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

[Wengang Wang]

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
>