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

[Wengang Wang]

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
> >