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SubjectRe: [PATCH v6 3/3] cgroups: make procs file writable
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@andrew.cmu.edu>
>
> 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@andrew.cmu.edu>
> ---
> 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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