cgroup: remove cgroup_root_mutex

cgroup_root_mutex was added to avoid deadlock involving namespace_sem
via cgroup_show_options().  It added a lot of overhead for the small
purpose of it and, because it's nested under cgroup_mutex, it has very
limited usefulness.  The previous patch made cgroup_show_options() not
use cgroup_root_mutex, so nobody needs it anymore.  Remove it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

kernel/cgroup.c

@@ -70,18 +70,6 @@
 /*
  * cgroup_mutex is the master lock.  Any modification to cgroup or its
  * hierarchy must be performed while holding it.
- *
- * cgroup_root_mutex nests inside cgroup_mutex and should be held to modify
- * cgroupfs_root of any cgroup hierarchy - subsys list, flags,
- * release_agent_path and so on.  Modifying requires both cgroup_mutex and
- * cgroup_root_mutex.  Readers can acquire either of the two.  This is to
- * break the following locking order cycle.
- *
- *  A. cgroup_mutex -> cred_guard_mutex -> s_type->i_mutex_key -> namespace_sem
- *  B. namespace_sem -> cgroup_mutex
- *
- * B happens only through cgroup_show_options() and using cgroup_root_mutex
- * breaks it.
  */
 #ifdef CONFIG_PROVE_RCU
 DEFINE_MUTEX(cgroup_mutex);
@@ -90,8 +78,6 @@ EXPORT_SYMBOL_GPL(cgroup_mutex);	/* only for lockdep */
 static DEFINE_MUTEX(cgroup_mutex);
 #endif
 
-static DEFINE_MUTEX(cgroup_root_mutex);
-
 /*
  * Protects cgroup_subsys->release_agent_path.  Modifying it also requires
  * cgroup_mutex.  Reading requires either cgroup_mutex or this spinlock.
@@ -103,14 +89,6 @@ static DEFINE_SPINLOCK(release_agent_path_lock);
 			   lockdep_is_held(&cgroup_mutex),		\
 			   "cgroup_mutex or RCU read lock required");
 
-#ifdef CONFIG_LOCKDEP
-#define cgroup_assert_mutex_or_root_locked()				\
-	WARN_ON_ONCE(debug_locks && (!lockdep_is_held(&cgroup_mutex) &&	\
-				     !lockdep_is_held(&cgroup_root_mutex)))
-#else
-#define cgroup_assert_mutex_or_root_locked()	do { } while (0)
-#endif
-
 /*
  * cgroup destruction makes heavy use of work items and there can be a lot
  * of concurrent destructions.  Use a separate workqueue so that cgroup
@@ -154,11 +132,7 @@ static struct cgroup * const cgroup_dummy_top = &cgroup_dummy_root.top_cgroup;
 static LIST_HEAD(cgroup_roots);
 static int cgroup_root_count;
 
-/*
- * Hierarchy ID allocation and mapping.  It follows the same exclusion
- * rules as other root ops - both cgroup_mutex and cgroup_root_mutex for
- * writes, either for reads.
- */
+/* hierarchy ID allocation and mapping, protected by cgroup_mutex */
 static DEFINE_IDR(cgroup_hierarchy_idr);
 
 static struct cgroup_name root_cgroup_name = { .name = "/" };
@@ -973,7 +947,6 @@ static int rebind_subsystems(struct cgroupfs_root *root,
 	int i, ret;
 
 	BUG_ON(!mutex_is_locked(&cgroup_mutex));
-	BUG_ON(!mutex_is_locked(&cgroup_root_mutex));
 
 	/* Check that any added subsystems are currently free */
 	for_each_subsys(ss, i)
@@ -1246,7 +1219,6 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
 
 	mutex_lock(&cgrp->dentry->d_inode->i_mutex);
 	mutex_lock(&cgroup_mutex);
-	mutex_lock(&cgroup_root_mutex);
 
