Merge branch 'for-3.3' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup

* 'for-3.3' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup: (21 commits)
  cgroup: fix to allow mounting a hierarchy by name
  cgroup: move assignement out of condition in cgroup_attach_proc()
  cgroup: Remove task_lock() from cgroup_post_fork()
  cgroup: add sparse annotation to cgroup_iter_start() and cgroup_iter_end()
  cgroup: mark cgroup_rmdir_waitq and cgroup_attach_proc() as static
  cgroup: only need to check oldcgrp==newgrp once
  cgroup: remove redundant get/put of task struct
  cgroup: remove redundant get/put of old css_set from migrate
  cgroup: Remove unnecessary task_lock before fetching css_set on migration
  cgroup: Drop task_lock(parent) on cgroup_fork()
  cgroups: remove redundant get/put of css_set from css_set_check_fetched()
  resource cgroups: remove bogus cast
  cgroup: kill subsys->can_attach_task(), pre_attach() and attach_task()
  cgroup, cpuset: don't use ss->pre_attach()
  cgroup: don't use subsys->can_attach_task() or ->attach_task()
  cgroup: introduce cgroup_taskset and use it in subsys->can_attach(), cancel_attach() and attach()
  cgroup: improve old cgroup handling in cgroup_attach_proc()
  cgroup: always lock threadgroup during migration
  threadgroup: extend threadgroup_lock() to cover exit and exec
  threadgroup: rename signal->threadgroup_fork_lock to ->group_rwsem
  ...

Fix up conflict in kernel/cgroup.c due to commit e0197aae59e5: "cgroups:
fix a css_set not found bug in cgroup_attach_proc" that already
mentioned that the bug is fixed (differently) in Tejun's cgroup
patchset. This one, in other words.

15 files changed, 470 insertions, 349 deletions

diff --git a/Documentation/cgroups/cgroups.txt b/Documentation/cgroups/cgroups.txt
index 9c452ef2328c..a7c96ae5557c 100644
--- a/Documentation/cgroups/cgroups.txt
+++ b/Documentation/cgroups/cgroups.txt
@@ -594,53 +594,44 @@ rmdir() will fail with it. From this behavior, pre_destroy() can be
 called multiple times against a cgroup.
 
 int can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
-	       struct task_struct *task)
+	       struct cgroup_taskset *tset)
 (cgroup_mutex held by caller)
 
-Called prior to moving a task into a cgroup; if the subsystem
-returns an error, this will abort the attach operation.  If a NULL
-task is passed, then a successful result indicates that *any*
-unspecified task can be moved into the cgroup. Note that this isn't
-called on a fork. If this method returns 0 (success) then this should
-remain valid while the caller holds cgroup_mutex and it is ensured that either
+Called prior to moving one or more tasks into a cgroup; if the
+subsystem returns an error, this will abort the attach operation.
+@tset contains the tasks to be attached and is guaranteed to have at
+least one task in it.
+
+If there are multiple tasks in the taskset, then:
+  - it's guaranteed that all are from the same thread group
+  - @tset contains all tasks from the thread group whether or not
+    they're switching cgroups
+  - the first task is the leader
+
+Each @tset entry also contains the task's old cgroup and tasks which
+aren't switching cgroup can be skipped easily using the
+cgroup_taskset_for_each() iterator. Note that this isn't called on a
+fork. If this method returns 0 (success) then this should remain valid
+while the caller holds cgroup_mutex and it is ensured that either
 attach() or cancel_attach() will be called in future.
 
-int can_attach_task(struct cgroup *cgrp, struct task_struct *tsk);
-(cgroup_mutex held by caller)
-
-As can_attach, but for operations that must be run once per task to be
-attached (possibly many when using cgroup_attach_proc). Called after
-can_attach.
-
 void cancel_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
-	       struct task_struct *task, bool threadgroup)
+		   struct cgroup_taskset *tset)
 (cgroup_mutex held by caller)
 
 Called when a task attach operation has failed after can_attach() has succeeded.
 A subsystem whose can_attach() has some side-effects should provide this
 function, so that the subsystem can implement a rollback. If not, not necessary.
 This will be called only about subsystems whose can_attach() operation have
-succeeded.
-
-void pre_attach(struct cgroup *cgrp);
-(cgroup_mutex held by caller)
-
-For any non-per-thread attachment work that needs to happen before
-attach_task. Needed by cpuset.
+succeeded. The parameters are identical to can_attach().
 
