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    Subject[PATCH 198/270] Revert "cgroup: Remove task_lock() from cgroup_post_fork()"
    Date
    3.5.7u1 -stable review patch.  If anyone has any objections, please let me know.

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

    From: Tejun Heo <tj@kernel.org>

    commit d87838321124061f6c935069d97f37010fa417e6 upstream.

    This reverts commit 7e3aa30ac8c904a706518b725c451bb486daaae9.

    The commit incorrectly assumed that fork path always performed
    threadgroup_change_begin/end() and depended on that for
    synchronization against task exit and cgroup migration paths instead
    of explicitly grabbing task_lock().

    threadgroup_change is not locked when forking a new process (as
    opposed to a new thread in the same process) and even if it were it
    wouldn't be effective as different processes use different threadgroup
    locks.

    Revert the incorrect optimization.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    LKML-Reference: <20121008020000.GB2575@localhost>
    Acked-by: Li Zefan <lizefan@huawei.com>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com>
    ---
    kernel/cgroup.c | 15 +++------------
    1 file changed, 3 insertions(+), 12 deletions(-)

    diff --git a/kernel/cgroup.c b/kernel/cgroup.c
    index 75d4318..a91aa0b 100644
    --- a/kernel/cgroup.c
    +++ b/kernel/cgroup.c
    @@ -4805,19 +4805,10 @@ void cgroup_post_fork(struct task_struct *child)
    */
    if (use_task_css_set_links) {
    write_lock(&css_set_lock);
    - if (list_empty(&child->cg_list)) {
    - /*
    - * It's safe to use child->cgroups without task_lock()
    - * here because we are protected through
    - * threadgroup_change_begin() against concurrent
    - * css_set change in cgroup_task_migrate(). Also
    - * the task can't exit at that point until
    - * wake_up_new_task() is called, so we are protected
    - * against cgroup_exit() setting child->cgroup to
    - * init_css_set.
    - */
    + task_lock(child);
    + if (list_empty(&child->cg_list))
    list_add(&child->cg_list, &child->cgroups->tasks);
    - }
    + task_unlock(child);
    write_unlock(&css_set_lock);
    }
    }
    --
    1.7.9.5


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