Makes procs file writable to move all threads by tgid at once From: Ben Blum 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 --- 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 Modified by Christoph Lameter +Modified by Ben Blum 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", -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/