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, 12 insertions, 10 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 8f296b83b6a3..67fc953c816a 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -986,17 +986,14 @@ static void cgroup_d_remove_dir(struct dentry *dentry)
  * returns an error, no reference counts are touched.
  */
 static int rebind_subsystems(struct cgroupfs_root *root,
-                              unsigned long final_subsys_mask)
+                             unsigned long added_mask, unsigned removed_mask)
 {
-        unsigned long added_mask, removed_mask;
         struct cgroup *cgrp = &root->top_cgroup;
         int i;
 
         BUG_ON(!mutex_is_locked(&cgroup_mutex));
         BUG_ON(!mutex_is_locked(&cgroup_root_mutex));
 
-        removed_mask = root->actual_subsys_mask & ~final_subsys_mask;
-        added_mask = final_subsys_mask & ~root->actual_subsys_mask;
         /* Check that any added subsystems are currently free */
         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                 unsigned long bit = 1UL << i;
@@ -1032,27 +1029,33 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                         BUG_ON(cgrp->subsys[i]);
                         BUG_ON(!cgroup_dummy_top->subsys[i]);
                         BUG_ON(cgroup_dummy_top->subsys[i]->cgroup != cgroup_dummy_top);
+
                         cgrp->subsys[i] = cgroup_dummy_top->subsys[i];
                         cgrp->subsys[i]->cgroup = cgrp;
                         list_move(&ss->sibling, &root->subsys_list);
                         ss->root = root;
                         if (ss->bind)
                                 ss->bind(cgrp);
+
                         /* refcount was already taken, and we're keeping it */
+                        root->subsys_mask |= bit;
                 } else if (bit & removed_mask) {
                         /* We're removing this subsystem */
                         BUG_ON(ss == NULL);
                         BUG_ON(cgrp->subsys[i] != cgroup_dummy_top->subsys[i]);
                         BUG_ON(cgrp->subsys[i]->cgroup != cgrp);
+
                         if (ss->bind)
                                 ss->bind(cgroup_dummy_top);
                         cgroup_dummy_top->subsys[i]->cgroup = cgroup_dummy_top;
                         cgrp->subsys[i] = NULL;
                         cgroup_subsys[i]->root = &cgroup_dummy_root;
                         list_move(&ss->sibling, &cgroup_dummy_root.subsys_list);
+
                         /* subsystem is now free - drop reference on module */
                         module_put(ss->module);
-                } else if (bit & final_subsys_mask) {
+                        root->subsys_mask &= ~bit;
+                } else if (bit & root->subsys_mask) {
                         /* Subsystem state should already exist */
                         BUG_ON(ss == NULL);
                         BUG_ON(!cgrp->subsys[i]);
@@ -1069,7 +1072,6 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                         BUG_ON(cgrp->subsys[i]);
                 }
         }
-        root->subsys_mask = root->actual_subsys_mask = final_subsys_mask;
 
         return 0;
 }
@@ -1343,7 +1345,7 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
         if (ret)
                 goto out_unlock;
 
-        if (opts.subsys_mask != root->actual_subsys_mask || opts.release_agent)
+        if (opts.subsys_mask != root->subsys_mask || opts.release_agent)
                 pr_warning("cgroup: option changes via remount are deprecated (pid=%d comm=%s)\n",
                            task_tgid_nr(current), current->comm);
 
@@ -1365,7 +1367,7 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
          */
         cgroup_clear_directory(cgrp->dentry, false, removed_mask);
 
-        ret = rebind_subsystems(root, opts.subsys_mask);
+        ret = rebind_subsystems(root, added_mask, removed_mask);
         if (ret) {
                 /* rebind_subsystems failed, re-populate the removed files */
                 cgroup_populate_dir(cgrp, false, removed_mask);
@@ -1634,7 +1636,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
                 if (ret)
                         goto unlock_drop;
 
-                ret = rebind_subsystems(root, root->subsys_mask);
+                ret = rebind_subsystems(root, root->subsys_mask, 0);
                 if (ret == -EBUSY) {
                         free_cgrp_cset_links(&tmp_links);
                         goto unlock_drop;
@@ -1727,7 +1729,7 @@ static void cgroup_kill_sb(struct super_block *sb) {
         mutex_lock(&cgroup_root_mutex);
 
         /* Rebind all subsystems back to the default hierarchy */
-        ret = rebind_subsystems(root, 0);
+        ret = rebind_subsystems(root, 0, root->subsys_mask);
         /* Shouldn't be able to fail ... */
         BUG_ON(ret);