[PATCH] Workqueue lockup: Circular dependency in threads

[Prateek Sood]

Hi,

While using Linux version 4.4 on my setup, I have observed a deadlock.

1) CPU3 is getting hot plugged from a worker thread(kworker/0:0) on CPU0.
2) Cpu hot plug flow needs to flush the work items on hot plugging CPU3,
   with a high priority worker from the corresponding CPU(cpu3) worker pool.
3) There is no high priority worker on CPU3, resulting in creation of worker
   thread with high priority from create_worker.
4) This creation is done by kthreadd, which got stuck while trying to acquire
   cgroup_threadgroup_rwsem during kernel thread creation.
5) Cgroup cgroup_threadgroup_rwsem is acquired by task init:729 and is waiting
   on cpuset_mutex.
6) cpuset_mutex is acquired by task init:1 and is waiting for cpuhotplug lock.
7) cpuhotplug lock is acquired by kworker/0:0 while doing hotplug of CPU3

Circular dependency:
kworker/0:0 => kthreadd => init:729 => init:1 => kworker/0:0

kworker/0:0
-000|__switch_to()
-001|context_switch(inline)
-001|__schedule()
-002|__preempt_count_sub(inline)
-002|schedule()
-003|schedule_timeout()
-004|do_wait_for_common(inline)
-004|__wait_for_common(inline)
-004|wait_for_common()
-005|wait_for_completion()
-006|flush_work()
-007|workqueue_cpu_down_callback()
-008|notifier_call_chain()
-009|__raw_notifier_call_chain()
-010|notifier_to_errno(inline)
-010|__cpu_notify()
-011|cpu_down()
-012|cpu_down()
-013|cpu_subsys_offline()
-014|device_offline()
-015|do_core_control()
-016|check_temp()
-017|__read_once_size(inline)
-017|static_key_count(inline)
-017|static_key_false(inline)
-017|trace_workqueue_execute_end(inline)
-017|process_one_work()
-018|worker_thread()
-019|kthread()
-020|ret_from_fork(asm)
 ---|end of frame

kthreadd
-000|__switch_to()
-001|context_switch(inline)
-001|__schedule()
-002|__preempt_count_sub(inline)
-002|schedule()
-003|rwsem_down_read_failed()
-004|current_thread_info(inline)
-004|preempt_count_ptr(inline)
-004|__preempt_count_add(inline)
-004|__percpu_down_read()
-005|current_thread_info(inline)
-005|preempt_count_ptr(inline)
-005|__preempt_count_dec_and_test(inline)
-005|percpu_down_read(inline)
-005|cgroup_threadgroup_change_begin(inline)
-005|threadgroup_change_begin(inline)
-005|copy_process.isra.60()
-006|do_fork()
-007|kernel_thread()
-008|create_kthread(inline)
-008|kthreadd()
-009|ret_from_fork(asm)
 ---|end of frame

init:729
-000|__switch_to()
-001|context_switch(inline)
-001|__schedule()
-002|__preempt_count_sub(inline)
-002|schedule()
-003|__preempt_count_add(inline)
-003|schedule_preempt_disabled()
-004|spin_lock(inline)
-004|__mutex_lock_common(inline)
-004|__mutex_lock_slowpath()
-005|current_thread_info(inline)
-005|mutex_set_owner(inline)
-005|mutex_lock()
-006|__read_once_size(inline)
-006|static_key_count(inline)
-006|cpuset_can_attach()
-007|cgroup_taskset_migrate()
-008|cgroup_migrate()
-009|cgroup_attach_task()
-010|__cgroup_procs_write.isra.32()
-011|cgroup_tasks_write()
-012|cgroup_file_write()
-013|kernfs_fop_write()
-014|__vfs_write()
-015|vfs_write()
-016|SYSC_write(inline)
-016|sys_write()
-017|el0_svc_naked(asm)
 -->|exception
-018|NUX:0x507970(asm)
 ---|end of frame

init:1
-000|__switch_to()
-001|context_switch(inline)
-001|__schedule()
-002|__preempt_count_sub(inline)
-002|schedule()
-003|__preempt_count_add(inline)
-003|schedule_preempt_disabled()
-004|spin_lock(inline)
-004|__mutex_lock_common(inline)
-004|__mutex_lock_slowpath()
-005|current_thread_info(inline)
-005|mutex_set_owner(inline)
-005|mutex_lock()
-006|atomic_add(inline)
-006|get_online_cpus()
-007|rebuild_sched_domains_locked()
-008|update_cpumask(inline)
-008|cpuset_write_resmask()
-009|cgroup_file_write()
-010|kernfs_fop_write()
-011|__vfs_write()
-012|vfs_write()
-013|SYSC_write(inline)
-013|sys_write()
-014|el0_svc_naked(asm)
 -->|exception
-015|NUX:0x507970(asm)
 ---|end of frame

We can reorder the sequence of locks as in the below diff to avoid this
deadlock. But I am looking for inputs/better solution to fix this deadlock.

---
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
 /**
  * update_tasks_cpumask - Update the cpumasks of tasks in the cpuset.
  * @cs: the cpuset in which each task's cpus_allowed mask needs to be changed
@@ -930,7 +946,7 @@ static void update_cpumasks_hier(struct cpuset *cs, struct cpumask *new_cpus)
 	rcu_read_unlock();
 
 	if (need_rebuild_sched_domains)
-		rebuild_sched_domains_locked();
+		rebuild_sched_domains_unlocked()(without taking cpuhotplug.lock)
 }
 
 /**
@@ -1719,6 +1735,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
+	get_online_cpus();
 	mutex_lock(&cpuset_mutex);
 	if (!is_cpuset_online(cs))
 		goto out_unlock;
@@ -1744,6 +1761,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
 	mutex_unlock(&cpuset_mutex);
+	put_online_cpus();
 	kernfs_unbreak_active_protection(of->kn);
 	css_put(&cs->css);
 	flush_workqueue(cpuset_migrate_mm_wq);
-- 
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