On Tue, Apr 02, 2024 at 11:30:11AM -0400, Waiman Long <longman@xxxxxxxxxx> wrote:
Yes, there is a potential that a cpus_read_lock() may be called leading to
deadlock. So unless we reverse the current cgroup_mutex --> cpu_hotplug_lock
ordering, it is not safe to call cgroup_transfer_tasks() directly.
I see that cgroup_transfer_tasks() has the only user -- cpuset. What
about bending it for the specific use like:
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 34aaf0e87def..64deb7212c5c 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -109,7 +109,7 @@ struct cgroup *cgroup_get_from_fd(int fd);
struct cgroup *cgroup_v1v2_get_from_fd(int fd);
int cgroup_attach_task_all(struct task_struct *from, struct task_struct *);
-int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from);
+int cgroup_transfer_tasks_locked(struct cgroup *to, struct cgroup *from);
int cgroup_add_dfl_cftypes(struct cgroup_subsys *ss, struct cftype *cfts);
int cgroup_add_legacy_cftypes(struct cgroup_subsys *ss, struct cftype *cfts);
diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 520a11cb12f4..f97025858c7a 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -91,7 +91,8 @@ EXPORT_SYMBOL_GPL(cgroup_attach_task_all);
*
* Return: %0 on success or a negative errno code on failure
*/
-int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
+int cgroup_transfer_tasks_locked(struct cgroup *to, struct cgroup *from)
{
DEFINE_CGROUP_MGCTX(mgctx);
struct cgrp_cset_link *link;
@@ -106,9 +106,11 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
if (ret)
return ret;
- cgroup_lock();
-
- cgroup_attach_lock(true);
+ /* The locking rules serve specific purpose of v1 cpuset hotplug
+ * migration, see hotplug_update_tasks_legacy() and
+ * cgroup_attach_lock() */
+ lockdep_assert_held(&cgroup_mutex);
+ lockdep_assert_cpus_held();
+ percpu_down_write(&cgroup_threadgroup_rwsem);
/* all tasks in @from are being moved, all csets are source */
spin_lock_irq(&css_set_lock);
@@ -144,8 +146,7 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
} while (task && !ret);
out_err:
cgroup_migrate_finish(&mgctx);
- cgroup_attach_unlock(true);
- cgroup_unlock();
+ percpu_up_write(&cgroup_threadgroup_rwsem);
return ret;
}
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 13d27b17c889..94fb8b26f038 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -4331,7 +4331,7 @@ static void remove_tasks_in_empty_cpuset(struct cpuset *cs)
nodes_empty(parent->mems_allowed))
parent = parent_cs(parent);
- if (cgroup_transfer_tasks(parent->css.cgroup, cs->css.cgroup)) {
+ if (cgroup_transfer_tasks_locked(parent->css.cgroup, cs->css.cgroup)) {
pr_err("cpuset: failed to transfer tasks out of empty cpuset ");
pr_cont_cgroup_name(cs->css.cgroup);
pr_cont("\n");
@@ -4376,21 +4376,9 @@ hotplug_update_tasks_legacy(struct cpuset *cs,
/*
* Move tasks to the nearest ancestor with execution resources,
- * This is full cgroup operation which will also call back into
- * cpuset. Execute it asynchronously using workqueue.
*/
- if (is_empty && css_tryget_online(&cs->css)) {
- struct cpuset_remove_tasks_struct *s;
-
- s = kzalloc(sizeof(*s), GFP_KERNEL);
- if (WARN_ON_ONCE(!s)) {
- css_put(&cs->css);
- return;
- }
-
- s->cs = cs;
- INIT_WORK(&s->work, cpuset_migrate_tasks_workfn);
- schedule_work(&s->work);
+ if (is_empty)
+ remove_tasks_in_empty_cpuset(cs);
}
}
