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

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

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)?

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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>