On 10/26/2017 07:35 PM, Waiman Long wrote:
> On 10/26/2017 07:52 AM, Prateek Sood wrote:
>> Remove circular dependency deadlock in a scenario where hotplug of CPU is
>> being done while there is updation in cgroup and cpuset triggered from
>> userspace.
>>
>> Process A => kthreadd => Process B => Process C => Process A
>>
>> Process A
>> cpu_subsys_offline();
>> cpu_down();
>> _cpu_down();
>> percpu_down_write(&cpu_hotplug_lock); //held
>> cpuhp_invoke_callback();
>> workqueue_offline_cpu();
>> wq_update_unbound_numa();
>> kthread_create_on_node();
>> wake_up_process(); //wakeup kthreadd
>> flush_work();
>> wait_for_completion();
>>
>> kthreadd
>> kthreadd();
>> kernel_thread();
>> do_fork();
>> copy_process();
>> percpu_down_read(&cgroup_threadgroup_rwsem);
>> __rwsem_down_read_failed_common(); //waiting
>>
>> Process B
>> kernfs_fop_write();
>> cgroup_file_write();
>> cgroup_procs_write();
>> percpu_down_write(&cgroup_threadgroup_rwsem); //held
>> cgroup_attach_task();
>> cgroup_migrate();
>> cgroup_migrate_execute();
>> cpuset_can_attach();
>> mutex_lock(&cpuset_mutex); //waiting
>>
>> Process C
>> kernfs_fop_write();
>> cgroup_file_write();
>> cpuset_write_resmask();
>> mutex_lock(&cpuset_mutex); //held
>> update_cpumask();
>> update_cpumasks_hier();
>> rebuild_sched_domains_locked();
>> get_online_cpus();
>> percpu_down_read(&cpu_hotplug_lock); //waiting
>>
>> Eliminating deadlock by reversing the locking order for cpuset_mutex and
>> cpu_hotplug_lock.
>
> General comments:
>
> Please add a version number of your patch. I have seen multiple versions
> of this patch and have lost track how many are there as there is no
> version number information. In addition, there are changes beyond just
> swapping the lock order and they are not documented in this change log.
> I would like to see you discuss about those additional changes here as well.
Thanks for the comments Longman. I will introduce patch versioning and update
commit text to document extra changes.
Explaintaion for extra changes in this patch:
After inverting the locking sequence of cpu_hotplug_lock and cpuset_mutex,
cpuset_hotplug_workfn() related functionality can be done synchronously from
the context doing cpu hotplug. Extra changes in this patch intend to remove
queuing of cpuset_hotplug_workfn() as a work item for cpu hotplug path. For
memory hotplug it still gets queued as a work item.
This suggestion came in from Peter.
Peter could you please elaborate if I have missed anything.
>
>> void rebuild_sched_domains(void)
>> {
>> + cpus_read_lock();
>> mutex_lock(&cpuset_mutex);
>> - rebuild_sched_domains_locked();
>> + rebuild_sched_domains_cpuslocked();
>> mutex_unlock(&cpuset_mutex);
>> + cpus_read_unlock();
>> }
>
> I saw a lot of instances where cpus_read_lock() and mutex_lock() come
> together. Maybe some new lock/unlock helper functions may help.
Ok, I will introduce a single wrapper for locking and unlocking
of both locks
>
>> @@ -2356,25 +2354,29 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
>> }
>>
>> /* rebuild sched domains if cpus_allowed has changed */
>> - if (cpus_updated || force_rebuild) {
>> - force_rebuild = false;
>> - rebuild_sched_domains();
>> + if (cpus_updated) {
>> + if (use_cpu_hp_lock)
>> + rebuild_sched_domains();
>> + else {
>> + /* When called during cpu hotplug cpu_hotplug_lock
>> + * is held by the calling thread, not
>> + * not cpuhp_thread_fun
>> + */
>
> ??? The comment is not clear.
Following is the scenario that is described by the comment
Process A
_cpu_down()
cpus_write_lock() //cpu_hotplug_lock held
cpuhp_kick_ap_work()
cpuhp_kick_ap()
wake_up_process() // wake up cpuhp_thread_fun
wait_for_ap_thread() //wait for hotplug thread to signal completion
cpuhp_thread_fun()
cpuhp_invoke_callback()
sched_cpu_deactivate()
cpuset_cpu_inactive()
cpuset_update_active_cpus()
cpuset_hotplug(false) \\ do not use cpu_hotplug_lock from _cpu_down() path
I will update the comment in next version of patch to elaborate more.
>
> Cheers,
> Longman
>
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