cgroups: make procs file writable

Make procs file writable to move all threads by tgid at once.

Add functionality that enables users to move all threads in a threadgroup
at once to a cgroup by writing the tgid to the 'cgroup.procs' file. This
current implementation makes use of a per-threadgroup rwsem that's taken
for reading in the fork() path to prevent newly forking threads within the
threadgroup from "escaping" while the move is in progress.

Signed-off-by: Ben Blum <[e-mail]>
Cc: "Eric W. Biederman" <[e-mail]>
Cc: Li Zefan <[e-mail]>
Cc: Matt Helsley <[e-mail]>
Reviewed-by: Paul Menage <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: David Rientjes <[e-mail]>
Cc: Miao Xie <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/cgroups/cgroups.txt

···
  - cgroup.procs: list of tgids in the cgroup.  This list is not
    guaranteed to be sorted or free of duplicate tgids, and userspace
    should sort/uniquify the list if this property is required.
-   This is a read-only file, for now.
+   Writing a thread group id into this file moves all threads in that
+   group into this cgroup.
  - notify_on_release flag: run the release agent on exit?
  - release_agent: the path to use for release notifications (this file
    exists in the top cgroup only)
···
 You can attach the current shell task by echoing 0:
 
 # echo 0 > tasks
+
+You can use the cgroup.procs file instead of the tasks file to move all
+threads in a threadgroup at once. Echoing the pid of any task in a
+threadgroup to cgroup.procs causes all tasks in that threadgroup to be
+be attached to the cgroup. Writing 0 to cgroup.procs moves all tasks
+in the writing task's threadgroup.
 
 Note: Since every task is always a member of exactly one cgroup in each
 mounted hierarchy, to remove a task from its current cgroup you must

kernel/cgroup.c

···
 }
 EXPORT_SYMBOL_GPL(cgroup_path);
 
+/*
+ * 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.
+ */
+static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
+			       struct task_struct *tsk, bool guarantee)
+{
+	struct css_set *oldcg;
+	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.
+	 */
+	task_lock(tsk);
+	oldcg = tsk->cgroups;
+	get_css_set(oldcg);
+	task_unlock(tsk);
+
+	/* locate or allocate a new css_set for this task. */
+	if (guarantee) {
+		/* we know the css_set we want already exists. */
+		struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];
+		read_lock(&css_set_lock);
+		newcg = find_existing_css_set(oldcg, cgrp, template);
+		BUG_ON(!newcg);
+		get_css_set(newcg);
+		read_unlock(&css_set_lock);
+	} else {
+		might_sleep();
+		/* find_css_set will give us newcg already referenced. */
+		newcg = find_css_set(oldcg, cgrp);
+		if (!newcg) {
+			put_css_set(oldcg);
+			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);
+
+	/* Update the css_set linked lists if we're using them */
+	write_lock(&css_set_lock);
+	if (!list_empty(&tsk->cg_list))
+		list_move(&tsk->cg_list, &newcg->tasks);
+	write_unlock(&css_set_lock);
+
+	/*
+	 * We just gained a reference on oldcg by taking it from the task. As
+	 * trading it for newcg is protected by cgroup_mutex, we're safe to drop
+	 * it here; it will be freed under RCU.
+	 */
+	put_css_set(oldcg);
+
+	set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
+	return 0;
+}
+
 /**
  * cgroup_attach_task - attach task 'tsk' to cgroup 'cgrp'
  * @cgrp: the cgroup the task is attaching to
···
  */
 int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 {
-	int retval = 0;
+	int retval;
 	struct cgroup_subsys *ss, *failed_ss = NULL;
 	struct cgroup *oldcgrp;
-	struct css_set *cg;
-	struct css_set *newcg;
 	struct cgroupfs_root *root = cgrp->root;
 
 	/* Nothing to do if the task is already in that cgroup */
···
 		}
 	}
 
-	task_lock(tsk);
-	cg = tsk->cgroups;
-	get_css_set(cg);
-	task_unlock(tsk);
-	/*
-	 * Locate or allocate a new css_set for this task,
-	 * based on its final set of cgroups
-	 */
-	newcg = find_css_set(cg, cgrp);
-	put_css_set(cg);
-	if (!newcg) {
-		retval = -ENOMEM;
+	retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, false);
+	if (retval)
 		goto out;
-	}
-
-	task_lock(tsk);
-	if (tsk->flags & PF_EXITING) {
-		task_unlock(tsk);
-		put_css_set(newcg);
-		retval = -ESRCH;
-		goto out;
-	}
-	rcu_assign_pointer(tsk->cgroups, newcg);
-	task_unlock(tsk);
-
-	/* Update the css_set linked lists if we're using them */
-	write_lock(&css_set_lock);
-	if (!list_empty(&tsk->cg_list))
-		list_move(&tsk->cg_list, &newcg->tasks);
-	write_unlock(&css_set_lock);
 
 	for_each_subsys(root, ss) {
 		if (ss->pre_attach)
···
 		if (ss->attach)
 			ss->attach(ss, cgrp, oldcgrp, tsk);
 	}
-	set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
+
 	synchronize_rcu();
-	put_css_set(cg);
 