 void attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
-	    struct cgroup *old_cgrp, struct task_struct *task)
+	    struct cgroup_taskset *tset)
 (cgroup_mutex held by caller)
 
 Called after the task has been attached to the cgroup, to allow any
 post-attachment activity that requires memory allocations or blocking.
-
-void attach_task(struct cgroup *cgrp, struct task_struct *tsk);
-(cgroup_mutex held by caller)
-
-As attach, but for operations that must be run once per task to be attached,
-like can_attach_task. Called before attach. Currently does not support any
-subsystem that might need the old_cgrp for every thread in the group.
+The parameters are identical to can_attach().
 
 void fork(struct cgroup_subsy *ss, struct task_struct *task)
 
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 8f630cec906e..b8c143d68ee0 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -30,8 +30,10 @@ EXPORT_SYMBOL_GPL(blkio_root_cgroup);
 
 static struct cgroup_subsys_state *blkiocg_create(struct cgroup_subsys *,
 						  struct cgroup *);
-static int blkiocg_can_attach_task(struct cgroup *, struct task_struct *);
-static void blkiocg_attach_task(struct cgroup *, struct task_struct *);
+static int blkiocg_can_attach(struct cgroup_subsys *, struct cgroup *,
+			      struct cgroup_taskset *);
+static void blkiocg_attach(struct cgroup_subsys *, struct cgroup *,
+			   struct cgroup_taskset *);
 static void blkiocg_destroy(struct cgroup_subsys *, struct cgroup *);
 static int blkiocg_populate(struct cgroup_subsys *, struct cgroup *);
 
@@ -44,8 +46,8 @@ static int blkiocg_populate(struct cgroup_subsys *, struct cgroup *);
 struct cgroup_subsys blkio_subsys = {
 	.name = "blkio",
 	.create = blkiocg_create,
-	.can_attach_task = blkiocg_can_attach_task,
-	.attach_task = blkiocg_attach_task,
+	.can_attach = blkiocg_can_attach,
+	.attach = blkiocg_attach,
 	.destroy = blkiocg_destroy,
 	.populate = blkiocg_populate,
 #ifdef CONFIG_BLK_CGROUP
@@ -1626,30 +1628,39 @@ done:
  * of the main cic data structures.  For now we allow a task to change
  * its cgroup only if it's the only owner of its ioc.
  */
-static int blkiocg_can_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
+static int blkiocg_can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
+			      struct cgroup_taskset *tset)
 {
+	struct task_struct *task;
 	struct io_context *ioc;
 	int ret = 0;
 
 	/* task_lock() is needed to avoid races with exit_io_context() */
-	task_lock(tsk);
-	ioc = tsk->io_context;
-	if (ioc && atomic_read(&ioc->nr_tasks) > 1)
-		ret = -EINVAL;
-	task_unlock(tsk);
-
+	cgroup_taskset_for_each(task, cgrp, tset) {
+		task_lock(task);
+		ioc = task->io_context;
+		if (ioc && atomic_read(&ioc->nr_tasks) > 1)
+			ret = -EINVAL;
+		task_unlock(task);
+		if (ret)
+			break;
+	}
 	return ret;
 }
 
-static void blkiocg_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
+static void blkiocg_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
+			   struct cgroup_taskset *tset)
 {
+	struct task_struct *task;
 	struct io_context *ioc;
 
-	task_lock(tsk);
-	ioc = tsk->io_context;
-	if (ioc)
-		ioc->cgroup_changed = 1;
-	task_unlock(tsk);
+	cgroup_taskset_for_each(task, cgrp, tset) {
+		task_lock(task);
+		ioc = task->io_context;
+		if (ioc)
+			ioc->cgroup_changed = 1;
+		task_unlock(task);
+	}
 }
 
 void blkio_policy_register(struct blkio_policy_type *blkiop)
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index a17becc36ca1..e9b602151caf 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -457,6 +457,28 @@ void cgroup_exclude_rmdir(struct cgroup_subsys_state *css);
 void cgroup_release_and_wakeup_rmdir(struct cgroup_subsys_state *css);
 
