Re: [PATCH] md: create new workqueue for object destruction

[NeilBrown <neilb@suse.com>]

[-- Attachment #1: Type: text/plain, Size: 1777 bytes --]

On Tue, Oct 17 2017, Shaohua Li wrote:

> On Tue, Oct 17, 2017 at 04:04:52PM +1100, Neil Brown wrote:
>> 
>> lockdep currently complains about a potential deadlock
>> with sysfs access taking reconfig_mutex, and that
>> waiting for a work queue to complete.
>> 
>> The cause is inappropriate overloading of work-items
>> on work-queues.
>> 
>> We currently have two work-queues: md_wq and md_misc_wq.
>> They service 5 different tasks:
>> 
>>   mddev->flush_work                       md_wq
>>   mddev->event_work (for dm-raid)         md_misc_wq
>>   mddev->del_work (mddev_delayed_delete)  md_misc_wq
>>   mddev->del_work (md_start_sync)         md_misc_wq
>>   rdev->del_work                          md_misc_wq
>> 
>> We need to call flush_workqueue() for md_start_sync and ->event_work
>> while holding reconfig_mutex, but mustn't hold it when
>> flushing mddev_delayed_delete or rdev->del_work.
>> 
>> md_wq is a bit special as it has WQ_MEM_RECLAIM so it is
>> best to leave that alone.
>> 
>> So create a new workqueue, md_del_wq, and a new work_struct,
>> mddev->sync_work, so we can keep two classes of work separate.
>> 
>> md_del_wq and ->del_work are used only for destroying rdev
>> and mddev.
>> md_misc_wq is used for event_work and sync_work.
>> 
>> Also document the purpose of each flush_workqueue() call.
>> 
>> This removes the lockdep warning.
>
> I had the exactly same patch queued internally,

Cool :-)

>                                                   but the mdadm test suite still
> shows lockdep warnning. I haven't time to check further.
>

The only other lockdep I've seen later was some ext4 thing, though I
haven't tried the full test suite.  I might have a look tomorrow.
Thanks,
NeilBrown

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 832 bytes --]