.
    >
    > Any other ideas?
    >

    The problem is, forks can happen at any time, so there's no way to prevent
    forks from happening while iterating tasks in a cgroup, so controllers
    have to deal with it. In fact freezer is somewhat aware of this issue,
    that's why it provides the ->fork callback, but there's race.

    This patch is not too bad (needs a bit modification). cgroup core will detect
    (via seqcount) if something's happened to a cgroup and the tasks in it, and
    then cgroup will notify controllers to check if newly-forked tasks should
    be adjusted accordingly, so they will have consistent status with other tasks
    in the same cgroup.


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