 /*
+ * Control Group taskset, used to pass around set of tasks to cgroup_subsys
+ * methods.
+ */
+struct cgroup_taskset;
+struct task_struct *cgroup_taskset_first(struct cgroup_taskset *tset);
+struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset);
+struct cgroup *cgroup_taskset_cur_cgroup(struct cgroup_taskset *tset);
+int cgroup_taskset_size(struct cgroup_taskset *tset);
+
+/**
+ * cgroup_taskset_for_each - iterate cgroup_taskset
+ * @task: the loop cursor
+ * @skip_cgrp: skip if task's cgroup matches this, %NULL to iterate through all
+ * @tset: taskset to iterate
+ */
+#define cgroup_taskset_for_each(task, skip_cgrp, tset)			\
+	for ((task) = cgroup_taskset_first((tset)); (task);		\
+	     (task) = cgroup_taskset_next((tset)))			\
+		if (!(skip_cgrp) ||					\
+		    cgroup_taskset_cur_cgroup((tset)) != (skip_cgrp))
+
+/*
  * Control Group subsystem type.
  * See Documentation/cgroups/cgroups.txt for details
  */
@@ -467,14 +489,11 @@ struct cgroup_subsys {
 	int (*pre_destroy)(struct cgroup_subsys *ss, struct cgroup *cgrp);
 	void (*destroy)(struct cgroup_subsys *ss, struct cgroup *cgrp);
 	int (*can_attach)(struct cgroup_subsys *ss, struct cgroup *cgrp,
-			  struct task_struct *tsk);
-	int (*can_attach_task)(struct cgroup *cgrp, struct task_struct *tsk);
+			  struct cgroup_taskset *tset);
 	void (*cancel_attach)(struct cgroup_subsys *ss, struct cgroup *cgrp,
-			      struct task_struct *tsk);
-	void (*pre_attach)(struct cgroup *cgrp);
-	void (*attach_task)(struct cgroup *cgrp, struct task_struct *tsk);
+			      struct cgroup_taskset *tset);
 	void (*attach)(struct cgroup_subsys *ss, struct cgroup *cgrp,
-		       struct cgroup *old_cgrp, struct task_struct *tsk);
+		       struct cgroup_taskset *tset);
 	void (*fork)(struct cgroup_subsys *ss, struct task_struct *task);
 	void (*exit)(struct cgroup_subsys *ss, struct cgroup *cgrp,
 			struct cgroup *old_cgrp, struct task_struct *task);
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 32574eef9394..9c66b1ada9d7 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -23,11 +23,10 @@ extern struct files_struct init_files;
 extern struct fs_struct init_fs;
 
 #ifdef CONFIG_CGROUPS
-#define INIT_THREADGROUP_FORK_LOCK(sig)					\
-	.threadgroup_fork_lock =					\
-		__RWSEM_INITIALIZER(sig.threadgroup_fork_lock),
+#define INIT_GROUP_RWSEM(sig)						\
+	.group_rwsem = __RWSEM_INITIALIZER(sig.group_rwsem),
 #else
-#define INIT_THREADGROUP_FORK_LOCK(sig)
+#define INIT_GROUP_RWSEM(sig)
 #endif
 
 #define INIT_SIGNALS(sig) {						\
@@ -46,7 +45,7 @@ extern struct fs_struct init_fs;
 	},								\
 	.cred_guard_mutex =						\
 		 __MUTEX_INITIALIZER(sig.cred_guard_mutex),		\
-	INIT_THREADGROUP_FORK_LOCK(sig)					\
+	INIT_GROUP_RWSEM(sig)						\
 }
 
 extern struct nsproxy init_nsproxy;
diff --git a/include/linux/sched.h b/include/linux/sched.h
index ad93e1ec8c65..f044f66018f2 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -637,13 +637,15 @@ struct signal_struct {
 #endif
 #ifdef CONFIG_CGROUPS
 	/*
-	 * The threadgroup_fork_lock prevents threads from forking with
-	 * CLONE_THREAD while held for writing. Use this for fork-sensitive
-	 * threadgroup-wide operations. It's taken for reading in fork.c in
-	 * copy_process().
-	 * Currently only needed write-side by cgroups.
+	 * group_rwsem prevents new tasks from entering the threadgroup and
+	 * member tasks from exiting,a more specifically, setting of
+	 * PF_EXITING.  fork and exit paths are protected with this rwsem
+	 * using threadgroup_change_begin/end().  Users which require
+	 * threadgroup to remain stable should use threadgroup_[un]lock()
+	 * which also takes care of exec path.  Currently, cgroup is the
+	 * only user.
 	 */
-	struct rw_semaphore threadgroup_fork_lock;
+	struct rw_semaphore group_rwsem;
 #endif
 
 	int oom_adj;		/* OOM kill score adjustment (bit shift) */
@@ -2394,29 +2396,62 @@ static inline void unlock_task_sighand(struct task_struct *tsk,
 	spin_unlock_irqrestore(&tsk->sighand->siglock, *flags);
 }
 
