cgroup: remove cgroupfs_root->refcnt

Currently, cgroupfs_root and its ->top_cgroup are separated reference
counted and the latter's is ignored.  There's no reason to do this
separately.  This patch removes cgroupfs_root->refcnt and destroys
cgroupfs_root when the top_cgroup is released.

* cgroup_put() updated to ignore cgroup_is_dead() test for top
  cgroups.  cgroup_free_fn() updated to handle root destruction when
  releasing a top cgroup.

* As root destruction is now bounced through cgroup destruction, it is
  asynchronous.  Update cgroup_mount() so that it waits for pending
  release which is currently implemented using msleep().  Converting
  this to proper wait_queue isn't hard but likely unnecessary.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

1 files changed, 1 insertions, 3 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index f0e6105bbbc1..298d616e8f40 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -282,12 +282,10 @@ struct cgroupfs_root {
         /* The bitmask of subsystems attached to this hierarchy */
         unsigned long subsys_mask;
 
-        atomic_t refcnt;
-
         /* Unique id for this hierarchy. */
         int hierarchy_id;
 
-        /* The root cgroup for this hierarchy */
+        /* The root cgroup.  Root is destroyed on its release. */
         struct cgroup top_cgroup;
 
         /* Number of cgroups in the hierarchy, used only for /proc/cgroups */