Re: CLOSE/OPEN race

[Jeff Layton <jlayton@redhat.com>]

On Sat, 2016-11-12 at 10:31 -0500, Benjamin Coddington wrote:
> On 12 Nov 2016, at 7:54, Jeff Layton wrote:
> 
> > 
> > On Sat, 2016-11-12 at 06:08 -0500, Benjamin Coddington wrote:
> > > 
> > > I've been seeing the following on a modified version of generic/089
> > > that gets the client stuck sending LOCK with NFS4ERR_OLD_STATEID.
> > > 
> > > 1. Client has open stateid A, sends a CLOSE
> > > 2. Client sends OPEN with same owner
> > > 3. Client sends another OPEN with same owner
> > > 4. Client gets a reply to OPEN in 3, stateid is B.2 (stateid B 
> > > sequence 2)
> > > 5. Client does LOCK,LOCKU,FREE_STATEID from B.2
> > > 6. Client gets a reply to CLOSE in 1
> > > 7. Client gets reply to OPEN in 2, stateid is B.1
> > > 8. Client sends LOCK with B.1 - OLD_STATEID, now stuck in a loop
> > > 
> > > The CLOSE response in 6 causes us to clear NFS_OPEN_STATE, so that 
> > > the OPEN
> > > response in 7 is able to update the open_stateid even though it has a 
> > > lower
> > > sequence number.
> > > 
> > > I think this case could be handled by never updating the open_stateid 
> > > if the
> > > stateids match but the sequence number of the new state is less than 
> > > the
> > > current open_state.
> > > 
> > 
> > What kernel is this on?
> 
> On v4.9-rc2 with a couple fixups.  Without them, I can't test long 
> enough to
> reproduce this race.  I don't think any of those are involved in this
> problem, though.
> 
> > 
> > Yes, that seems wrong. The client should be picking B.2 for the open
> > stateid to use. I think that decision of whether to take a seqid is 
> > made
> > in nfs_need_update_open_stateid. The logic in there looks correct to 
> > me
> > at first glance though.
> 
> nfs_need_update_open_stateid() will return true if NFS_OPEN_STATE is 
> unset.
> That's the precondition set up by steps 1-6.  Perhaps it should not 
> update
> the stateid if they match but the sequence number is less, and still set
> NFS_OPEN_STATE once more.  That will fix _this_ case.  Are there other 
> cases
> where that would be a problem?
> 
> Ben

That seems wrong. The only close was sent in step 1, and that was for a
completely different stateid (A rather than B). It seems likely that
that is where the bug is.

-- 
Jeff Layton <jlayton@redhat.com>