-/* See the declaration of threadgroup_fork_lock in signal_struct. */
 #ifdef CONFIG_CGROUPS
-static inline void threadgroup_fork_read_lock(struct task_struct *tsk)
+static inline void threadgroup_change_begin(struct task_struct *tsk)
 {
-	down_read(&tsk->signal->threadgroup_fork_lock);
+	down_read(&tsk->signal->group_rwsem);
 }
-static inline void threadgroup_fork_read_unlock(struct task_struct *tsk)
+static inline void threadgroup_change_end(struct task_struct *tsk)
 {
-	up_read(&tsk->signal->threadgroup_fork_lock);
+	up_read(&tsk->signal->group_rwsem);
 }
-static inline void threadgroup_fork_write_lock(struct task_struct *tsk)
+
+/**
+ * threadgroup_lock - lock threadgroup
+ * @tsk: member task of the threadgroup to lock
+ *
+ * Lock the threadgroup @tsk belongs to.  No new task is allowed to enter
+ * and member tasks aren't allowed to exit (as indicated by PF_EXITING) or
+ * perform exec.  This is useful for cases where the threadgroup needs to
+ * stay stable across blockable operations.
+ *
+ * fork and exit paths explicitly call threadgroup_change_{begin|end}() for
+ * synchronization.  While held, no new task will be added to threadgroup
+ * and no existing live task will have its PF_EXITING set.
+ *
+ * During exec, a task goes and puts its thread group through unusual
+ * changes.  After de-threading, exclusive access is assumed to resources
+ * which are usually shared by tasks in the same group - e.g. sighand may
+ * be replaced with a new one.  Also, the exec'ing task takes over group
+ * leader role including its pid.  Exclude these changes while locked by
+ * grabbing cred_guard_mutex which is used to synchronize exec path.
+ */
+static inline void threadgroup_lock(struct task_struct *tsk)
 {
-	down_write(&tsk->signal->threadgroup_fork_lock);
+	/*
+	 * exec uses exit for de-threading nesting group_rwsem inside
+	 * cred_guard_mutex. Grab cred_guard_mutex first.
+	 */
+	mutex_lock(&tsk->signal->cred_guard_mutex);
+	down_write(&tsk->signal->group_rwsem);
 }
-static inline void threadgroup_fork_write_unlock(struct task_struct *tsk)
+
+/**
+ * threadgroup_unlock - unlock threadgroup
+ * @tsk: member task of the threadgroup to unlock
+ *
+ * Reverse threadgroup_lock().
+ */
+static inline void threadgroup_unlock(struct task_struct *tsk)
 {
-	up_write(&tsk->signal->threadgroup_fork_lock);
+	up_write(&tsk->signal->group_rwsem);
+	mutex_unlock(&tsk->signal->cred_guard_mutex);
 }
 #else
-static inline void threadgroup_fork_read_lock(struct task_struct *tsk) {}
-static inline void threadgroup_fork_read_unlock(struct task_struct *tsk) {}
-static inline void threadgroup_fork_write_lock(struct task_struct *tsk) {}
-static inline void threadgroup_fork_write_unlock(struct task_struct *tsk) {}
+static inline void threadgroup_change_begin(struct task_struct *tsk) {}
+static inline void threadgroup_change_end(struct task_struct *tsk) {}
+static inline void threadgroup_lock(struct task_struct *tsk) {}
+static inline void threadgroup_unlock(struct task_struct *tsk) {}
 #endif
 
 #ifndef __HAVE_THREAD_FUNCTIONS
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 7cab65f83f1d..a5d3b5325f77 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -63,7 +63,24 @@
 
 #include <linux/atomic.h>
 
+/*
+ * 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.
+ */
 static DEFINE_MUTEX(cgroup_mutex);
+static DEFINE_MUTEX(cgroup_root_mutex);
 
 /*
  * Generate an array of cgroup subsystem pointers. At boot time, this is
@@ -921,7 +938,7 @@ static void cgroup_d_remove_dir(struct dentry *dentry)
  *
  * CGRP_WAIT_ON_RMDIR flag is set under cgroup's inode->i_mutex;
  */
-DECLARE_WAIT_QUEUE_HEAD(cgroup_rmdir_waitq);
+static DECLARE_WAIT_QUEUE_HEAD(cgroup_rmdir_waitq);
 
 static void cgroup_wakeup_rmdir_waiter(struct cgroup *cgrp)
 {
@@ -953,6 +970,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
 	int i;
 
