[Devel] [PATCH 5/6] Makes procs file writable to move all threads by tgid at once
Ben Blum
bblum at google.com
Thu Jul 23 20:22:00 PDT 2009
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 at google.com>
---
Documentation/cgroups/cgroups.txt | 12 +
include/linux/cgroup.h | 2
kernel/cgroup.c | 422 +++++++++++++++++++++++++++++++++----
kernel/fork.c | 2
4 files changed, 393 insertions(+), 45 deletions(-)
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
@@ -228,6 +228,7 @@ Each cgroup is represented by a directory in the cgroup file system
containing the following files describing that cgroup:
- tasks: list of tasks (by pid) attached to that cgroup
+ - cgroup.procs: list of unique tgids in the 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)
@@ -374,7 +375,7 @@ Now you want to do something with this cgroup.
In this directory you can find several files:
# ls
-notify_on_release tasks
+cgroup.procs notify_on_release tasks
(plus whatever files added by the attached subsystems)
Now attach your shell to this cgroup:
@@ -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.
+
3. Kernel API
=============
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 24e3f1a..cae7d3e 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -34,6 +34,7 @@ extern void cgroup_fork(struct task_struct *p);
extern void cgroup_fork_callbacks(struct task_struct *p);
extern void cgroup_post_fork(struct task_struct *p);
extern void cgroup_exit(struct task_struct *p, int run_callbacks);
+extern void cgroup_fork_failed(struct task_struct *p, int run_callbacks);
extern int cgroupstats_build(struct cgroupstats *stats,
struct dentry *dentry);
@@ -554,6 +555,7 @@ static inline void cgroup_fork(struct task_struct *p) {}
static inline void cgroup_fork_callbacks(struct task_struct *p) {}
static inline void cgroup_post_fork(struct task_struct *p) {}
static inline void cgroup_exit(struct task_struct *p, int callbacks) {}
+static inline void cgroup_fork_failed(struct task_struct *p, int callbacks) {}
static inline void cgroup_lock(void) {}
static inline void cgroup_unlock(void) {}
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 637a54e..3f8d323 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -254,6 +254,18 @@ static struct hlist_head *css_set_hash(struct cgroup_subsys_state *css[])
* reduces the fork()/exit() overhead for people who have cgroups
* compiled into their kernel but not actually in use */
static int use_task_css_set_links __read_mostly;
+/* This rwsem locks out cgroup_attach_proc() from races with fork().
+ * If a thread with a tgid that's being moved via the procs file tries
+ * to fork, its child thread could escape the iteration across the
+ * threadgroup if it copies its parent cgroup pointer before the parent
+ * gets moved but doesn't add itself to the threadgroup list or finish
+ * cgroup fork routines until afterwards. The way we solve this is by
+ * taking this lock in read mode in the fork path across the sensitive
+ * section (at the cost of a cache miss when there's no contention),
+ * and as a write lock in cgroup_attach_proc(). Note of course that
+ * there will never be more than one cgroup_attach_proc contending, as
+ * it needs to be holding the cgroup_mutex to begin with. */
+static DECLARE_RWSEM(cgroup_fork_mutex);
/* When we create or destroy a css_set, the operation simply
* takes/releases a reference count on all the cgroups referenced
@@ -1297,6 +1309,87 @@ static void get_first_subsys(const struct cgroup *cgrp,
*subsys_id = test_ss->subsys_id;
}
+/*
+ * 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, 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_del(&tsk->cg_list);
+ list_add(&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
@@ -1307,11 +1400,9 @@ static void get_first_subsys(const struct cgroup *cgrp,
*/
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;
int subsys_id;
@@ -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;
+ }
}
- 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;
+ }
+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
+ * 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
+ * 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);
}
- 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);
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