cgroup/cpuset: Fix "suspicious RCU usage" lockdep warning

It was found that a "suspicious RCU usage" lockdep warning was issued
with the rcu_read_lock() call in update_sibling_cpumasks().  It is
because the update_cpumasks_hier() function may sleep. So we have
to release the RCU lock, call update_cpumasks_hier() and reacquire
it afterward.

Also add a percpu_rwsem_assert_held() in update_sibling_cpumasks()
instead of stating that in the comment.

Fixes: 4716909cc5c5 ("cpuset: Track cpusets that use parent's effective_cpus")
Signed-off-by: Waiman Long <longman@redhat.com>
Tested-by: Phil Auld <pauld@redhat.com>
Reviewed-by: Phil Auld <pauld@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 804ff57..4c7254e 100644 (file)
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1550,10 +1550,15 @@ static void update_sibling_cpumasks(struct cpuset *parent, struct cpuset *cs,
        struct cpuset *sibling;
        struct cgroup_subsys_state *pos_css;
 
+       percpu_rwsem_assert_held(&cpuset_rwsem);
+
        /*
         * Check all its siblings and call update_cpumasks_hier()
         * if their use_parent_ecpus flag is set in order for them
         * to use the right effective_cpus value.
+        *
+        * The update_cpumasks_hier() function may sleep. So we have to
+        * release the RCU read lock before calling it.
         */
        rcu_read_lock();
        cpuset_for_each_child(sibling, pos_css, parent) {
@@ -1561,8 +1566,13 @@ static void update_sibling_cpumasks(struct cpuset *parent, struct cpuset *cs,
                        continue;
                if (!sibling->use_parent_ecpus)
                        continue;
+               if (!css_tryget_online(&sibling->css))
+                       continue;
 
+               rcu_read_unlock();
                update_cpumasks_hier(sibling, tmp);
+               rcu_read_lock();
+               css_put(&sibling->css);
        }
        rcu_read_unlock();
 }