[Ocfs2-devel] avoid being purged when queued for assert_master

[Sunil Mushran <sunil.mushran@oracle.com>]

That's what ovm does. Have you reproduced it with ovm3 kernel?

On 10/12/2011 07:07 PM, Wengang Wang wrote:
> On 11-10-13 09:51, Wengang Wang wrote:
>> On 11-10-12 18:47, Sunil Mushran wrote:
>>> I meant master_request (not query). We set refmap _before_
>>> asserting. So that should not happen.
>> Why can't the remote node requested deref (DLM_DEREF_LOCKRES_MSG)?
> The problem can easily happen on this dlmfs useage:
>
> reopen:
> 	open(create) /dlm/dirxx/filexx
> 	close	     /dlm/dirxx/filexx
> 	sleep 60
> 	goto reopen
>
>> thanks,
>> wengang.
>>> On 10/12/2011 06:02 PM, Wengang Wang wrote:
>>>> Hi Sunil,
>>>>
>>>> On 11-10-12 17:32, Sunil Mushran wrote:
>>>>> So you are saying a lockres can get purged before the node is asserting
>>>>> master to other nodes?
>>>>>
>>>>> The main place where we dispatch assert is during master_query.
>>>>> There we set refmap before dispatching. Meaning refmap will protect
>>>>> us from purging.
>>>>>
>>>>> But I think it could happen in master_requery, which only comes into
>>>>> play if a node dies during migration.
>>>>>
>>>>> Is that the case here?
>>>> I think this can mainly include the response for a master_request.
>>>> in dlm_master_request_handler(), the master node quques assert_master.
>>>> The node which requested a master_request knows the master by receving
>>>> response values. It doesn't need to wait until the assert_master come.
>>>> As you know, the asserting master work is done in a workqueue. And the
>>>> work item in it can be heavily delayed. So in the duriation from the
>>>> (old) master responding with "Yes, I am master" to it sending assert_master,
>>>> Anything can heppan, the worse case is the lockres on the (old) master
>>>> get purged and is remasted by another node. So in this case,
>>>> apparently, the old master shouldn't send the assert_master any longer.
>>>> To prevent that from happening, we should keep the lockres un-purged as
>>>> long as it's queued for master_request.
>>>>
>>>> #the problem is what my flush_workqueue patch tries to fix.
>>>>
>>>> thanks,
>>>> wengang.
>>>>
>>>>> On 10/12/2011 12:04 AM, Wengang Wang wrote:
>>>>>> Hi Sunil/Joel/Mark and anyone who has interest,
>>>>>>
>>>>>> This is not a patch but a discuss.
>>>>>>
>>>>>> Currently we have a problem:
>>>>>> When a lockres is still queued(in dlm->work_list) for sending an
>>>>>> assert_master(or in processing of sending), the lockres can't be
>>>>>> purged(removed from hash). there is no flag/state,on lockres its self,dinotes
>>>>>> this situation.
>>>>>>
>>>>>> The badness is that if the lockres is purged(surely not the owner at the
>>>>>> moment), and the assert_master is after the purge. it can confuse other
>>>>>> nodes. On another node, the owner now can be any other nodes, thus on
>>>>>> receiving the assert_master, it can trigger a BUG() because 'owner'
>>>>>> doesn't match.
>>>>>>
>>>>>> So we'd better to prevent the lockres from be purged when it's queued
>>>>>> for something(assert_master).
>>>>>>
>>>>>> Srini and I discussed some possible fixes:
>>>>>> 1) adding a flag to lockres->state.
>>>>>>     this does not work. A lockres can have multiple instances in the queue list.
>>>>>>     A simple flag is not safe. And the instances are not nested, so even
>>>>>>     saving a previous flags doesn't work. Neither can we merge the instances
>>>>>>     because they can be for different purposes.
>>>>>>
>>>>>> 2) checking if the lockres if queued before purging it.
>>>>>>    this works, but doesn't sounds good. it needs changes of current behaviour
>>>>>>    on the queue list.   Also, we have no idea on the performance of the checking
>>>>>>    (searching list).
>>>>>>
>>>>>> 3) making use of lockres->inflight_locks.
>>>>>>    this works, but seems to be a mis-use of inflight_locks.
>>>>>>
>>>>>> 4) adding a new member to lockres counting the queued time.
>>>>>>     this works and simple. but needs extra memory.
>>>>>>
>>>>>> I prefer to the 4).
>>>>>>
>>>>>> What's your idea?
>>>>>>
>>>>>> thanks,
>>>>>> wengang.
>>>>>>
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