cgroup: make sure a parent css isn't offlined before its children

There are three subsystem callbacks in css shutdown path -
css_offline(), css_released() and css_free().  Except for
css_released(), cgroup core didn't guarantee the order of invocation.
css_offline() or css_free() could be called on a parent css before its
children.  This behavior is unexpected and led to bugs in cpu and
memory controller.

This patch updates offline path so that a parent css is never offlined
before its children.  Each css keeps online_cnt which reaches zero iff
itself and all its children are offline and offline_css() is invoked
only after online_cnt reaches zero.

This fixes the memory controller bug and allows the fix for cpu
controller.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-and-tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reported-by: Brian Christiansen <brian.o.christiansen@gmail.com>
Link: http://lkml.kernel.org/g/5698A023.9070703@de.ibm.com
Link: http://lkml.kernel.org/g/CAKB58ikDkzc8REt31WBkD99+hxNzjK4+FBmhkgS+NVrC9vjMSg@mail.gmail.com
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 7f540f7f588d8c8461af975a5ebd21a08e6cf14b..789471dba6fb30f15c752fbca46ddbefe5bfe850 100644 (file)
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -127,6 +127,12 @@ struct cgroup_subsys_state {
         */
        u64 serial_nr;
 
+       /*
+        * Incremented by online self and children.  Used to guarantee that
+        * parents are not offlined before their children.
+        */
+       atomic_t online_cnt;
+
        /* percpu_ref killing and RCU release */
        struct rcu_head rcu_head;
        struct work_struct destroy_work;

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 88abd4d076d89835f7741962b0ffdf036c7dc950..d01587793865e71acf9d6810c02446ab353d989d 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4760,6 +4760,7 @@ static void init_and_link_css(struct cgroup_subsys_state *css,
        INIT_LIST_HEAD(&css->sibling);
        INIT_LIST_HEAD(&css->children);
        css->serial_nr = css_serial_nr_next++;
+       atomic_set(&css->online_cnt, 0);
 
        if (cgroup_parent(cgrp)) {
                css->parent = cgroup_css(cgroup_parent(cgrp), ss);
@@ -4782,6 +4783,10 @@ static int online_css(struct cgroup_subsys_state *css)
        if (!ret) {
                css->flags |= CSS_ONLINE;
                rcu_assign_pointer(css->cgroup->subsys[ss->id], css);
+
+               atomic_inc(&css->online_cnt);
+               if (css->parent)
+                       atomic_inc(&css->parent->online_cnt);
        }
        return ret;
 }
@@ -5019,10 +5024,15 @@ static void css_killed_work_fn(struct work_struct *work)
                container_of(work, struct cgroup_subsys_state, destroy_work);
 
        mutex_lock(&cgroup_mutex);
-       offline_css(css);
-       mutex_unlock(&cgroup_mutex);
 
-       css_put(css);
+       do {
+               offline_css(css);
+               css_put(css);
+               /* @css can't go away while we're holding cgroup_mutex */
+               css = css->parent;
+       } while (css && atomic_dec_and_test(&css->online_cnt));
+
+       mutex_unlock(&cgroup_mutex);
 }
 
 /* css kill confirmation processing requires process context, bounce */
@@ -5031,8 +5041,10 @@ static void css_killed_ref_fn(struct percpu_ref *ref)
        struct cgroup_subsys_state *css =
                container_of(ref, struct cgroup_subsys_state, refcnt);
 
-       INIT_WORK(&css->destroy_work, css_killed_work_fn);
-       queue_work(cgroup_destroy_wq, &css->destroy_work);
+       if (atomic_dec_and_test(&css->online_cnt)) {
+               INIT_WORK(&css->destroy_work, css_killed_work_fn);
+               queue_work(cgroup_destroy_wq, &css->destroy_work);
+       }
 }
 
 /**