cgroup: stash cgroup_root reference into cgroup_fs_context

Note that this reference is *NOT* contributing to refcount of
cgroup_root in question and is valid only until cgroup_do_mount()
returns.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/cgroup/cgroup-internal.h

@@ -41,6 +41,7 @@ extern void __init enable_debug_cgroup(void);
  * The cgroup filesystem superblock creation/mount context.
  */
 struct cgroup_fs_context {
+	struct cgroup_root	*root;
 	unsigned int	flags;			/* CGRP_ROOT_* flags */
 
 	/* cgroup1 bits */
@@ -208,7 +209,7 @@ int cgroup_path_ns_locked(struct cgroup *cgrp, char *buf, size_t buflen,
 			  struct cgroup_namespace *ns);
 
 void cgroup_free_root(struct cgroup_root *root);
-void init_cgroup_root(struct cgroup_root *root, struct cgroup_fs_context *ctx);
+void init_cgroup_root(struct cgroup_fs_context *ctx);
 int cgroup_setup_root(struct cgroup_root *root, u16 ss_mask);
 int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask);
 struct dentry *cgroup_do_mount(struct file_system_type *fs_type, int flags,

kernel/cgroup/cgroup-v1.c

@@ -1208,6 +1208,7 @@ int cgroup1_get_tree(struct fs_context *fc)
 		if (root->flags ^ ctx->flags)
 			pr_warn("new mount options do not match the existing superblock, will be ignored\n");
 
+		ctx->root = root;
 		ret = 0;
 		goto out_unlock;
 	}
@@ -1234,7 +1235,8 @@ int cgroup1_get_tree(struct fs_context *fc)
 		goto out_unlock;
 	}
 
-	init_cgroup_root(root, ctx);
+	ctx->root = root;
+	init_cgroup_root(ctx);
 
 	ret = cgroup_setup_root(root, ctx->subsys_mask);
 	if (ret)

kernel/cgroup/cgroup.c

@@ -1915,8 +1915,9 @@ static void init_cgroup_housekeeping(struct cgroup *cgrp)
 	INIT_WORK(&cgrp->release_agent_work, cgroup1_release_agent);
 }
 
-void init_cgroup_root(struct cgroup_root *root, struct cgroup_fs_context *ctx)
+void init_cgroup_root(struct cgroup_fs_context *ctx)
 {
+	struct cgroup_root *root = ctx->root;
 	struct cgroup *cgrp = &root->cgrp;
 
 	INIT_LIST_HEAD(&root->root_list);
@@ -2098,6 +2099,7 @@ static int cgroup_get_tree(struct fs_context *fc)
 
 	cgrp_dfl_visible = true;
 	cgroup_get_live(&cgrp_dfl_root.cgrp);
+	ctx->root = &cgrp_dfl_root;
 
 	root = cgroup_do_mount(&cgroup2_fs_type, fc->sb_flags, &cgrp_dfl_root,
 					 CGROUP2_SUPER_MAGIC, ns);
@@ -5374,7 +5376,8 @@ int __init cgroup_init_early(void)
 	struct cgroup_subsys *ss;
 	int i;
 
-	init_cgroup_root(&cgrp_dfl_root, &ctx);
+	ctx.root = &cgrp_dfl_root;
+	init_cgroup_root(&ctx);
 	cgrp_dfl_root.cgrp.self.flags |= CSS_NO_REF;
 
 	RCU_INIT_POINTER(init_task.cgroups, &init_css_set);