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    Subject[RFC] [PATCH 2/2] cgroups: make procs file writable
    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.

    Signed-off-by: Ben Blum <bblum@andrew.cmu.edu>
    ---
    Documentation/cgroups/cgroups.txt | 7 +
    kernel/cgroup.c | 426 ++++++++++++++++++++++++++++++++++---
    2 files changed, 393 insertions(+), 40 deletions(-)

    diff --git a/Documentation/cgroups/cgroups.txt b/Documentation/cgroups/cgroups.txt
    index 7527bac..a5f1e6a 100644
    --- a/Documentation/cgroups/cgroups.txt
    +++ b/Documentation/cgroups/cgroups.txt
    @@ -9,6 +9,7 @@ Portions Copyright (C) 2004 BULL SA.
    Portions Copyright (c) 2004-2006 Silicon Graphics, Inc.
    Modified by Paul Jackson <pj@sgi.com>
    Modified by Christoph Lameter <clameter@sgi.com>
    +Modified by Ben Blum <bblum@google.com>

    CONTENTS:
    =========
    @@ -415,6 +416,12 @@ 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
    --------------------------------

    diff --git a/kernel/cgroup.c b/kernel/cgroup.c
    index 99782a0..f79d70b 100644
    --- a/kernel/cgroup.c
    +++ b/kernel/cgroup.c
    @@ -1622,6 +1622,87 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
    return 0;
    }

    +/*
    + * 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, int 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. 'guarantee' tells
    + * us whether or not we are sure that a new css_set already exists;
    + * in that case, we are not allowed to fail or sleep, as we won't need
    + * malloc.
    + */
    + if (guarantee) {
    + /*
    + * our caller promises us that the css_set we want already
    + * exists, so we use find_existing_css_set directly.
    + */
    + 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);
    +
    + /*
    + * we cannot move a task that's declared itself as exiting, as once
    + * 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;
    +}
    +
    /**
    * threadgroup_fork_lock - block all CLONE_THREAD forks in the threadgroup
    * @tsk: the task whose threadgroup should be locked
    @@ -1697,11 +1778,9 @@ void threadgroup_fork_unlock(struct sighand_struct *sighand)
    */
    int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
    {
    - int retval = 0;
    + int retval;
    struct cgroup_subsys *ss;
    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 */
    @@ -1717,75 +1796,326 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
    }
    }

    - task_lock(tsk);
    - cg = tsk->cgroups;
    - get_css_set(cg);
    - task_unlock(tsk);
    + retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, 0);
    + if (retval)
    + return retval;
    +
    + for_each_subsys(root, ss) {
    + if (ss->attach)
    + ss->attach(ss, cgrp, oldcgrp, tsk, false);
    + }
    +
    + synchronize_rcu();
    +
    /*
    - * Locate or allocate a new css_set for this task,
    - * based on its final set of cgroups
    + * wake up rmdir() waiter. the rmdir should fail since the cgroup
    + * is no longer empty.
    */
    + cgroup_wakeup_rmdir_waiter(cgrp);
    + return 0;
    +}
    +
    +/*
    + * 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 on success, -ENOMEM if we
    + * run out of memory.
    + */
    +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);
    - put_css_set(cg);
    if (!newcg)
    return -ENOMEM;
    + /* add new element to 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;
    +}

    - task_lock(tsk);
    - if (tsk->flags & PF_EXITING) {
    +/**
    + * 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;
    + 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;
    + /* needed for locking the threadgroup */
    + struct sighand_struct *threadgroup_sighand;
    +
    + /*
    + * because of possible races with de_thread() we can't distinguish
    + * between the case where the user gives a non-leader tid and the case
    + * where it changes out from under us.
    + */
    + leader = leader->group_leader;
    +
    + /*
    + * 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)
    + return retval;
    + }
    + }
    +
    + /*
    + * 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();
    + /*
    + * 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 only has one css_set for all of them, usually O(n).
    + */
    + 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);
    - put_css_set(newcg);
    - return -ESRCH;
    + /* 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_assign_pointer(tsk->cgroups, newcg);
    - task_unlock(tsk);
    + rcu_read_unlock();

