Re: [PATCH v6 3/3] cgroups: make procs file writable

[Paul E. McKenney]

On Fri, Dec 24, 2010 at 03:24:45AM -0500, Ben Blum wrote:
> Makes procs file writable to move all threads by tgid at once
> 
> From: Ben Blum <bblum@xxxxxxxxxxxxxx>
> 
> This patch adds 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.

One minor nit below.

							Thanx, Paul

> Signed-off-by: Ben Blum <bblum@xxxxxxxxxxxxxx>
> ---
>  Documentation/cgroups/cgroups.txt |   13 +
>  kernel/cgroup.c                   |  424 +++++++++++++++++++++++++++++++++----
>  2 files changed, 387 insertions(+), 50 deletions(-)
> 
> diff --git a/Documentation/cgroups/cgroups.txt b/Documentation/cgroups/cgroups.txt
> index 190018b..07674e5 100644
> --- a/Documentation/cgroups/cgroups.txt
> +++ b/Documentation/cgroups/cgroups.txt
> @@ -236,7 +236,8 @@ containing the following files describing that cgroup:
>   - 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)
> @@ -426,6 +427,12 @@ 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.
> +
>  2.3 Mounting hierarchies by name
>  --------------------------------
> 
> @@ -574,7 +581,9 @@ 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. If threadgroup is
>  true, then a successful result indicates that all threads in the given
> -thread's threadgroup can be moved together.
> +thread's threadgroup can be moved together. If the subsystem wants to
> +iterate over task->thread_group, it must take rcu_read_lock then check
> +if thread_group_leader(task), returning -EAGAIN if that fails.
> 
>  void cancel_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
>  	       struct task_struct *task, bool threadgroup)
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index f86dd9c..74be02c 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1771,6 +1771,76 @@ int cgroup_can_attach_per_thread(struct cgroup *cgrp, struct task_struct *task,
>  }
>  EXPORT_SYMBOL_GPL(cgroup_can_attach_per_thread);
> 
> +/*
> + * 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 exit. 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
> @@ -1781,11 +1851,9 @@ EXPORT_SYMBOL_GPL(cgroup_can_attach_per_thread);
>   */
>  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 */
> @@ -1809,46 +1877,16 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
>  		}
>  	}
> 
> -	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;
> -		goto out;
> -	}
> -
> -	task_lock(tsk);
> -	if (tsk->flags & PF_EXITING) {
> -		task_unlock(tsk);
> -		put_css_set(newcg);
> -		retval = -ESRCH;
> +	retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, false);
> +	if (retval)
>  		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_del(&tsk->cg_list);
> -		list_add(&tsk->cg_list, &newcg->tasks);
> -	}
> -	write_unlock(&css_set_lock);
> 
>  	for_each_subsys(root, ss) {
>  		if (ss->attach)
>  			ss->attach(ss, cgrp, oldcgrp, tsk, false);
>  	}
> -	set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
> +
>  	synchronize_rcu();
> -	put_css_set(cg);
> 
>  	/*
>  	 * wake up rmdir() waiter. the rmdir should fail since the cgroup
> @@ -1898,49 +1936,339 @@ int cgroup_attach_task_all(struct task_struct *from, struct task_struct *tsk)
>  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
> + */
> +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 attach_task_by_pid(struct cgroup *cgrp, u64 pid)
> +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. 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;
> +	struct cgroup_subsys *ss, *failed_ss = NULL;
> +	struct cgroup *oldcgrp;
> +	struct css_set *oldcg;
> +	struct cgroupfs_root *root = cgrp->root;
> +	/* threadgroup list cursor */
> +	struct task_struct *tsk;
> +	/*
> +	 * 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;
> +
> +	/* 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, true);
> +			if (retval) {
> +				failed_ss = ss;
> +				goto out;
> +			}
> +		}
> +	}
> +
> +	/*
> +	 * step 1: 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);
> +	oldcgrp = task_cgroup_from_root(leader, root);
> +	if (cgrp != oldcgrp) {
> +		/* get old css_set */
> +		task_lock(leader);
> +		if (leader->flags & PF_EXITING) {
> +			task_unlock(leader);
> +			goto prefetch_loop;
> +		}
> +		oldcg = leader->cgroups;
> +		get_css_set(oldcg);
> +		task_unlock(leader);
> +		/* acquire new one */
> +		retval = css_set_prefetch(cgrp, oldcg, &newcg_list);
> +		put_css_set(oldcg);
> +		if (retval)
> +			goto list_teardown;
> +	}
> +prefetch_loop:
> +	rcu_read_lock();
> +	/* sanity check - if we raced with de_thread, we must abort */
> +	if (!thread_group_leader(leader)) {
> +		retval = -EAGAIN;
> +		goto list_teardown;
> +	}
> +	/*
> +	 * if we need to fetch a new css_set for this task, we must exit the
> +	 * rcu_read section because allocating it can sleep. afterwards, we'll
> +	 * need to restart iteration on the threadgroup list - the whole thing
> +	 * will be O(nm) in the number of threads and css_sets; as the typical
> +	 * case has only one css_set for all of them, usually O(n). which ones
> +	 * we need allocated won't change as long as we hold cgroup_mutex.
> +	 */
> +	list_for_each_entry_rcu(tsk, &leader->thread_group, thread_group) {
> +		/* 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. */
> +			rcu_read_unlock();
> +			retval = css_set_prefetch(cgrp, oldcg, &newcg_list);
> +			put_css_set(oldcg);
> +			if (retval)
> +				goto list_teardown;
> +			/* begin iteration again. */
> +			goto prefetch_loop;
> +		}
> +	}
> +	rcu_read_unlock();
> +
> +	/*
> +	 * step 2: now that we're guaranteed success wrt the css_sets, proceed
> +	 * to move all tasks to the new cgroup. we need to lock against possible
> +	 * races with fork(). note: we can safely take the threadgroup_fork_lock
> +	 * of leader since attach_task_by_pid took a reference.
> +	 * threadgroup_fork_lock must be taken outside of tasklist_lock to match
> +	 * the order in the fork path.
> +	 */
> +	threadgroup_fork_write_lock(leader);
> +	read_lock(&tasklist_lock);
> +	/* sanity check - if we raced with de_thread, we must abort */
> +	if (!thread_group_leader(leader)) {
> +		retval = -EAGAIN;
> +		read_unlock(&tasklist_lock);
> +		threadgroup_fork_write_unlock(leader);
> +		goto list_teardown;
> +	}
> +	/*
> +	 * No failure cases left, so this is the commit point.
> +	 *
> +	 * If the leader is already there, skip moving him. Note: even if the
> +	 * leader is PF_EXITING, we still move all other threads; if everybody
> +	 * is PF_EXITING, we end up doing nothing, which is ok.
> +	 */
> +	oldcgrp = task_cgroup_from_root(leader, root);
> +	if (cgrp != oldcgrp) {
> +		retval = cgroup_task_migrate(cgrp, oldcgrp, leader, true);
> +		BUG_ON(retval != 0 && retval != -ESRCH);
> +	}
> +	/* Now iterate over each thread in the group. */
> +	list_for_each_entry_rcu(tsk, &leader->thread_group, thread_group) {