 	BUG_ON(!mutex_is_locked(&cgroup_mutex));
+	BUG_ON(!mutex_is_locked(&cgroup_root_mutex));
 
 	removed_bits = root->actual_subsys_bits & ~final_bits;
 	added_bits = final_bits & ~root->actual_subsys_bits;
@@ -1043,7 +1061,7 @@ static int cgroup_show_options(struct seq_file *seq, struct dentry *dentry)
 	struct cgroupfs_root *root = dentry->d_sb->s_fs_info;
 	struct cgroup_subsys *ss;
 
-	mutex_lock(&cgroup_mutex);
+	mutex_lock(&cgroup_root_mutex);
 	for_each_subsys(root, ss)
 		seq_printf(seq, ",%s", ss->name);
 	if (test_bit(ROOT_NOPREFIX, &root->flags))
@@ -1054,7 +1072,7 @@ static int cgroup_show_options(struct seq_file *seq, struct dentry *dentry)
 		seq_puts(seq, ",clone_children");
 	if (strlen(root->name))
 		seq_printf(seq, ",name=%s", root->name);
-	mutex_unlock(&cgroup_mutex);
+	mutex_unlock(&cgroup_root_mutex);
 	return 0;
 }
 
@@ -1175,10 +1193,10 @@ static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
 
 	/*
 	 * If the 'all' option was specified select all the subsystems,
-	 * otherwise 'all, 'none' and a subsystem name options were not
-	 * specified, let's default to 'all'
+	 * otherwise if 'none', 'name=' and a subsystem name options
+	 * were not specified, let's default to 'all'
 	 */
-	if (all_ss || (!all_ss && !one_ss && !opts->none)) {
+	if (all_ss || (!one_ss && !opts->none && !opts->name)) {
 		for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
 			struct cgroup_subsys *ss = subsys[i];
 			if (ss == NULL)
@@ -1269,6 +1287,7 @@ 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);
@@ -1297,6 +1316,7 @@ 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;
@@ -1481,6 +1501,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 	int ret = 0;
 	struct super_block *sb;
 	struct cgroupfs_root *new_root;
+	struct inode *inode;
 
 	/* First find the desired set of subsystems */
 	mutex_lock(&cgroup_mutex);
@@ -1514,7 +1535,6 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 		/* We used the new root structure, so this is a new hierarchy */
 		struct list_head tmp_cg_links;
 		struct cgroup *root_cgrp = &root->top_cgroup;
-		struct inode *inode;
 		struct cgroupfs_root *existing_root;
 		const struct cred *cred;
 		int i;
@@ -1528,18 +1548,14 @@ 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);
 
-		if (strlen(root->name)) {
-			/* Check for name clashes with existing mounts */
-			for_each_active_root(existing_root) {
-				if (!strcmp(existing_root->name, root->name)) {
-					ret = -EBUSY;
-					mutex_unlock(&cgroup_mutex);
-					mutex_unlock(&inode->i_mutex);
-					goto drop_new_super;
-				}
-			}
-		}
+		/* Check for name clashes with existing mounts */
+		ret = -EBUSY;
+		if (strlen(root->name))
+			for_each_active_root(existing_root)
+				if (!strcmp(existing_root->name, root->name))
+					goto unlock_drop;
 
 		/*
 		 * We're accessing css_set_count without locking
@@ -1549,18 +1565,13 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 		 * have some link structures left over
 		 */
 		ret = allocate_cg_links(css_set_count, &tmp_cg_links);
-		if (ret) {
-			mutex_unlock(&cgroup_mutex);
-			mutex_unlock(&inode->i_mutex);
-			goto drop_new_super;
-		}
+		if (ret)
+			goto unlock_drop;
 
 		ret = rebind_subsystems(root, root->subsys_bits);
 		if (ret == -EBUSY) {
-			mutex_unlock(&cgroup_mutex);
-			mutex_unlock(&inode->i_mutex);
 			free_cg_links(&tmp_cg_links);
-			goto drop_new_super;
+			goto unlock_drop;
 		}
 		/*
 		 * There must be no failure case after here, since rebinding
@@ -1599,6 +1610,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 		cred = override_creds(&init_cred);
 		cgroup_populate_dir(root_cgrp);
 		revert_creds(cred);
+		mutex_unlock(&cgroup_root_mutex);
 		mutex_unlock(&cgroup_mutex);
 		mutex_unlock(&inode->i_mutex);
 	} else {
@@ -1615,6 +1627,10 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 	kfree(opts.name);
 	return dget(sb->s_root);
 
