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    Subject[PATCH 2/2] cgroup: Drop task_lock(parent) on cgroup_fork()
    Date
    We don't need to hold the parent task_lock() on the
    parent in cgroup_fork() because we are already synchronized
    against the two places that may change the parent css_set
    concurrently:

    - cgroup_exit(), but the parent obviously can't exit concurrently
    - cgroup migration: we are synchronized against threadgroup_lock()

    So we can safely remove the task_lock() there.

    Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: Li Zefan <lizf@cn.fujitsu.com>
    Cc: Containers <containers@lists.linux-foundation.org>
    Cc: Cgroups <cgroups@vger.kernel.org>
    Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Paul Menage <paul@paulmenage.org>
    Cc: Mandeep Singh Baines <msb@chromium.org>
    ---
    kernel/cgroup.c | 10 +++++++---
    1 files changed, 7 insertions(+), 3 deletions(-)

    diff --git a/kernel/cgroup.c b/kernel/cgroup.c
    index 6b5f693..a1e87a4 100644
    --- a/kernel/cgroup.c
    +++ b/kernel/cgroup.c
    @@ -4544,7 +4544,7 @@ static const struct file_operations proc_cgroupstats_operations = {
    *
    * A pointer to the shared css_set was automatically copied in
    * fork.c by dup_task_struct(). However, we ignore that copy, since
    - * it was not made under the protection of RCU or cgroup_mutex, so
    + * it was not made under the protection of threadgroup_change_begin(), so
    * might no longer be a valid cgroup pointer. cgroup_attach_task() might
    * have already changed current->cgroups, allowing the previously
    * referenced cgroup group to be removed and freed.
    @@ -4554,10 +4554,14 @@ static const struct file_operations proc_cgroupstats_operations = {
    */
    void cgroup_fork(struct task_struct *child)
    {
    - task_lock(current);
    + /*
    + * We don't need to task_lock() current because current->cgroups
    + * can't be changed concurrently here. The parent obviously hasn't
    + * exited and called cgroup_exit(), and we are synchronized against
    + * cgroup migration through threadgroup_change_begin().
    + */
    child->cgroups = current->cgroups;
    get_css_set(child->cgroups);
    - task_unlock(current);
    INIT_LIST_HEAD(&child->cg_list);
    }

    --
    1.7.5.4


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