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.

1 files changed, 231 insertions, 170 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 7cab65f..a5d3b53 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_list);
 	for (i = 0; i < group_size; i++) {
-		tsk = flex_array_get_ptr(group, i);
-		/* nothing to do if this task is already in the cgroup */
-		oldcgrp = task_cgroup_from_root(tsk, root);
-		if (cgrp == oldcgrp)
-			continue;
-		/* get old css_set pointer */
-		task_lock(tsk);
-		oldcg = tsk->cgroups;
-		get_css_set(oldcg);
-		task_unlock(tsk);
-		/* see if the new one for us is already in the list? */
-		if (css_set_check_fetched(cgrp, tsk, oldcg, &newcg_list)) {
-			/* was already there, nothing to do. */
-			put_css_set(oldcg);
-		} else {
-			/* we don't already have it. get new one. */
+		tc = flex_array_get(group, i);
+		oldcg = tc->task->cgroups;
+
+		/* if we don't already have it in the list get a new one */
+		if (!css_set_check_fetched(cgrp, tc->task, oldcg,
+					   &newcg_list)) {
 			retval = css_set_prefetch(cgrp, oldcg, &newcg_list);
-			put_css_set(oldcg);
 			if (retval)
 				goto out_list_teardown;
 		}
 	}
 
 	/*
-	 * step 3: now that we're guaranteed success wrt the css_sets, proceed
-	 * to move all tasks to the new cgroup, calling ss->attach_task for each
-	 * one along the way. there are no failure cases after here, so this is
-	 * the commit point.
+	 * step 3: now that we're guaranteed success wrt the css_sets,
+	 * proceed to move all tasks to the new cgroup.  There are no
+	 * failure cases after here, so this is the commit point.
 	 */
-	for_each_subsys(root, ss) {
-		if (ss->pre_attach)
-			ss->pre_attach(cgrp);
-	}
 	for (i = 0; i < group_size; i++) {
-		tsk = flex_array_get_ptr(group, i);
-		/* leave current thread as it is if it's already there */
-		oldcgrp = task_cgroup_from_root(tsk, root);
-		if (cgrp == oldcgrp)
-			continue;
-		/* if the thread is PF_EXITING, it can just get skipped. */
-		retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, true);
-		if (retval == 0) {
-			/* attach each task to each subsystem */
-			for_each_subsys(root, ss) {
-				if (ss->attach_task)
-					ss->attach_task(cgrp, tsk);
-			}
-		} else {
-			BUG_ON(retval != -ESRCH);
-		}
+		tc = flex_array_get(group, i);
+		retval = cgroup_task_migrate(cgrp, tc->cgrp, tc->task, true);
+		BUG_ON(retval);
 	}
 	/* nothing is sensitive to fork() after this point. */
 
 	/*
-	 * step 4: do expensive, non-thread-specific subsystem callbacks.
-	 * TODO: if ever a subsystem needs to know the oldcgrp for each task
-	 * being moved, this call will need to be reworked to communicate that.
+	 * step 4: do subsystem attach callbacks.
 	 */
 	for_each_subsys(root, ss) {
 		if (ss->attach)
-			ss->attach(ss, cgrp, oldcgrp, leader);
+			ss->attach(ss, cgrp, &tset);
 	}
 
 	/*
@@ -2171,20 +2220,12 @@ out_cancel_attach:
 	/* same deal as in cgroup_attach_task */
 	if (retval) {
 		for_each_subsys(root, ss) {
-			if (ss == failed_ss) {
-				if (cancel_failed_ss && ss->cancel_attach)
-					ss->cancel_attach(ss, cgrp, leader);
+			if (ss == failed_ss)
 				break;
-			}
 			if (ss->cancel_attach)
-				ss->cancel_attach(ss, cgrp, leader);
+				ss->cancel_attach(ss, cgrp, &tset);
 		}
 	}
-	/* clean up the array of referenced threads in the group. */
-	for (i = 0; i < group_size; i++) {
-		tsk = flex_array_get_ptr(group, i);
-		put_task_struct(tsk);
-	}
 out_free_group_list:
 	flex_array_free(group);
 	return retval;
@@ -2192,8 +2233,8 @@ out_free_group_list:
 
 /*
  * Find the task_struct of the task to attach by vpid and pass it along to the
- * function to attach either it or all tasks in its threadgroup. Will take
- * cgroup_mutex; may take task_lock of task.
+ * function to attach either it or all tasks in its threadgroup. Will lock
+ * cgroup_mutex and threadgroup; may take task_lock of task.
  */
 static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
 {
@@ -2220,13 +2261,7 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
 			 * detect it later.
 			 */
 			tsk = tsk->group_leader;
-		} else if (tsk->flags & PF_EXITING) {
-			/* optimization for the single-task-only case */
-			rcu_read_unlock();
-			cgroup_unlock();
-			return -ESRCH;
 		}
-
 		/*
 		 * even if we're attaching all tasks in the thread group, we
 		 * only need to check permissions on one of them.
@@ -2249,13 +2284,15 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
 		get_task_struct(tsk);
 	}
 
