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    Subject[PATCH 1/2 v3] cgroup: Remove unnecessary task_lock before fetching css_set on migration
    Date
    When we fetch the css_set of the tasks on cgroup migration, we don't need
    anymore to synchronize against cgroup_exit() that could swap the old one
    with init_css_set. Now that we are using threadgroup_lock() during
    the migrations, we don't need to worry about it anymore.

    Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    Reviewed-by: Mandeep Singh Baines <msb@chromium.org>
    Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
    Cc: Tejun Heo <tj@kernel.org>
    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>
    ---
    kernel/cgroup.c | 20 ++++++++++----------
    1 files changed, 10 insertions(+), 10 deletions(-)

    diff --git a/kernel/cgroup.c b/kernel/cgroup.c
    index bc3caff..24f6d6f 100644
    --- a/kernel/cgroup.c
    +++ b/kernel/cgroup.c
    @@ -1850,14 +1850,14 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
    struct css_set *newcg;

    /*
    - * get old css_set. we need to take task_lock and refcount it, because
    - * an exiting task can change its css_set to init_css_set and drop its
    - * old one without taking cgroup_mutex.
    + * get old css_set. We are synchronized through threadgroup_lock()
    + * against PF_EXITING setting such that we can't race against
    + * cgroup_exit() changing the css_set to init_css_set and dropping the
    + * old one.
    */
    - task_lock(tsk);
    + WARN_ON_ONCE(tsk->flags & PF_EXITING);
    oldcg = tsk->cgroups;
    get_css_set(oldcg);
    - task_unlock(tsk);

    /* locate or allocate a new css_set for this task. */
    if (guarantee) {
    @@ -1879,9 +1879,7 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
    }
    put_css_set(oldcg);

    - /* @tsk can't exit as its threadgroup is locked */
    task_lock(tsk);
    - WARN_ON_ONCE(tsk->flags & PF_EXITING);
    rcu_assign_pointer(tsk->cgroups, newcg);
    task_unlock(tsk);

    @@ -2182,11 +2180,13 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
    /* nothing to do if this task is already in the cgroup */
    if (tc->cgrp == cgrp)
    continue;
    - /* get old css_set pointer */
    - task_lock(tc->task);
    + /*
    + * get old css_set pointer. threadgroup is locked so this is
    + * safe against concurrent cgroup_exit() changing this to
    + * init_css_set.
    + */
    oldcg = tc->task->cgroups;
    get_css_set(oldcg);
    - task_unlock(tc->task);
    /* see if the new one for us is already in the list? */
    if (css_set_check_fetched(cgrp, tc->task, oldcg, &newcg_list)) {
    /* was already there, nothing to do. */
    --
    1.7.5.4


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