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Commit 6be96a5c authored by Li Zefan's avatar Li Zefan Committed by Tejun Heo
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cgroup: remove hierarchy_mutex 



It was introduced for memcg to iterate cgroup hierarchy without
holding cgroup_mutex, but soon after that it was replaced with
a lockless way in memcg.

No one used hierarchy_mutex since that, so remove it.

Signed-off-by: default avatarLi Zefan <lizefan@huawei.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 967db0ea

Documentation/cgroups/cgroups.txt

+1 −1
Original line number Diff line number Diff line
@@ -656,7 +656,7 @@ example in cpusets, no task may attach before 'cpus' and 'mems' are set 
up.



void bind(struct cgroup *root)

(cgroup_mutex and ss->hierarchy_mutex held by caller)

(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

include/linux/cgroup.h

+2 −15
Original line number Diff line number Diff line
@@ -499,22 +499,9 @@ struct cgroup_subsys { 
#define MAX_CGROUP_TYPE_NAMELEN 32

	const char *name;



	/*

	 * Protects sibling/children links of cgroups in this

	 * hierarchy, plus protects which hierarchy (or none) the

	 * subsystem is a part of (i.e. root/sibling).  To avoid

	 * potential deadlocks, the following operations should not be

	 * undertaken while holding any hierarchy_mutex:

	 *

	 * - allocating memory

	 * - initiating hotplug events

	 */

	struct mutex hierarchy_mutex;

	struct lock_class_key subsys_key;



	/*

	 * Link to parent, and list entry in parent's children.

	 * Protected by this->hierarchy_mutex and cgroup_lock()

	 * Protected by cgroup_lock()

	 */

	struct cgroupfs_root *root;

	struct list_head sibling;
@@ -602,7 +589,7 @@ int cgroup_attach_task_all(struct task_struct *from, struct task_struct *); 
 * the lifetime of cgroup_subsys_state is subsys's matter.

 *

 * Looking up and scanning function should be called under rcu_read_lock().

 * Taking cgroup_mutex()/hierarchy_mutex() is not necessary for following calls.

 * Taking cgroup_mutex is not necessary for following calls.

 * But the css returned by this routine can be "not populated yet" or "being

 * destroyed". The caller should check css and cgroup's status.

 */

kernel/cgroup.c

+0 −45
Original line number Diff line number Diff line
@@ -1073,28 +1073,24 @@ static int rebind_subsystems(struct cgroupfs_root *root, 
			BUG_ON(cgrp->subsys[i]);

			BUG_ON(!dummytop->subsys[i]);

			BUG_ON(dummytop->subsys[i]->cgroup != dummytop);

			mutex_lock(&ss->hierarchy_mutex);

			cgrp->subsys[i] = dummytop->subsys[i];

			cgrp->subsys[i]->cgroup = cgrp;

			list_move(&ss->sibling, &root->subsys_list);

			ss->root = root;

			if (ss->bind)

				ss->bind(cgrp);

			mutex_unlock(&ss->hierarchy_mutex);

			/* refcount was already taken, and we're keeping it */

		} else if (bit & removed_bits) {

			/* We're removing this subsystem */

			BUG_ON(ss == NULL);

			BUG_ON(cgrp->subsys[i] != dummytop->subsys[i]);

			BUG_ON(cgrp->subsys[i]->cgroup != cgrp);

			mutex_lock(&ss->hierarchy_mutex);

			if (ss->bind)

				ss->bind(dummytop);

			dummytop->subsys[i]->cgroup = dummytop;

			cgrp->subsys[i] = NULL;

			subsys[i]->root = &rootnode;

			list_move(&ss->sibling, &rootnode.subsys_list);

			mutex_unlock(&ss->hierarchy_mutex);

			/* subsystem is now free - drop reference on module */

			module_put(ss->module);

		} else if (bit & final_bits) {
@@ -3917,37 +3913,6 @@ static void init_cgroup_css(struct cgroup_subsys_state *css, 
		set_bit(CSS_CLEAR_CSS_REFS, &css->flags);

}



static void cgroup_lock_hierarchy(struct cgroupfs_root *root)

{

	/* We need to take each hierarchy_mutex in a consistent order */

	int i;



	/*

	 * No worry about a race with rebind_subsystems that might mess up the

	 * locking order, since both parties are under cgroup_mutex.

	 */

	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {

		struct cgroup_subsys *ss = subsys[i];

		if (ss == NULL)

			continue;

		if (ss->root == root)

			mutex_lock(&ss->hierarchy_mutex);

	}

}



static void cgroup_unlock_hierarchy(struct cgroupfs_root *root)

{

	int i;



	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {

		struct cgroup_subsys *ss = subsys[i];

		if (ss == NULL)

			continue;

		if (ss->root == root)

			mutex_unlock(&ss->hierarchy_mutex);

	}

}



/*

 * cgroup_create - create a cgroup

 * @parent: cgroup that will be parent of the new cgroup
@@ -4008,9 +3973,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry, 
			ss->post_clone(cgrp);

	}



	cgroup_lock_hierarchy(root);

	list_add(&cgrp->sibling, &cgrp->parent->children);

	cgroup_unlock_hierarchy(root);

	root->number_of_cgroups++;



	err = cgroup_create_dir(cgrp, dentry, mode);
@@ -4037,9 +4000,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry, 


 err_remove:



	cgroup_lock_hierarchy(root);

	list_del(&cgrp->sibling);

	cgroup_unlock_hierarchy(root);

	root->number_of_cgroups--;



 err_destroy:
@@ -4247,10 +4208,8 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry) 
		list_del_init(&cgrp->release_list);

	raw_spin_unlock(&release_list_lock);



	cgroup_lock_hierarchy(cgrp->root);

	/* delete this cgroup from parent->children */

	list_del_init(&cgrp->sibling);

	cgroup_unlock_hierarchy(cgrp->root);



	list_del_init(&cgrp->allcg_node);
@@ -4324,8 +4283,6 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss) 
	 * need to invoke fork callbacks here. */

	BUG_ON(!list_empty(&init_task.tasks));



	mutex_init(&ss->hierarchy_mutex);

	lockdep_set_class(&ss->hierarchy_mutex, &ss->subsys_key);

	ss->active = 1;



	/* this function shouldn't be used with modular subsystems, since they
@@ -4452,8 +4409,6 @@ int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss) 
	}

	write_unlock(&css_set_lock);



	mutex_init(&ss->hierarchy_mutex);

	lockdep_set_class(&ss->hierarchy_mutex, &ss->subsys_key);

	ss->active = 1;



	/* success! */