--- a/include/linux/cgroup.h	Thu Apr 09 12:06:38 2009 +0200
+++ b/include/linux/cgroup.h	Thu Apr 09 12:07:21 2009 +0200
@@ -52,9 +52,9 @@
 	 * hierarchy structure */
 	struct cgroup *cgroup;
 
-	/* State maintained by the cgroup system to allow
-	 * subsystems to be "busy". Should be accessed via css_get()
-	 * and css_put() */
+	/* State maintained by the cgroup system to allow subsystems
+	 * to be "busy". Should be accessed via css_get(),
+	 * css_tryget() and and css_put(). */
 
 	atomic_t refcnt;
 
@@ -64,11 +64,14 @@
 /* bits in struct cgroup_subsys_state flags field */
 enum {
 	CSS_ROOT, /* This CSS is the root of the subsystem */
+	CSS_REMOVED, /* This CSS is dead */
 };
 
 /*
- * Call css_get() to hold a reference on the cgroup;
- *
+ * Call css_get() to hold a reference on the css; it can be used
+ * for a reference obtained via:
+ * - an existing ref-counted reference to the css
+ * - task->cgroups for a locked task
  */
 
 static inline void css_get(struct cgroup_subsys_state *css)
@@ -77,9 +80,33 @@
 	if (!test_bit(CSS_ROOT, &css->flags))
 		atomic_inc(&css->refcnt);
 }
+
+static inline bool css_is_removed(struct cgroup_subsys_state *css)
+{
+	return test_bit(CSS_REMOVED, &css->flags);
+}
+
+/*
+ * Call css_tryget() to take a reference on a css if your existing
+ * (known-valid) reference isn't already ref-counted. Returns false if
+ * the css has been destroyed.
+ */
+
+static inline bool css_tryget(struct cgroup_subsys_state *css)
+{
+	if (test_bit(CSS_ROOT, &css->flags))
+		return true;
+	while (!atomic_inc_not_zero(&css->refcnt)) {
+		if (test_bit(CSS_REMOVED, &css->flags))
+			return false;
+		cpu_relax();
+	}
+	return true;
+}
+
 /*
  * css_put() should be called to release a reference taken by
- * css_get()
+ * css_get() or css_tryget()
  */
 
 extern void __css_put(struct cgroup_subsys_state *css);
@@ -116,7 +143,7 @@
 	struct list_head children;	/* my children */
 
 	struct cgroup *parent;	/* my parent */
-	struct dentry *dentry;	  	/* cgroup fs entry */
+	struct dentry *dentry;	  	/* cgroup fs entry, RCU protected */
 
 	/* Private pointers for each registered subsystem */
 	struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];
@@ -145,6 +172,9 @@
 	int pids_use_count;
 	/* Length of the current tasks_pids array */
 	int pids_length;
+
+	/* For RCU-protected deletion */
+	struct rcu_head rcu_head;
 };
 
 /* A css_set is a structure holding pointers to a set of
@@ -329,13 +359,7 @@
 			struct cgroup *cgrp);
 	void (*post_clone)(struct cgroup_subsys *ss, struct cgroup *cgrp);
 	void (*bind)(struct cgroup_subsys *ss, struct cgroup *root);
-	/*
-	 * This routine is called with the task_lock of mm->owner held
-	 */
-	void (*mm_owner_changed)(struct cgroup_subsys *ss,
-					struct cgroup *old,
-					struct cgroup *new,
-					struct task_struct *p);
+
 	int subsys_id;
 	int active;
 	int disabled;
@@ -343,9 +367,24 @@
 #define MAX_CGROUP_TYPE_NAMELEN 32
 	const char *name;
 
-	/* Protected by RCU */
+	/*
+	 * 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()
+	 */
 	struct cgroupfs_root *root;
-
 	struct list_head sibling;
 };
 
@@ -400,9 +439,6 @@
 int cgroup_scan_tasks(struct cgroup_scanner *scan);
 int cgroup_attach_task(struct cgroup *, struct task_struct *);
 
-void cgroup_mm_owner_callbacks(struct task_struct *old,
-			       struct task_struct *new);
-
 #else /* !CONFIG_CGROUPS */
 
 static inline int cgroup_init_early(void) { return 0; }
@@ -420,9 +456,6 @@
 	return -EINVAL;
 }
 
-static inline void cgroup_mm_owner_callbacks(struct task_struct *old,
-					     struct task_struct *new) {}
-
 #endif /* !CONFIG_CGROUPS */
 
 #endif /* _LINUX_CGROUP_H */