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SubjectRe: [PATCH 5/6] Makes procs file writable to move all threads by tgid at once
Hi Ben,

On 23/07/09 20:22 -0700, Ben Blum wrote:
> Makes procs file writable to move all threads by tgid at once
>
> 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 rwsem that's taken for
> reading in the fork() path to prevent newly forking threads within the
> threadgroup from "escaping" while moving is in progress.
>
> Signed-off-by: Ben Blum <bblum@google.com>

Thank you for working on this interface. This can indeed be very useful. Please
find comments below, hoping that this will help making it better.

[...]

> diff --git a/Documentation/cgroups/cgroups.txt b/Documentation/cgroups/cgroups.txt
> index 6eb1a97..d579346 100644
> --- a/Documentation/cgroups/cgroups.txt
> +++ b/Documentation/cgroups/cgroups.txt

[...]

> @@ -408,6 +409,15 @@ You can attach the current shell task by echoing 0:
>
> # echo 0 > tasks
>
> +The cgroup.procs file is useful for managing all tasks in a threadgroup at
> +once. It works the same way as the tasks file, but moves all tasks in the
> +threadgroup with the specified tgid.
> +
> +Writing the pid of a task that's not the threadgroup leader (i.e., a pid
> +that isn't a tgid) is treated as invalid. Writing a '0' to cgroup.procs will
> +attach the writing task and all tasks in its threadgroup, but is invalid if
> +the writing task is not the leader of the threadgroup.
> +

This restriction sounds unfortunate and I'm not sure that there are good reasons
for it (see below).

[...]

> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 637a54e..3f8d323 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c

[...]

> @@ -1330,75 +1421,294 @@ 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);
> + }
> +
> + 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_waiters(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 int 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 1;
> + /* 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 0;
> + }
> + }
> + /* not found */
> + put_css_set(newcg);
> + return 1;
> +}
> +
> +/*
> + * 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;
> -
> - task_lock(tsk);
> - if (tsk->flags & PF_EXITING) {
> - task_unlock(tsk);
> + /* add new element to list */
> + cg_entry = kmalloc(sizeof(struct cg_list_entry), GFP_KERNEL);
> + if (!cg_entry) {
> put_css_set(newcg);
> - return -ESRCH;
> + return -ENOMEM;
> }
> - rcu_assign_pointer(tsk->cgroups, newcg);
> - task_unlock(tsk);
> + cg_entry->cg = newcg;
> + list_add(&cg_entry->links, newcg_list);
> + return 0;
> +}
>
> - /* 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);
> +/**
> + * 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;
> + int subsys_id;
> + /* 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;
> +
> + /* first, make sure this came from a valid tgid */
> + if (!thread_group_leader(leader))
> + return -EINVAL;
> + /*
> + * 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)
> + return retval;
> + }
> }

So the semantics of ->can_attach() becomes: if called for a thread group leader,
the result should be valid for the whole thread group, even if only the thread
group leader is being attached. This looks a bit fuzzy and thus not desirable.
Why not checking ->can_attach() for all threads (and lock cgroup_fork_mutex
earlier)?

> - write_unlock(&css_set_lock);
>
> + get_first_subsys(cgrp, NULL, &subsys_id);
> +
> + /*
> + * 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(leader, subsys_id);
> + if (cgrp != oldcgrp) {
> + /* get old css_set */
> + task_lock(leader);
> + if (leader->flags & PF_EXITING) {
> + task_unlock(leader);
> + retval = -ESRCH;
> + goto list_teardown;
> + }
> + 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;
> + }

The only difference between leader's case (above) and other threads' case
(below) is the check for PF_EXITING. If all threads were handled equally in the
->can_attach check, this special handling would be pointless.

> +again:
> + 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(tsk, subsys_id);
> + 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? */
> + retval = css_set_check_fetched(cgrp, tsk, oldcg, &newcg_list);
> + if (retval) {
> + /* 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 again;
> + } else {
> + /* was already there, nothing to do. */
> + put_css_set(oldcg);
> + }
> + }
> + rcu_read_unlock();
> +
> + /*
> + * step 2: now that we're guaranteed success wrt the css_sets, proceed

I don't see how css_sets are guaranteed while cgroup_fork_mutex is not held and
thus does not prevent new threads from being created right now. Could you
elaborate on that?

> + * to move all tasks to the new cgroup. the only fail case henceforth
> + * is if the threadgroup leader has PF_EXITING set (in which case all
> + * the other threads get killed) - if other threads happen to be

This statement is wrong. A thread group leader can have PF_EXITING (and even
become zombie) while other sub-threads continue their execution. For instance it
is perfectly valid for a thread group leader to call sys_exit(), and no other
thread will be affected.

