From: Prateek Sood <prsood@codeaurora.org>
To: Waiman Long <longman@redhat.com>,
peterz@infradead.org, tj@kernel.org, lizefan@huawei.com,
mingo@kernel.org, boqun.feng@gmail.com, tglx@linutronix.de
Cc: cgroups@vger.kernel.org, sramana@codeaurora.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] cgroup/cpuset: remove circular dependency deadlock
Date: Fri, 27 Oct 2017 13:33:41 +0530 [thread overview]
Message-ID: <45cdac2f-4462-e5b5-d724-8cca58e3932a@codeaurora.org> (raw)
In-Reply-To: <dc80ad9d-5b3d-b991-76c8-35630bc139c5@redhat.com>
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
>
--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation
Center, Inc., is a member of Code Aurora Forum, a Linux Foundation
Collaborative Project
next prev parent reply other threads:[~2017-10-27 8:04 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-07 13:56 [PATCH] cgroup/cpuset: remove circular dependency deadlock Prateek Sood
2017-09-07 17:45 ` Peter Zijlstra
2017-09-08 2:13 ` Prateek Sood
2017-09-07 17:51 ` Peter Zijlstra
2017-10-09 13:27 ` Prateek Sood
2017-10-11 9:48 ` Peter Zijlstra
2017-10-25 8:39 ` Prateek Sood
2017-10-25 9:30 ` Peter Zijlstra
2017-10-26 11:52 ` Prateek Sood
2017-10-26 14:05 ` Waiman Long
2017-10-27 8:03 ` Prateek Sood [this message]
2017-10-30 7:16 ` [PATCH v2] " Prateek Sood
2017-11-06 4:01 ` Prateek Sood
2017-11-15 10:26 ` Prateek Sood
2017-11-15 10:37 ` Peter Zijlstra
2017-11-15 14:20 ` [PATCH v3 0/2] Invert cpu_hotplug_lock and cpuset_mutex locking order Prateek Sood
2017-11-15 14:20 ` [PATCH v3 1/2] cgroup/cpuset: remove circular dependency deadlock Prateek Sood
2017-11-15 14:20 ` [PATCH v3 2/2] cpuset: Make cpuset hotplug synchronous Prateek Sood
2017-11-27 16:48 ` [PATCH v3 0/2] Invert cpu_hotplug_lock and cpuset_mutex locking order Tejun Heo
2017-11-15 17:05 ` [PATCH v2] cgroup/cpuset: remove circular dependency deadlock Tejun Heo
2017-11-15 17:18 ` Prateek Sood
-- strict thread matches above, loose matches on Subject: below --
2017-09-07 6:04 [PATCH] " Prateek Sood
2017-09-07 7:28 ` Peter Zijlstra
2017-09-07 8:56 ` Boqun Feng
2017-09-07 9:07 ` Prateek Sood
2017-09-07 9:05 ` Prateek Sood
2017-09-06 11:48 Prateek Sood
2017-09-06 12:56 ` Waiman Long
2017-09-06 14:23 ` Prateek Sood
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