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    Subject[PATCH 2/2] cgroup: Walk task list under tasklist_lock in cgroup_enable_task_cg_list
    Date
    Walking through the tasklist in cgroup_enable_task_cg_list() inside
    an RCU read side critical section is not enough because:

    - RCU is not (yet) safe against while_each_thread()

    - If we use only RCU, a forking task that has passed cgroup_post_fork()
    without seeing use_task_css_set_links == 1 is not guaranteed to have
    its child immediately visible in the tasklist if we walk through it
    remotely with RCU. In this case it will be missing in its css_set's
    task list.

    Thus we need to traverse the list (unfortunately) under the
    tasklist_lock. It makes us safe against while_each_thread() and also
    make sure we see all forked task that have been added to the tasklist.

    As a secondary effect, reading and writing use_task_css_set_links are
    now well ordered against tasklist traversing and modification. The new
    layout is:

    CPU 0 CPU 1

    use_task_css_set_links = 1 write_lock(tasklist_lock)
    read_lock(tasklist_lock) add task to tasklist
    do_each_thread() { write_unlock(tasklist_lock)
    add thread to css set links if (use_task_css_set_links)
    } while_each_thread() add thread to css set links
    read_unlock(tasklist_lock)

    If CPU 0 traverse the list after the task has been added to the tasklist
    then it is correctly added to the css set links. OTOH if CPU 0 traverse
    the tasklist before the new task had the opportunity to be added to the
    tasklist because it was too early in the fork process, then CPU 1
    catches up and add the task to the css set links after it added the task
    to the tasklist. The right value of use_task_css_set_links is guaranteed
    to be visible from CPU 1 due to the LOCK/UNLOCK implicit barrier properties:
    the read_unlock on CPU 0 makes the write on use_task_css_set_links happening
    and the write_lock on CPU 1 make the read of use_task_css_set_links that comes
    afterward to return the correct value.

    Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: Li Zefan <lizf@cn.fujitsu.com>
    Cc: Mandeep Singh Baines <msb@chromium.org>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    ---
    kernel/cgroup.c | 20 ++++++++++++++++++++
    1 files changed, 20 insertions(+), 0 deletions(-)

    diff --git a/kernel/cgroup.c b/kernel/cgroup.c
    index 6e4eb43..c6877fe 100644
    --- a/kernel/cgroup.c
    +++ b/kernel/cgroup.c
    @@ -2707,6 +2707,14 @@ static void cgroup_enable_task_cg_lists(void)
    struct task_struct *p, *g;
    write_lock(&css_set_lock);
    use_task_css_set_links = 1;
    + /*
    + * We need tasklist_lock because RCU is not safe against
    + * while_each_thread(). Besides, a forking task that has passed
    + * cgroup_post_fork() without seeing use_task_css_set_links = 1
    + * is not guaranteed to have its child immediately visible in the
    + * tasklist if we walk through it with RCU.
    + */
    + read_lock(&tasklist_lock);
    do_each_thread(g, p) {
    task_lock(p);
    /*
    @@ -2718,6 +2726,7 @@ static void cgroup_enable_task_cg_lists(void)
    list_add(&p->cg_list, &p->cgroups->tasks);
    task_unlock(p);
    } while_each_thread(g, p);
    + read_unlock(&tasklist_lock);
    write_unlock(&css_set_lock);
    }

    @@ -4522,6 +4531,17 @@ void cgroup_fork_callbacks(struct task_struct *child)
    */
    void cgroup_post_fork(struct task_struct *child)
    {
    + /*
    + * use_task_css_set_links is set to 1 before we walk the tasklist
    + * under the tasklist_lock and we read it here after we added the child
    + * to the tasklist under the tasklist_lock as well. If the child wasn't
    + * yet in the tasklist when we walked through it from
    + * cgroup_enable_task_cg_lists(), then use_task_css_set_links value
    + * should be visible now due to the paired locking and barriers implied
    + * by LOCK/UNLOCK: it is written before the tasklist_lock unlock
    + * in cgroup_enable_task_cg_lists() and read here after the tasklist_lock
    + * lock on fork.
    + */
    if (use_task_css_set_links) {
    write_lock(&css_set_lock);
    if (list_empty(&child->cg_list)) {
    --
    1.7.5.4


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