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    Subject[PATCH 3.8 40/81] cgroup: update cgroup_enable_task_cg_lists() to grab siglock
    Date
    3.8.13.20 -stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Tejun Heo <tj@kernel.org>

    commit 532de3fc72adc2a6525c4d53c07bf81e1732083d upstream.

    Currently, there's nothing preventing cgroup_enable_task_cg_lists()
    from missing set PF_EXITING and race against cgroup_exit(). Depending
    on the timing, cgroup_exit() may finish with the task still linked on
    css_set leading to list corruption. Fix it by grabbing siglock in
    cgroup_enable_task_cg_lists() so that PF_EXITING is guaranteed to be
    visible.

    This whole on-demand cg_list optimization is extremely fragile and has
    ample possibility to lead to bugs which can cause things like
    once-a-year oops during boot. I'm wondering whether the better
    approach would be just adding "cgroup_disable=all" handling which
    disables the whole cgroup rather than tempting fate with this
    on-demand craziness.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Acked-by: Li Zefan <lizefan@huawei.com>
    Signed-off-by: Kamal Mostafa <kamal@canonical.com>
    ---
    kernel/cgroup.c | 5 +++++
    1 file changed, 5 insertions(+)

    diff --git a/kernel/cgroup.c b/kernel/cgroup.c
    index ba9e28c..a2d2c80 100644
    --- a/kernel/cgroup.c
    +++ b/kernel/cgroup.c
    @@ -2982,9 +2982,14 @@ static void cgroup_enable_task_cg_lists(void)
    * We should check if the process is exiting, otherwise
    * it will race with cgroup_exit() in that the list
    * entry won't be deleted though the process has exited.
    + * Do it while holding siglock so that we don't end up
    + * racing against cgroup_exit().
    */
    + spin_lock_irq(&p->sighand->siglock);
    if (!(p->flags & PF_EXITING) && list_empty(&p->cg_list))
    list_add(&p->cg_list, &p->cgroups->tasks);
    + spin_unlock_irq(&p->sighand->siglock);
    +
    task_unlock(p);
    } while_each_thread(g, p);
    read_unlock(&tasklist_lock);
    --
    1.8.3.2


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