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    Subject[24/67] cgroups: fix a css_set not found bug in cgroup_attach_proc
    3.0-stable review patch.  If anyone has any objections, please let me know.

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

    From: Mandeep Singh Baines <msb@chromium.org>

    commit e0197aae59e55c06db172bfbe1a1cdb8c0e1cab3 upstream.

    There is a BUG when migrating a PF_EXITING proc. Since css_set_prefetch()
    is not called for the PF_EXITING case, find_existing_css_set() will return
    NULL inside cgroup_task_migrate() causing a BUG.

    This bug is easy to reproduce. Create a zombie and echo its pid to
    cgroup.procs.

    $ cat zombie.c
    \#include <unistd.h>

    int main()
    {
    if (fork())
    pause();
    return 0;
    }
    $

    We are hitting this bug pretty regularly on ChromeOS.

    This bug is already fixed by Tejun Heo's cgroup patchset which is
    targetted for the next merge window:

    https://lkml.org/lkml/2011/11/1/356

    I've create a smaller patch here which just fixes this bug so that a
    fix can be merged into the current release and stable.

    Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
    Downstream-Bug-Report: http://crosbug.com/23953
    Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
    Signed-off-by: Tejun Heo <tj@kernel.org>
    Cc: containers@lists.linux-foundation.org
    Cc: cgroups@vger.kernel.org
    Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Paul Menage <paul@paulmenage.org>
    Cc: Olof Johansson <olofj@chromium.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    kernel/cgroup.c | 5 -----
    1 file changed, 5 deletions(-)

    --- a/kernel/cgroup.c
    +++ b/kernel/cgroup.c
    @@ -2095,11 +2095,6 @@ int cgroup_attach_proc(struct cgroup *cg
    continue;
    /* get old css_set pointer */
    task_lock(tsk);
    - if (tsk->flags & PF_EXITING) {
    - /* ignore this task if it's going away */
    - task_unlock(tsk);
    - continue;
    - }
    oldcg = tsk->cgroups;
    get_css_set(oldcg);
    task_unlock(tsk);



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