+ unlock_drop:
+	mutex_unlock(&cgroup_root_mutex);
+	mutex_unlock(&cgroup_mutex);
+	mutex_unlock(&inode->i_mutex);
  drop_new_super:
 	deactivate_locked_super(sb);
  drop_modules:
@@ -1639,6 +1655,7 @@ static void cgroup_kill_sb(struct super_block *sb) {
 	BUG_ON(!list_empty(&cgrp->sibling));
 
 	mutex_lock(&cgroup_mutex);
+	mutex_lock(&cgroup_root_mutex);
 
 	/* Rebind all subsystems back to the default hierarchy */
 	ret = rebind_subsystems(root, 0);
@@ -1664,6 +1681,7 @@ static void cgroup_kill_sb(struct super_block *sb) {
 		root_count--;
 	}
 
+	mutex_unlock(&cgroup_root_mutex);
 	mutex_unlock(&cgroup_mutex);
 
 	kill_litter_super(sb);
@@ -1740,11 +1758,90 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
 EXPORT_SYMBOL_GPL(cgroup_path);
 
 /*
+ * Control Group taskset
+ */
+struct task_and_cgroup {
+	struct task_struct	*task;
+	struct cgroup		*cgrp;
+};
+
+struct cgroup_taskset {
+	struct task_and_cgroup	single;
+	struct flex_array	*tc_array;
+	int			tc_array_len;
+	int			idx;
+	struct cgroup		*cur_cgrp;
+};
+
+/**
+ * cgroup_taskset_first - reset taskset and return the first task
+ * @tset: taskset of interest
+ *
+ * @tset iteration is initialized and the first task is returned.
+ */
+struct task_struct *cgroup_taskset_first(struct cgroup_taskset *tset)
+{
+	if (tset->tc_array) {
+		tset->idx = 0;
+		return cgroup_taskset_next(tset);
+	} else {
+		tset->cur_cgrp = tset->single.cgrp;
+		return tset->single.task;
+	}
+}
+EXPORT_SYMBOL_GPL(cgroup_taskset_first);
+
+/**
+ * cgroup_taskset_next - iterate to the next task in taskset
+ * @tset: taskset of interest
+ *
+ * Return the next task in @tset.  Iteration must have been initialized
+ * with cgroup_taskset_first().
+ */
+struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset)
+{
+	struct task_and_cgroup *tc;
+
+	if (!tset->tc_array || tset->idx >= tset->tc_array_len)
+		return NULL;
+
+	tc = flex_array_get(tset->tc_array, tset->idx++);
+	tset->cur_cgrp = tc->cgrp;
+	return tc->task;
+}
+EXPORT_SYMBOL_GPL(cgroup_taskset_next);
+
+/**
+ * cgroup_taskset_cur_cgroup - return the matching cgroup for the current task
+ * @tset: taskset of interest
+ *
+ * Return the cgroup for the current (last returned) task of @tset.  This
+ * function must be preceded by either cgroup_taskset_first() or
+ * cgroup_taskset_next().
+ */
+struct cgroup *cgroup_taskset_cur_cgroup(struct cgroup_taskset *tset)
+{
+	return tset->cur_cgrp;
+}
+EXPORT_SYMBOL_GPL(cgroup_taskset_cur_cgroup);
+
+/**
+ * cgroup_taskset_size - return the number of tasks in taskset
+ * @tset: taskset of interest
+ */
+int cgroup_taskset_size(struct cgroup_taskset *tset)
+{
+	return tset->tc_array ? tset->tc_array_len : 1;
+}
+EXPORT_SYMBOL_GPL(cgroup_taskset_size);
+
+
+/*
  * cgroup_task_migrate - move a task from one cgroup to another.
  *
  * 'guarantee' is set if the caller promises that a new css_set for the task
  * will already exist. If not set, this function might sleep, and can fail with
- * -ENOMEM. Otherwise, it can only fail with -ESRCH.
+ * -ENOMEM. Must be called with cgroup_mutex and threadgroup locked.
  */
 static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
 			       struct task_struct *tsk, bool guarantee)
@@ -1753,14 +1850,12 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
 	struct css_set *newcg;
 
 	/*
-	 * get old css_set. we need to take task_lock and refcount it, because
-	 * an exiting task can change its css_set to init_css_set and drop its
-	 * old one without taking cgroup_mutex.
+	 * We are synchronized through threadgroup_lock() against PF_EXITING
+	 * setting such that we can't race against cgroup_exit() changing the
+	 * css_set to init_css_set and dropping the old one.
 	 */
-	task_lock(tsk);
+	WARN_ON_ONCE(tsk->flags & PF_EXITING);
 	oldcg = tsk->cgroups;
-	get_css_set(oldcg);
-	task_unlock(tsk);
 
