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); -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc., is a member of Code Aurora Forum, a Linux Foundation Collaborative Project. -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