Why can't we just use list_for_each_entry() here?  Unless I am confused
(quite possible!), we are not in an RCU read-side critical section.

> +		/* leave current thread as it is if it's already there */
> +		oldcgrp = task_cgroup_from_root(tsk, root);
> +		if (cgrp == oldcgrp)
> +			continue;
> +		/* we don't care whether these threads are exiting */
> +		retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, true);
> +		BUG_ON(retval != 0 && retval != -ESRCH);
> +	}
> +
> +	/*
> +	 * step 3: attach whole threadgroup to each subsystem
> +	 * 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, true);
> +	}
> +	/* holding these until here keeps us safe from exec() and fork(). */
> +	read_unlock(&tasklist_lock);
> +	threadgroup_fork_write_unlock(leader);
> +
> +	/*
> +	 * step 4: success! and cleanup
> +	 */
> +	synchronize_rcu();
> +	cgroup_wakeup_rmdir_waiter(cgrp);
> +	retval = 0;
> +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:
> +	if (retval) {
> +		/* same deal as in cgroup_attach_task, with threadgroup=true */
> +		for_each_subsys(root, ss) {
> +			if (ss == failed_ss)
> +				break;
> +			if (ss->cancel_attach)
> +				ss->cancel_attach(ss, cgrp, leader, true);
> +		}
> +	}
> +	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) {
> +			/*
> +			 * it is safe to find group_leader because tsk was found
> +			 * in the tid map, meaning it can't have been unhashed
> +			 * by someone in de_thread changing the leadership.
> +			 */
> +			tsk = tsk->group_leader;
> +			BUG_ON(!thread_group_leader(tsk));
> +		} 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)
> +		ret = cgroup_attach_proc(cgrp, 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;
>  }
> 
> @@ -3294,9 +3622,9 @@ static struct cftype files[] = {
>  	{
>  		.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",
> --
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