[Ocfs2-devel] [PATCH 1/2] ocfs2 fix o2dlm dlm run purgelist

[Wengang Wang]

On 10-06-18 09:37, Sunil Mushran wrote:
> On 06/17/2010 07:37 PM, Wengang Wang wrote:
> >On 10-06-17 08:06, Sunil Mushran wrote:
> >>On 06/15/2010 11:06 PM, Wengang Wang wrote:
> >>>still the question.
> >>>If you have sent DEREF request to the master, and the lockres became in-use
> >>>again, then the lockres remains in the hash table and also in the purge list.
> >>>So
> >>>1) If this node is the last ref, there is a possibility that the master
> >>>purged the lockres after receiving DEREF request from this node. In this
> >>>case, when this node does dlmlock_remote(), the lockres won't be found on the
> >>>master. How to deal with it?
> >>>
> >>>2) The lockres on this node is going to be purged again, it means it will send
> >>>secondary DEREFs to the master. This is not good I think.
> >>>
> >>>A thought is setting lockres->owner to DLM_LOCK_RES_OWNER_UNKNOWN after
> >>>sending a DEREF request againt this lockres. Also redo master reqeust
> >>>before locking on it.
> >>The fix we are working towards is to ensure that we set
> >>DLM_LOCK_RES_DROPPING_REF once we are determined
> >>to purge the lockres. As in, we should not let go of the spinlock
> >>before we have either set the flag or decided against purging
> >>that resource.
> >>
> >>Once the flag is set, new users looking up the resource via
> >>dlm_get_lock_resource() will notice the flag and will then wait
> >>for that flag to be cleared before looking up the lockres hash
> >>again. If all goes well, the lockres will not be found (because it
> >>has since been unhashed) and it will be forced to go thru the
> >>full mastery process.
> >That is ideal.
> >In many cases the lockres is not got via dlm_get_lock_resource(), but
> >via dlm_lookup_lockres()/__dlm_lookup_lockres, which doesn't set the new
> >IN-USE state, directly. dlm_lookup_lockres() takes and drops
> >dlm->spinlock. And some of caller of __dlm_lookup_lockres() drops the
> >spinlock as soon as it got the lockres. Such paths access the lockres
> >later after dropping dlm->spinlock and res->spinlock.
> >So there is a window that dlm_thread() get a chance to take the
> >dlm->spinlock and res->spinlock and set the DROPPING_REF state.
> >So whether new users can get the lockres depends on how "new" it is. If
> >finds the lockres after DROPPING_REF state is set, sure it works well. But
> >if it find it before DROPPING_REF is set, it won't protect the lockres
> >from purging since even it "gets" the lockres, the lockres can still in
> >unused state.
> 
> dlm_lookup_lockres() and friends just looks up the lockres hash.
> dlm_get_lock_resource() also calls it. It inturn is called by dlmlock()
> to find and/or create lockres and create a lock on that resource.

Yes you are right.
> The other calls to dlm_lookup_lockres() are from handlers and those
> handlers can only be tickled if a lock already exists. And if a lock
> exits, then we cannot be purging the lockres.
> 
> The one exception is the create_lock handler and that only comes
> into play on the lockres master. The inflight ref blocks removal of
> such lockres in the window before the lock is created.

I think there is another exception, dlm_mig_lockres_handler(). Could you check it
in my email(in this thread, to Srini, Message-ID:
<20100617110548.GA3178 at laptop.us.oracle.com>).

> DROPPING_REF is only valid for non-master nodes. As in, only
> a non-master node has to send a deref message to the master node.

Yes I know.
> Confused? Well, I think this needs to be documented. I guess I will
> do that after I am done with the global heartbeat business.

No, I am clear. Well a document will greatly helpful!

regards,
wengang.