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

[Wengang Wang <wen.gang.wang@oracle.com>]

On 11-10-13 09:09, Sunil Mushran wrote:
> The last email you said it reproduced. Now you say it did not.
> I'm confused.

Oh? Did I. If I did, I meant it had reproductions in different customers's ENV,
I had no reproduction in house.

Sorry for confusion :P

thanks,
wengang.
> 
> On 10/12/2011 07:13 PM, Wengang Wang wrote:
> >On 11-10-12 19:11, Sunil Mushran wrote:
> >>That's what ovm does. Have you reproduced it with ovm3 kernel?
> >>
> >No, I have no reproductions.
> >
> >thanks,
> >wengang.
> >>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.
> >>>>>>>>
> >>>>>>>>_______________________________________________
> >>>>>>>>Ocfs2-devel mailing list
> >>>>>>>>Ocfs2-devel at oss.oracle.com
> >>>>>>>>http://oss.oracle.com/mailman/listinfo/ocfs2-devel
>