 	/*
 	 * wake up rmdir() waiter. the rmdir should fail since the cgroup
···
 EXPORT_SYMBOL_GPL(cgroup_attach_task_all);
 
 /*
- * Attach task with pid 'pid' to cgroup 'cgrp'. Call with cgroup_mutex
- * held. May take task_lock of task
+ * cgroup_attach_proc works in two stages, the first of which prefetches all
+ * new css_sets needed (to make sure we have enough memory before committing
+ * to the move) and stores them in a list of entries of the following type.
+ * TODO: possible optimization: use css_set->rcu_head for chaining instead
  */
-static int attach_task_by_pid(struct cgroup *cgrp, u64 pid)
+struct cg_list_entry {
+	struct css_set *cg;
+	struct list_head links;
+};
+
+static bool css_set_check_fetched(struct cgroup *cgrp,
+				  struct task_struct *tsk, struct css_set *cg,
+				  struct list_head *newcg_list)
+{
+	struct css_set *newcg;
+	struct cg_list_entry *cg_entry;
+	struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];
+
+	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);
+			return true;
+		}
+	}
+
+	/* not found */
+	put_css_set(newcg);
+	return false;
+}
+
+/*
+ * Find the new css_set and store it in the list in preparation for moving the
+ * given task to the given cgroup. Returns 0 or -ENOMEM.
+ */
+static int css_set_prefetch(struct cgroup *cgrp, struct css_set *cg,
+			    struct list_head *newcg_list)
+{
+	struct css_set *newcg;
+	struct cg_list_entry *cg_entry;
+
+	/* ensure a new css_set will exist for this thread */
+	newcg = find_css_set(cg, cgrp);
+	if (!newcg)
+		return -ENOMEM;
+	/* add it to the list */
+	cg_entry = kmalloc(sizeof(struct cg_list_entry), GFP_KERNEL);
+	if (!cg_entry) {
+		put_css_set(newcg);
+		return -ENOMEM;
+	}
+	cg_entry->cg = newcg;
+	list_add(&cg_entry->links, newcg_list);
+	return 0;
+}
+
+/**
+ * cgroup_attach_proc - attach all threads in a threadgroup to a cgroup
+ * @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.
+ */
+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_struct **group;
+	/*
+	 * 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
+	 * case we get an ENOMEM we can bail out before making any changes.
+	 */
+	struct list_head newcg_list;
+	struct cg_list_entry *cg_entry, *temp_nobe;
+
+	/*
+	 * 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_size = get_nr_threads(leader);
+	group = kmalloc(group_size * sizeof(*group), GFP_KERNEL);
+	if (!group)
+		return -ENOMEM;
+
+	/* prevent changes to the threadgroup list while we take a snapshot. */
+	rcu_read_lock();
+	if (!thread_group_leader(leader)) {
+		/*
+		 * a race with de_thread from another thread's exec() may strip
+		 * us of our leadership, making while_each_thread unsafe to use
+		 * on this task. if this happens, there is no choice but to
+		 * throw this task away and try again (from cgroup_procs_write);
+		 * this is "double-double-toil-and-trouble-check locking".
+		 */
+		rcu_read_unlock();
+		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 {
+		/* as per above, nr_threads may decrease, but not increase. */
+		BUG_ON(i >= group_size);
+		get_task_struct(tsk);
+		group[i] = tsk;
+		i++;
+	} while_each_thread(leader, tsk);
+	/* remember the number of threads in the array for later. */
+	group_size = i;
+	rcu_read_unlock();
+
+	/*
+	 * 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);
+			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++) {
+				retval = ss->can_attach_task(cgrp, group[i]);
+				if (retval) {
+					failed_ss = ss;
+					cancel_failed_ss = true;
+					goto out_cancel_attach;
+				}
+			}
+		}
+	}
+
+	/*
+	 * step 2: make sure css_sets exist for all threads to be migrated.
+	 * we use find_css_set, which allocates a new one if necessary.
+	 */
+	INIT_LIST_HEAD(&newcg_list);
+	for (i = 0; i < group_size; i++) {
+		tsk = 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);
+		if (tsk->flags & PF_EXITING) {
+			/* ignore this task if it's going away */
+			task_unlock(tsk);
+			continue;
+		}
+		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. */
+			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.
+	 */
+	for_each_subsys(root, ss) {
+		if (ss->pre_attach)
+			ss->pre_attach(cgrp);
+	}
+	for (i = 0; i < group_size; i++) {
+		tsk = group[i];
+		/* leave current thread as it is if it's already there */
+		oldcgrp = task_cgroup_from_root(tsk, root);
+		if (cgrp == oldcgrp)
+			continue;
+		/* attach each task to each subsystem */
+		for_each_subsys(root, ss) {
+			if (ss->attach_task)
+				ss->attach_task(cgrp, tsk);
+		}
+		/* if the thread is PF_EXITING, it can just get skipped. */
+		retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, true);
+		BUG_ON(retval != 0 && retval != -ESRCH);
+	}
+	/* 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.
+	 */
+	for_each_subsys(root, ss) {
+		if (ss->attach)
+			ss->attach(ss, cgrp, oldcgrp, leader);
+	}
+
+	/*
+	 * step 5: success! and cleanup
+	 */
+	synchronize_rcu();
+	cgroup_wakeup_rmdir_waiter(cgrp);
+	retval = 0;
+out_list_teardown:
+	/* clean up the list of prefetched css_sets. */
+	list_for_each_entry_safe(cg_entry, temp_nobe, &newcg_list, links) {
+		list_del(&cg_entry->links);
+		put_css_set(cg_entry->cg);
+		kfree(cg_entry);
+	}
+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);
+				break;
+			}
+			if (ss->cancel_attach)
+				ss->cancel_attach(ss, cgrp, leader);
+		}
+	}
+	/* clean up the array of referenced threads in the group. */
+	for (i = 0; i < group_size; i++)
+		put_task_struct(group[i]);
+out_free_group_list:
+	kfree(group);
+	return retval;
+}
+
+/*
+ * 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.
+ */
+static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
 {
 	struct task_struct *tsk;
 	const struct cred *cred = current_cred(), *tcred;
 	int ret;
 
