}
EXPORT_SYMBOL_GPL(task_cgroup_path);
+/**
+ * cgroup_attach_lock - Lock for ->attach()
+ * @lock_threadgroup: whether to down_write cgroup_threadgroup_rwsem
+ *
+ * cgroup migration sometimes needs to stabilize threadgroups against forks and
+ * exits by write-locking cgroup_threadgroup_rwsem. However, some ->attach()
+ * implementations (e.g. cpuset), also need to disable CPU hotplug.
+ * Unfortunately, letting ->attach() operations acquire cpus_read_lock() can
+ * lead to deadlocks.
+ *
+ * Bringing up a CPU may involve creating and destroying tasks which requires
+ * read-locking threadgroup_rwsem, so threadgroup_rwsem nests inside
+ * cpus_read_lock(). If we call an ->attach() which acquires the cpus lock while
+ * write-locking threadgroup_rwsem, the locking order is reversed and we end up
+ * waiting for an on-going CPU hotplug operation which in turn is waiting for
+ * the threadgroup_rwsem to be released to create new tasks. For more details:
+ *
+ * http://lkml.kernel.org/r/20220711174629.uehfmqegcwn2lqzu@wubuntu
+ *
+ * Resolve the situation by always acquiring cpus_read_lock() before optionally
+ * write-locking cgroup_threadgroup_rwsem. This allows ->attach() to assume that
+ * CPU hotplug is disabled on entry.
+ */
+static void cgroup_attach_lock(bool lock_threadgroup)
+{
+ cpus_read_lock();
+ if (lock_threadgroup)
+ percpu_down_write(&cgroup_threadgroup_rwsem);
+}
+
+/**
+ * cgroup_attach_unlock - Undo cgroup_attach_lock()
+ * @lock_threadgroup: whether to up_write cgroup_threadgroup_rwsem
+ */
+static void cgroup_attach_unlock(bool lock_threadgroup)
+{
+ if (lock_threadgroup)
+ percpu_up_write(&cgroup_threadgroup_rwsem);
+ cpus_read_unlock();
+}
+
/**
* cgroup_migrate_add_task - add a migration target task to a migration context
* @task: target task
}
struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
- bool *locked)
- __acquires(&cgroup_threadgroup_rwsem)
+ bool *threadgroup_locked)
{
struct task_struct *tsk;
pid_t pid;
* Therefore, we can skip the global lock.
*/
lockdep_assert_held(&cgroup_mutex);
- if (pid || threadgroup) {
- percpu_down_write(&cgroup_threadgroup_rwsem);
- *locked = true;
- } else {
- *locked = false;
- }
+ *threadgroup_locked = pid || threadgroup;
+ cgroup_attach_lock(*threadgroup_locked);
rcu_read_lock();
if (pid) {
goto out_unlock_rcu;
out_unlock_threadgroup:
- if (*locked) {
- percpu_up_write(&cgroup_threadgroup_rwsem);
- *locked = false;
- }
+ cgroup_attach_unlock(*threadgroup_locked);
+ *threadgroup_locked = false;
out_unlock_rcu:
rcu_read_unlock();
return tsk;
}
-void cgroup_procs_write_finish(struct task_struct *task, bool locked)
- __releases(&cgroup_threadgroup_rwsem)
+void cgroup_procs_write_finish(struct task_struct *task, bool threadgroup_locked)
{
struct cgroup_subsys *ss;
int ssid;
/* release reference from cgroup_procs_write_start() */
put_task_struct(task);
- if (locked)
- percpu_up_write(&cgroup_threadgroup_rwsem);
+ cgroup_attach_unlock(threadgroup_locked);
+
for_each_subsys(ss, ssid)
if (ss->post_attach)
ss->post_attach();
* write-locking can be skipped safely.
*/
has_tasks = !list_empty(&mgctx.preloaded_src_csets);
- if (has_tasks)
- percpu_down_write(&cgroup_threadgroup_rwsem);
+ cgroup_attach_lock(has_tasks);
/* NULL dst indicates self on default hierarchy */
ret = cgroup_migrate_prepare_dst(&mgctx);
ret = cgroup_migrate_execute(&mgctx);
out_finish:
cgroup_migrate_finish(&mgctx);
- if (has_tasks)
- percpu_up_write(&cgroup_threadgroup_rwsem);
+ cgroup_attach_unlock(has_tasks);
return ret;
}
struct task_struct *task;
const struct cred *saved_cred;
ssize_t ret;
- bool locked;
+ bool threadgroup_locked;
dst_cgrp = cgroup_kn_lock_live(of->kn, false);
if (!dst_cgrp)
return -ENODEV;
- task = cgroup_procs_write_start(buf, threadgroup, &locked);
+ task = cgroup_procs_write_start(buf, threadgroup, &threadgroup_locked);
ret = PTR_ERR_OR_ZERO(task);
if (ret)
goto out_unlock;
ret = cgroup_attach_task(dst_cgrp, task, threadgroup);
out_finish:
- cgroup_procs_write_finish(task, locked);
+ cgroup_procs_write_finish(task, threadgroup_locked);
out_unlock:
cgroup_kn_unlock(of->kn);