cgroups: add a per-subsystem hierarchy_mutex

These patches introduce new locking/refcount support for cgroups to
reduce the need for subsystems to call cgroup_lock(). This will
ultimately allow the atomicity of cgroup_rmdir() (which was removed
recently) to be restored.

These three patches give:

1/3 - introduce a per-subsystem hierarchy_mutex which a subsystem can
     use to prevent changes to its own cgroup tree

2/3 - use hierarchy_mutex in place of calling cgroup_lock() in the
     memory controller

3/3 - introduce a css_tryget() function similar to the one recently
      proposed by Kamezawa, but avoiding spurious refcount failures in
      the event of a race between a css_tryget() and an unsuccessful
      cgroup_rmdir()

Future patches will likely involve:

- using hierarchy mutex in place of cgroup_lock() in more subsystems
 where appropriate

- restoring the atomicity of cgroup_rmdir() with respect to cgroup_create()

This patch:

Add a hierarchy_mutex to the cgroup_subsys object that protects changes to
the hierarchy observed by that subsystem.  It is taken by the cgroup
subsystem (in addition to cgroup_mutex) for the following operations:

- linking a cgroup into that subsystem's cgroup tree
- unlinking a cgroup from that subsystem's cgroup tree
- moving the subsystem to/from a hierarchy (including across the
  bind() callback)

Thus if the subsystem holds its own hierarchy_mutex, it can safely
traverse its own hierarchy.

Signed-off-by: Paul Menage <menage@google.com>
Tested-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 52 insertions, 4 deletions

diff --git a/Documentation/cgroups/cgroups.txt b/Documentation/cgroups/cgroups.txt
index 60287e9e9d27..e33ee74eee77 100644
--- a/Documentation/cgroups/cgroups.txt
+++ b/Documentation/cgroups/cgroups.txt
@@ -528,7 +528,7 @@ example in cpusets, no task may attach before 'cpus' and 'mems' are set
 up.
 
 void bind(struct cgroup_subsys *ss, struct cgroup *root)
-(cgroup_mutex held by caller)
+(cgroup_mutex and ss->hierarchy_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 73d1c730c3c4..ce1c1f34c30c 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -340,8 +340,23 @@ struct cgroup_subsys {
 #define MAX_CGROUP_TYPE_NAMELEN 32
         const char *name;
 
-        struct cgroupfs_root *root;
+        /*
+         * 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;
 
+        /*
+         * Link to parent, and list entry in parent's children.
+         * Protected by this->hierarchy_mutex and cgroup_lock()
+         */
+        struct cgroupfs_root *root;
         struct list_head sibling;
 };
 
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 83ea4f524be5..8b6379cdf637 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -722,23 +722,26 @@ 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(ss, cgrp);
-
+                        mutex_unlock(&ss->hierarchy_mutex);
                 } else if (bit & removed_bits) {
                         /* We're removing this subsystem */
                         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(ss, 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);
                 } else if (bit & final_bits) {
                         /* Subsystem state should already exist */
                         BUG_ON(!cgrp->subsys[i]);
@@ -2338,6 +2341,29 @@ static void init_cgroup_css(struct cgroup_subsys_state *css,
         cgrp->subsys[ss->subsys_id] = css;
 }
 
+static void cgroup_lock_hierarchy(struct cgroupfs_root *root)
+{
+        /* We need to take each hierarchy_mutex in a consistent order */
+        int i;
+
+        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+                struct cgroup_subsys *ss = subsys[i];
+                if (ss->root == root)
+                        mutex_lock_nested(&ss->hierarchy_mutex, i);
+        }
+}
+
+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->root == root)
+                        mutex_unlock(&ss->hierarchy_mutex);
+        }
+}
+
 /*
  * cgroup_create - create a cgroup
  * @parent: cgroup that will be parent of the new cgroup
@@ -2386,7 +2412,9 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
                 init_cgroup_css(css, ss, 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);
@@ -2504,8 +2532,12 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
         if (!list_empty(&cgrp->release_list))
                 list_del(&cgrp->release_list);
         spin_unlock(&release_list_lock);
-        /* delete my sibling from parent->children */
+
+        cgroup_lock_hierarchy(cgrp->root);
+        /* delete this cgroup from parent->children */
         list_del(&cgrp->sibling);
+        cgroup_unlock_hierarchy(cgrp->root);
+
         spin_lock(&cgrp->dentry->d_lock);
         d = dget(cgrp->dentry);
         spin_unlock(&d->d_lock);
@@ -2547,6 +2579,7 @@ 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);
         ss->active = 1;
 }