Revert "cgroup: remove bind() method from cgroup_subsys."

This reverts commit 84cfb6ab484b442d5115eb3baf9db7d74a3ea626.  There
are scheduled changes which make use of the removed callback.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Rami Rosen <ramirose@gmail.com>
Cc: Li Zefan <lizefan@huawei.com>

Documentation/cgroups/cgroups.txt

@@ -211,9 +211,10 @@ matches, and any of the requested subsystems are in use in an existing
 hierarchy, the mount will fail with -EBUSY. Otherwise, a new hierarchy
 is activated, associated with the requested subsystems.
 
-It's not possible to bind a new subsystem to an active cgroup
-hierarchy, or to unbind a subsystem from an active cgroup
-hierarchy.
+It's not currently possible to bind a new subsystem to an active
+cgroup hierarchy, or to unbind a subsystem from an active cgroup
+hierarchy. This may be possible in future, but is fraught with nasty
+error-recovery issues.
 
 When a cgroup filesystem is unmounted, if there are any
 child cgroups created below the top-level cgroup, that hierarchy
@@ -381,8 +382,10 @@ To Specify a hierarchy's release_agent:
 
 Note that specifying 'release_agent' more than once will return failure.
 
-Note that changing the set of subsystems is only supported when the
-hierarchy consists of a single (root) cgroup.
+Note that changing the set of subsystems is currently only supported
+when the hierarchy consists of a single (root) cgroup. Supporting
+the ability to arbitrarily bind/unbind subsystems from an existing
+cgroup hierarchy is intended to be implemented in the future.
 
 Then under /sys/fs/cgroup/rg1 you can find a tree that corresponds to the
 tree of the cgroups in the system. For instance, /sys/fs/cgroup/rg1
@@ -640,6 +643,13 @@ void exit(struct task_struct *task)
 
 Called during task exit.
 
+void bind(struct cgroup *root)
+(cgroup_mutex held by caller)
+
+Called when a cgroup subsystem is rebound to a different hierarchy
+and root cgroup. Currently this will only involve movement between
+the default hierarchy (which never has sub-cgroups) and a hierarchy
+that is being created/destroyed (and hence has no sub-cgroups).
 
 4. Extended attribute usage
 ===========================

include/linux/cgroup.h

@@ -484,6 +484,8 @@ struct cgroup_subsys {
 	void (*fork)(struct task_struct *task);
 	void (*exit)(struct cgroup *cgrp, struct cgroup *old_cgrp,
 		     struct task_struct *task);
+	void (*bind)(struct cgroup *root);
+
 	int subsys_id;
 	int active;
 	int disabled;

kernel/cgroup.c

@@ -1091,12 +1091,16 @@ static int rebind_subsystems(struct cgroupfs_root *root,
 			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 */
 		} else if (bit & removed_mask) {
 			/* We're removing this subsystem */
 			BUG_ON(ss == NULL);
 			BUG_ON(cgrp->subsys[i] != dummytop->subsys[i]);
 			BUG_ON(cgrp->subsys[i]->cgroup != cgrp);
+			if (ss->bind)
+				ss->bind(dummytop);
 			dummytop->subsys[i]->cgroup = dummytop;
 			cgrp->subsys[i] = NULL;
 			subsys[i]->root = &rootnode;