 	/* See what subsystems are wanted */
 	ret = parse_cgroupfs_options(data, &opts);
@@ -1288,7 +1260,6 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
  out_unlock:
 	kfree(opts.release_agent);
 	kfree(opts.name);
-	mutex_unlock(&cgroup_root_mutex);
 	mutex_unlock(&cgroup_mutex);
 	mutex_unlock(&cgrp->dentry->d_inode->i_mutex);
 	return ret;
@@ -1331,7 +1302,6 @@ static int cgroup_init_root_id(struct cgroupfs_root *root, int start, int end)
 	int id;
 
 	lockdep_assert_held(&cgroup_mutex);
-	lockdep_assert_held(&cgroup_root_mutex);
 
 	id = idr_alloc_cyclic(&cgroup_hierarchy_idr, root, start, end,
 			      GFP_KERNEL);
@@ -1345,7 +1315,6 @@ static int cgroup_init_root_id(struct cgroupfs_root *root, int start, int end)
 static void cgroup_exit_root_id(struct cgroupfs_root *root)
 {
 	lockdep_assert_held(&cgroup_mutex);
-	lockdep_assert_held(&cgroup_root_mutex);
 
 	if (root->hierarchy_id) {
 		idr_remove(&cgroup_hierarchy_idr, root->hierarchy_id);
@@ -1524,7 +1493,6 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 
 		mutex_lock(&inode->i_mutex);
 		mutex_lock(&cgroup_mutex);
-		mutex_lock(&cgroup_root_mutex);
 
 		root_cgrp->id = idr_alloc(&root->cgroup_idr, root_cgrp,
 					   0, 1, GFP_KERNEL);
@@ -1597,7 +1565,6 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 		BUG_ON(!list_empty(&root_cgrp->children));
 		BUG_ON(root->number_of_cgroups != 1);
 
-		mutex_unlock(&cgroup_root_mutex);
 		mutex_unlock(&cgroup_mutex);
 		mutex_unlock(&inode->i_mutex);
 	} else {
@@ -1628,7 +1595,6 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 	revert_creds(cred);
  unlock_drop:
 	cgroup_exit_root_id(root);
-	mutex_unlock(&cgroup_root_mutex);
 	mutex_unlock(&cgroup_mutex);
 	mutex_unlock(&inode->i_mutex);
  drop_new_super:
@@ -1653,7 +1619,6 @@ static void cgroup_kill_sb(struct super_block *sb)
 
 	mutex_lock(&cgrp->dentry->d_inode->i_mutex);
 	mutex_lock(&cgroup_mutex);
-	mutex_lock(&cgroup_root_mutex);
 
 	/* Rebind all subsystems back to the default hierarchy */
 	if (root->flags & CGRP_ROOT_SUBSYS_BOUND) {
@@ -1682,7 +1647,6 @@ static void cgroup_kill_sb(struct super_block *sb)
 
 	cgroup_exit_root_id(root);
 
-	mutex_unlock(&cgroup_root_mutex);
 	mutex_unlock(&cgroup_mutex);
 	mutex_unlock(&cgrp->dentry->d_inode->i_mutex);
 
@@ -2193,11 +2157,9 @@ static int cgroup_release_agent_write(struct cgroup_subsys_state *css,
 		return -EINVAL;
 	if (!cgroup_lock_live_group(css->cgroup))
 		return -ENODEV;
-	mutex_lock(&cgroup_root_mutex);
 	spin_lock(&release_agent_path_lock);
 	strcpy(css->cgroup->root->release_agent_path, buffer);
 	spin_unlock(&release_agent_path_lock);
-	mutex_unlock(&cgroup_root_mutex);
 	mutex_unlock(&cgroup_mutex);
 	return 0;
 }
@@ -4588,7 +4550,6 @@ int __init cgroup_init(void)
 
 	/* allocate id for the dummy hierarchy */
 	mutex_lock(&cgroup_mutex);
-	mutex_lock(&cgroup_root_mutex);
 
 	/* Add init_css_set to the hash table */
 	key = css_set_hash(init_css_set.subsys);
@@ -4600,7 +4561,6 @@ int __init cgroup_init(void)
 			0, 1, GFP_KERNEL);
 	BUG_ON(err < 0);
 
-	mutex_unlock(&cgroup_root_mutex);
 	mutex_unlock(&cgroup_mutex);
 
 	cgroup_kobj = kobject_create_and_add("cgroup", fs_kobj);