> + * exiting, we just ignore them and move on.
> + */
> + oldcgrp = task_cgroup(leader, subsys_id);
> + /* if leader is already there, skip moving him */
> + if (cgrp != oldcgrp) {
> + retval = cgroup_task_migrate(cgrp, oldcgrp, leader, 1);
> + if (retval) {
> + BUG_ON(retval != -ESRCH);
> + goto list_teardown;
> + }
> + }
> + /*
> + * now move all the rest of the threads - need to lock against
> + * possible races with fork().
> + */
> + down_write(&cgroup_fork_mutex);
> + rcu_read_lock();
> + 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(tsk, subsys_id);
> + 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);
> + }
> + rcu_read_unlock();
> + up_write(&cgroup_fork_mutex);
> +
> + /*
> + * step 3: attach whole threadgroup to each subsystem
> + */
> for_each_subsys(root, ss) {
> if (ss->attach)
> - ss->attach(ss, cgrp, oldcgrp, tsk);
> + ss->attach(ss, cgrp, oldcgrp, leader);
> }

So ->attach called for leader should attach all sub-threads too? Does this mean
that all subsystems should be changed accordingly? Again this looks making
->attach semantics fuzzy, and thus not desirable.

Thank,

Louis

> - set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
> - synchronize_rcu();
> - put_css_set(cg);
>
> /*
> - * 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_waiters(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);
> @@ -1408,19 +1718,25 @@ 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)
> {
> - int ret;
> - if (!cgroup_lock_live_group(cgrp))
> - return -ENODEV;
> - ret = attach_task_by_pid(cgrp, pid);
> - cgroup_unlock();
> - return ret;
> + return attach_task_by_pid(cgrp, pid, cgroup_attach_task);
> +}
> +
> +static int cgroup_procs_write(struct cgroup *cgrp, struct cftype *cft, u64 tgid)
> +{
> + return attach_task_by_pid(cgrp, tgid, cgroup_attach_proc);
> }
>
> /**
> @@ -2580,9 +2896,9 @@ 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",
> @@ -3185,6 +3501,7 @@ static struct file_operations proc_cgroupstats_operations = {
> */
> void cgroup_fork(struct task_struct *child)
> {
> + down_read(&cgroup_fork_mutex);
> task_lock(current);
> child->cgroups = current->cgroups;
> get_css_set(child->cgroups);
> @@ -3231,6 +3548,7 @@ void cgroup_post_fork(struct task_struct *child)
> task_unlock(child);
> write_unlock(&css_set_lock);
> }
> + up_read(&cgroup_fork_mutex);
> }
> /**
> * cgroup_exit - detach cgroup from exiting task
> @@ -3302,6 +3620,24 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
> }
>
> /**
> + * cgroup_fork_failed - undo operations for fork failure
> + * @tsk: pointer to task_struct of exiting process
> + * @run_callback: run exit callbacks?
> + *
> + * Description: Undo cgroup operations after cgroup_fork in fork failure.
> + *
> + * We release the read lock that was taken in cgroup_fork(), since it is
> + * supposed to be dropped in cgroup_post_fork in the success case. The other
> + * thing that wants to be done is detaching the failed child task from the
> + * cgroup, so we wrap cgroup_exit.
> + */
> +void cgroup_fork_failed(struct task_struct *tsk, int run_callbacks)
> +{
> + up_read(&cgroup_fork_mutex);
> + cgroup_exit(tsk, run_callbacks);
> +}
> +
> +/**
> * cgroup_clone - clone the cgroup the given subsystem is attached to
> * @tsk: the task to be moved
> * @subsys: the given subsystem
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 926c117..027ec16 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1300,7 +1300,7 @@ bad_fork_cleanup_policy:
> mpol_put(p->mempolicy);
> bad_fork_cleanup_cgroup:
> #endif
> - cgroup_exit(p, cgroup_callbacks_done);
> + cgroup_fork_failed(p, cgroup_callbacks_done);
> delayacct_tsk_free(p);
> if (p->binfmt)
> module_put(p->binfmt->module);
>
> _______________________________________________
> Containers mailing list
> Containers@lists.linux-foundation.org
> https://lists.linux-foundation.org/mailman/listinfo/containers

--
Dr Louis Rilling Kerlabs
Skype: louis.rilling Batiment Germanium
Phone: (+33|0) 6 80 89 08 23 80 avenue des Buttes de Coesmes
http://www.kerlabs.com/ 35700 Rennes[unhandled content-type:application/pgp-signature]
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