cgroup: remove cgroup->actual_subsys_mask

cgroup curiously has two subsystem masks, ->subsys_mask and
->actual_subsys_mask.  The latter only exists because the new target
subsys_mask is passed into rebind_subsystems() via @root>subsys_mask.
rebind_subsystems() needs to know what the current mask is to decide
how to reach the target mask so ->actual_subsys_mask is used as the
temp location to remember the current state.

Adding a temporary field to a permanent data structure is rather silly
and can be misleading.  Update rebind_subsystems() to take @added_mask
and @removed_mask instead and remove @root->actual_subsys_mask.

This patch shouldn't introduce any behavior changes.

v2: Comment and description updated as suggested by Li.

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

1 files changed, 1 insertions, 7 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index ab27001a2c4a..4c1eceb8c439 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -286,18 +286,12 @@ enum {
 struct cgroupfs_root {
         struct super_block *sb;
 
-        /*
-         * The bitmask of subsystems intended to be attached to this
-         * hierarchy
-         */
+        /* The bitmask of subsystems attached to this hierarchy */
         unsigned long subsys_mask;
 
         /* Unique id for this hierarchy. */
         int hierarchy_id;
 
-        /* The bitmask of subsystems currently attached to this hierarchy */
-        unsigned long actual_subsys_mask;
-
         /* A list running through the attached subsystems */
         struct list_head subsys_list;