-	if (threadgroup) {
-		threadgroup_fork_write_lock(tsk);
+	threadgroup_lock(tsk);
+
+	if (threadgroup)
 		ret = cgroup_attach_proc(cgrp, tsk);
-		threadgroup_fork_write_unlock(tsk);
-	} else {
+	else
 		ret = cgroup_attach_task(cgrp, tsk);
-	}
+
+	threadgroup_unlock(tsk);
+
 	put_task_struct(tsk);
 	cgroup_unlock();
 	return ret;
@@ -2306,7 +2343,9 @@ static int cgroup_release_agent_write(struct cgroup *cgrp, struct cftype *cft,
 		return -EINVAL;
 	if (!cgroup_lock_live_group(cgrp))
 		return -ENODEV;
+	mutex_lock(&cgroup_root_mutex);
 	strcpy(cgrp->root->release_agent_path, buffer);
+	mutex_unlock(&cgroup_root_mutex);
 	cgroup_unlock();
 	return 0;
 }
@@ -2789,6 +2828,7 @@ static void cgroup_enable_task_cg_lists(void)
 }
 
 void cgroup_iter_start(struct cgroup *cgrp, struct cgroup_iter *it)
+	__acquires(css_set_lock)
 {
 	/*
 	 * The first time anyone tries to iterate across a cgroup,
@@ -2828,6 +2868,7 @@ struct task_struct *cgroup_iter_next(struct cgroup *cgrp,
 }
 
 void cgroup_iter_end(struct cgroup *cgrp, struct cgroup_iter *it)
+	__releases(css_set_lock)
 {
 	read_unlock(&css_set_lock);
 }
@@ -4491,20 +4532,31 @@ static const struct file_operations proc_cgroupstats_operations = {
  *
  * A pointer to the shared css_set was automatically copied in
  * fork.c by dup_task_struct().  However, we ignore that copy, since
- * it was not made under the protection of RCU or cgroup_mutex, so
- * might no longer be a valid cgroup pointer.  cgroup_attach_task() might
- * have already changed current->cgroups, allowing the previously
- * referenced cgroup group to be removed and freed.
+ * it was not made under the protection of RCU, cgroup_mutex or
+ * threadgroup_change_begin(), so it might no longer be a valid
+ * cgroup pointer.  cgroup_attach_task() might have already changed
+ * current->cgroups, allowing the previously referenced cgroup
+ * group to be removed and freed.
+ *
+ * Outside the pointer validity we also need to process the css_set
+ * inheritance between threadgoup_change_begin() and
+ * threadgoup_change_end(), this way there is no leak in any process
+ * wide migration performed by cgroup_attach_proc() that could otherwise
+ * miss a thread because it is too early or too late in the fork stage.
  *
  * At the point that cgroup_fork() is called, 'current' is the parent
  * task, and the passed argument 'child' points to the child task.
  */
 void cgroup_fork(struct task_struct *child)
 {
-	task_lock(current);
+	/*
+	 * We don't need to task_lock() current because current->cgroups
+	 * can't be changed concurrently here. The parent obviously hasn't
+	 * exited and called cgroup_exit(), and we are synchronized against
+	 * cgroup migration through threadgroup_change_begin().
+	 */
 	child->cgroups = current->cgroups;
 	get_css_set(child->cgroups);
-	task_unlock(current);
 	INIT_LIST_HEAD(&child->cg_list);
 }
 
@@ -4546,10 +4598,19 @@ void cgroup_post_fork(struct task_struct *child)
 {
 	if (use_task_css_set_links) {
 		write_lock(&css_set_lock);
-		task_lock(child);
-		if (list_empty(&child->cg_list))
+		if (list_empty(&child->cg_list)) {
+			/*
+			 * It's safe to use child->cgroups without task_lock()
+			 * here because we are protected through
+			 * threadgroup_change_begin() against concurrent
+			 * css_set change in cgroup_task_migrate(). Also
+			 * the task can't exit at that point until
+			 * wake_up_new_task() is called, so we are protected
+			 * against cgroup_exit() setting child->cgroup to
+			 * init_css_set.
+			 */
 			list_add(&child->cg_list, &child->cgroups->tasks);
-		task_unlock(child);
+		}
 		write_unlock(&css_set_lock);
 	}
 }