    - /* 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);
    + /*
    + * step 2: now that we're guaranteed success wrt the css_sets, proceed
    + * to move all tasks to the new cgroup. Even if the threadgroup leader
    + * is PF_EXITING, we still proceed to move all of its sub-threads to
    + * the new cgroup; if everybody is PF_EXITING, we'll just end up doing
    + * nothing, which is ok.
    + */
    + oldcgrp = task_cgroup_from_root(leader, root);
    + /* if leader is already there, skip moving him */
    + if (cgrp != oldcgrp) {
    + retval = cgroup_task_migrate(cgrp, oldcgrp, leader, 1);
    + BUG_ON(retval != 0 && retval != -ESRCH);
    }
    - write_unlock(&css_set_lock);

    + /*
    + * now move all the rest of the threads - need to lock against
    + * possible races with fork(). (Remember, the sighand's lock needs
    + * to be taken outside of tasklist_lock.)
    + */
    + threadgroup_sighand = threadgroup_fork_lock(leader);
    + if (unlikely(IS_ERR(threadgroup_sighand))) {
    + /*
    + * this happens with either ESRCH or EAGAIN; either way, the
    + * calling function takes care of it.
    + */
    + retval = PTR_ERR(threadgroup_sighand);
    + goto list_teardown;
    + }
    + read_lock(&tasklist_lock);
    + /*
    + * Finally, before we can continue, make sure the threadgroup is sane.
    + * First, if de_thread() changed the leader, then no guarantees on the
    + * safety of iterating leader->thread_group. Second, regardless of
    + * leader, de_thread() can change the sighand since we grabbed a
    + * reference on it. Either case is a race with exec() and therefore
    + * not safe to proceed.
    + */
    + if (!thread_group_leader(leader) ||
    + (leader->sighand && leader->sighand != threadgroup_sighand)) {
    + retval = -EAGAIN;
    + read_unlock(&tasklist_lock);
    + threadgroup_fork_unlock(threadgroup_sighand);
    + goto list_teardown;
    + }
    +
    + list_for_each_entry_rcu(tsk, &leader->thread_group, thread_group) {
    + /* 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, 1);
    + 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
    + * information.
    + */
    for_each_subsys(root, ss) {
    if (ss->attach)
    - ss->attach(ss, cgrp, oldcgrp, tsk, false);
    + ss->attach(ss, cgrp, oldcgrp, tsk, true);
    }
    - set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
    - synchronize_rcu();
    - put_css_set(cg);
    +
    + /* holding these until here keeps us safe from exec() and fork(). */
    + read_unlock(&tasklist_lock);
    + threadgroup_fork_unlock(threadgroup_sighand);

    /*
    - * wake up rmdir() waiter. the rmdir should fail since the cgroup
    - * is no longer empty.
    + * step 4: success! ...and cleanup
    */
    + synchronize_rcu();
    cgroup_wakeup_rmdir_waiter(cgrp);
    - return 0;
    + retval = 0;
    +list_teardown:
    + /* no longer need the list of css_sets, so get rid of it */
    + while (!list_empty(&newcg_list)) {
    + /* pop from the list */
    + cg_entry = list_first_entry(&newcg_list, struct cg_list_entry,
    + links);
    + list_del(&cg_entry->links);
    + /* drop the refcount */
    + put_css_set(cg_entry->cg);
    + kfree(cg_entry);
    + }
    + /* done! */
    + return retval;
    }

    /*
    - * Attach task with pid 'pid' to cgroup 'cgrp'. Call with cgroup_mutex
    - * held. May take task_lock of task
    + * 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)
    +static int attach_task_by_pid(struct cgroup *cgrp, u64 pid,
    + int attach(struct cgroup *,
    + struct task_struct *))
    {
    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) {
    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 the group leader, because
    + * even if another task has different permissions, the group
    + * leader will have sufficient access to change it.
    + */
    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);
    @@ -1795,18 +2125,34 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid)
    get_task_struct(tsk);
    }

    - ret = cgroup_attach_task(cgrp, tsk);
    + /*
    + * Note that the check for whether the task is its threadgroup leader
    + * is done in cgroup_attach_proc. This means that writing 0 to the
    + * procs file will only work if the writing task is the leader.
    + */
    + ret = attach(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, cgroup_attach_task);
    +}
    +
    +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 {
    + /*
    + * Nobody lower than us can handle the EAGAIN, since if a race
    + * with de_thread() changes the group leader, the task_struct
    + * matching the given tgid will have changed, and we'll need
    + * to find it again.
    + */
    + ret = attach_task_by_pid(cgrp, tgid, cgroup_attach_proc);
    + } while (ret == -EAGAIN);
    return ret;
    }

    @@ -2966,9 +3312,9 @@ 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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