 	/* locate or allocate a new css_set for this task. */
 	if (guarantee) {
@@ -1775,20 +1870,11 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
 		might_sleep();
 		/* find_css_set will give us newcg already referenced. */
 		newcg = find_css_set(oldcg, cgrp);
-		if (!newcg) {
-			put_css_set(oldcg);
+		if (!newcg)
 			return -ENOMEM;
-		}
 	}
-	put_css_set(oldcg);
 
-	/* if PF_EXITING is set, the tsk->cgroups pointer is no longer safe. */
 	task_lock(tsk);
-	if (tsk->flags & PF_EXITING) {
-		task_unlock(tsk);
-		put_css_set(newcg);
-		return -ESRCH;
-	}
 	rcu_assign_pointer(tsk->cgroups, newcg);
 	task_unlock(tsk);
 
@@ -1814,8 +1900,8 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
  * @cgrp: the cgroup the task is attaching to
  * @tsk: the task to be attached
  *
- * Call holding cgroup_mutex. May take task_lock of
- * the task 'tsk' during call.
+ * Call with cgroup_mutex and threadgroup locked. May take task_lock of
+ * @tsk during call.
  */
 int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 {
@@ -1823,15 +1909,23 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 	struct cgroup_subsys *ss, *failed_ss = NULL;
 	struct cgroup *oldcgrp;
 	struct cgroupfs_root *root = cgrp->root;
+	struct cgroup_taskset tset = { };
+
+	/* @tsk either already exited or can't exit until the end */
+	if (tsk->flags & PF_EXITING)
+		return -ESRCH;
 
 	/* Nothing to do if the task is already in that cgroup */
 	oldcgrp = task_cgroup_from_root(tsk, root);
 	if (cgrp == oldcgrp)
 		return 0;
 
+	tset.single.task = tsk;
+	tset.single.cgrp = oldcgrp;
+
 	for_each_subsys(root, ss) {
 		if (ss->can_attach) {
-			retval = ss->can_attach(ss, cgrp, tsk);
+			retval = ss->can_attach(ss, cgrp, &tset);
 			if (retval) {
 				/*
 				 * Remember on which subsystem the can_attach()
@@ -1843,13 +1937,6 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 				goto out;
 			}
 		}
-		if (ss->can_attach_task) {
-			retval = ss->can_attach_task(cgrp, tsk);
-			if (retval) {
-				failed_ss = ss;
-				goto out;
-			}
-		}
 	}
 
 	retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, false);
@@ -1857,12 +1944,8 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 		goto out;
 
 	for_each_subsys(root, ss) {
-		if (ss->pre_attach)
-			ss->pre_attach(cgrp);
-		if (ss->attach_task)
-			ss->attach_task(cgrp, tsk);
 		if (ss->attach)
-			ss->attach(ss, cgrp, oldcgrp, tsk);
+			ss->attach(ss, cgrp, &tset);
 	}
 
 	synchronize_rcu();
@@ -1884,7 +1967,7 @@ out:
 				 */
 				break;
 			if (ss->cancel_attach)
-				ss->cancel_attach(ss, cgrp, tsk);
+				ss->cancel_attach(ss, cgrp, &tset);
 		}
 	}
 	return retval;
@@ -1935,23 +2018,17 @@ static bool css_set_check_fetched(struct cgroup *cgrp,
 
 	read_lock(&css_set_lock);
 	newcg = find_existing_css_set(cg, cgrp, template);
-	if (newcg)
-		get_css_set(newcg);
 	read_unlock(&css_set_lock);
 
 	/* doesn't exist at all? */
 	if (!newcg)
 		return false;
 	/* see if it's already in the list */
-	list_for_each_entry(cg_entry, newcg_list, links) {
-		if (cg_entry->cg == newcg) {
-			put_css_set(newcg);
+	list_for_each_entry(cg_entry, newcg_list, links)
+		if (cg_entry->cg == newcg)
 			return true;
-		}
-	}
 