+	if (!cgroup_lock_live_group(cgrp))
+		return -ENODEV;
+
 	if (pid) {
 		rcu_read_lock();
 		tsk = find_task_by_vpid(pid);
-		if (!tsk || tsk->flags & PF_EXITING) {
+		if (!tsk) {
 			rcu_read_unlock();
+			cgroup_unlock();
+			return -ESRCH;
+		}
+		if (threadgroup) {
+			/*
+			 * RCU protects this access, since tsk was found in the
+			 * tid map. a race with de_thread may cause group_leader
+			 * to stop being the leader, but cgroup_attach_proc will
+			 * 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.
+		 */
 		tcred = __task_cred(tsk);
 		if (cred->euid &&
 		    cred->euid != tcred->uid &&
 		    cred->euid != tcred->suid) {
 			rcu_read_unlock();
+			cgroup_unlock();
 			return -EACCES;
 		}
 		get_task_struct(tsk);
 		rcu_read_unlock();
 	} else {
-		tsk = current;
+		if (threadgroup)
+			tsk = current->group_leader;
+		else
+			tsk = current;
 		get_task_struct(tsk);
 	}
 
-	ret = cgroup_attach_task(cgrp, tsk);
+	if (threadgroup) {
+		threadgroup_fork_write_lock(tsk);
+		ret = cgroup_attach_proc(cgrp, tsk);
+		threadgroup_fork_write_unlock(tsk);
+	} else {
+		ret = cgroup_attach_task(cgrp, tsk);
+	}
 	put_task_struct(tsk);
+	cgroup_unlock();
 	return ret;
 }
 
 static int cgroup_tasks_write(struct cgroup *cgrp, struct cftype *cft, u64 pid)
 {
+	return attach_task_by_pid(cgrp, pid, false);
+}
+
+static int cgroup_procs_write(struct cgroup *cgrp, struct cftype *cft, u64 tgid)
+{
 	int ret;
-	if (!cgroup_lock_live_group(cgrp))
-		return -ENODEV;
-	ret = attach_task_by_pid(cgrp, pid);
-	cgroup_unlock();
+	do {
+		/*
+		 * attach_proc fails with -EAGAIN if threadgroup leadership
+		 * changes in the middle of the operation, in which case we need
+		 * to find the task_struct for the new leader and start over.
+		 */
+		ret = attach_task_by_pid(cgrp, tgid, true);
+	} while (ret == -EAGAIN);
 	return ret;
 }
 
···
 	{
 		.name = CGROUP_FILE_GENERIC_PREFIX "procs",
 		.open = cgroup_procs_open,
-		/* .write_u64 = cgroup_procs_write, TODO */
+		.write_u64 = cgroup_procs_write,
 		.release = cgroup_pidlist_release,
-		.mode = S_IRUGO,
+		.mode = S_IRUGO | S_IWUSR,
 	},
 	{
 		.name = "notify_on_release",