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: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/Documentation/cgroups/cgroups.txt b/Documentation/cgroups/cgroups.txt
index 8e74980..e86faae 100644 (file)
--- a/Documentation/cgroups/cgroups.txt
+++ b/Documentation/cgroups/cgroups.txt
@@ -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

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index d3f5fba..c90eaa8 100644 (file)
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -500,21 +500,8 @@ struct cgroup_subsys {
        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.
  */

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index ceeafe8..dec62f5 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -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 @@ again:
                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! */