 	/* not found */
-	put_css_set(newcg);
 	return false;
 }
 
@@ -1985,21 +2062,21 @@ static int css_set_prefetch(struct cgroup *cgrp, struct css_set *cg,
  * @cgrp: the cgroup to attach to
  * @leader: the threadgroup leader task_struct of the group to be attached
  *
- * Call holding cgroup_mutex and the threadgroup_fork_lock of the leader. Will
- * take task_lock of each thread in leader's threadgroup individually in turn.
+ * Call holding cgroup_mutex and the group_rwsem of the leader. Will take
+ * task_lock of each thread in leader's threadgroup individually in turn.
  */
-int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
+static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 {
 	int retval, i, group_size;
 	struct cgroup_subsys *ss, *failed_ss = NULL;
-	bool cancel_failed_ss = false;
 	/* guaranteed to be initialized later, but the compiler needs this */
-	struct cgroup *oldcgrp = NULL;
 	struct css_set *oldcg;
 	struct cgroupfs_root *root = cgrp->root;
 	/* threadgroup list cursor and array */
 	struct task_struct *tsk;
+	struct task_and_cgroup *tc;
 	struct flex_array *group;
+	struct cgroup_taskset tset = { };
 	/*
 	 * we need to make sure we have css_sets for all the tasks we're
 	 * going to move -before- we actually start moving them, so that in
@@ -2012,13 +2089,12 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 	 * step 0: in order to do expensive, possibly blocking operations for
 	 * every thread, we cannot iterate the thread group list, since it needs
 	 * rcu or tasklist locked. instead, build an array of all threads in the
-	 * group - threadgroup_fork_lock prevents new threads from appearing,
-	 * and if threads exit, this will just be an over-estimate.
+	 * group - group_rwsem prevents new threads from appearing, and if
+	 * threads exit, this will just be an over-estimate.
 	 */
 	group_size = get_nr_threads(leader);
 	/* flex_array supports very large thread-groups better than kmalloc. */
-	group = flex_array_alloc(sizeof(struct task_struct *), group_size,
-				 GFP_KERNEL);
+	group = flex_array_alloc(sizeof(*tc), group_size, GFP_KERNEL);
 	if (!group)
 		return -ENOMEM;
 	/* pre-allocate to guarantee space while iterating in rcu read-side. */
@@ -2040,49 +2116,53 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 		retval = -EAGAIN;
 		goto out_free_group_list;
 	}
-	/* take a reference on each task in the group to go in the array. */
+
 	tsk = leader;
 	i = 0;
 	do {
+		struct task_and_cgroup ent;
+
+		/* @tsk either already exited or can't exit until the end */
+		if (tsk->flags & PF_EXITING)
+			continue;
+
 		/* as per above, nr_threads may decrease, but not increase. */
 		BUG_ON(i >= group_size);
-		get_task_struct(tsk);
 		/*
 		 * saying GFP_ATOMIC has no effect here because we did prealloc
 		 * earlier, but it's good form to communicate our expectations.
 		 */
-		retval = flex_array_put_ptr(group, i, tsk, GFP_ATOMIC);
+		ent.task = tsk;
+		ent.cgrp = task_cgroup_from_root(tsk, root);
+		/* nothing to do if this task is already in the cgroup */
+		if (ent.cgrp == cgrp)
+			continue;
+		retval = flex_array_put(group, i, &ent, GFP_ATOMIC);
 		BUG_ON(retval != 0);
 		i++;
 	} while_each_thread(leader, tsk);
 	/* remember the number of threads in the array for later. */
 	group_size = i;
+	tset.tc_array = group;
+	tset.tc_array_len = group_size;
 	read_unlock(&tasklist_lock);
 
+	/* methods shouldn't be called if no task is actually migrating */
+	retval = 0;
+	if (!group_size)
+		goto out_free_group_list;
+
 	/*
 	 * step 1: check that we can legitimately attach to the cgroup.
 	 */
 	for_each_subsys(root, ss) {
 		if (ss->can_attach) {
-			retval = ss->can_attach(ss, cgrp, leader);
+			retval = ss->can_attach(ss, cgrp, &tset);
 			if (retval) {
 				failed_ss = ss;
 				goto out_cancel_attach;
 			}
 		}
-		/* a callback to be run on every thread in the threadgroup. */
-		if (ss->can_attach_task) {
-			/* run on each task in the threadgroup. */
-			for (i = 0; i < group_size; i++) {
-				tsk = flex_array_get_ptr(group, i);
-				retval = ss->can_attach_task(cgrp, tsk);
-				if (retval) {
-					failed_ss = ss;
-					cancel_failed_ss = true;
-					goto out_cancel_attach;
-				}
-			}
-		}
 	}
 
 	/*
@@ -2091,67 +2171,36 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 	 */
 	INIT_LIST_HEAD(&newcg