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

[Wengang Wang]

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.

regards,
wengang.