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Commit 3417ae1f authored by Tejun Heo's avatar Tejun Heo
Browse files

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: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarLi Zefan <lizefan@huawei.com>
parent 69e943b7

kernel/cgroup.c

+1 −41
Original line number Diff line number Diff line
@@ -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);