Re: [PATCH] nfsd: CB_RECALL can race with FREE_STATEID

[Scott Mayhew <smayhew@redhat.com>]

On Thu, 18 Apr 2019, J. Bruce Fields wrote:

> On Thu, Apr 18, 2019 at 09:24:00AM -0400, Scott Mayhew wrote:
> > While trying to track down some issues involving large numbers of
> > delegations being recalled/revoked, I caught the server setting
> > SEQ4_STATUS_CB_PATH_DOWN while the client was actively responding to
> > CB_RECALLs.  It turns out that the client had already done a
> > TEST_STATEID and FREE_STATEID for a delegation being recalled by the
> > time it received the CB_RECALL.
> 
> That's interesting, thanks!
> 
> This exception seems awfully narrow, though.
> 
> If we get back any NFS-level error at all, then I think the callback
> channel is working (am I wrong?)

Correct, if the client replies with either NFS4ERR_DELAY or
NFS4ERR_BAD_STATEID, the server will retry 1 time (see dl_retries).
After that, we fall thru and nfsd4_cb_recall_done() returns -1 which
causes the SEQ4_STATUS_CB_PATH_DOWN flag to be set.

> and telling the client to set up a new
> one is probably not going to help.  The best we can do is probably just
> give up

That's what the patch is essentially doing.  Or are you saying don't
even bother with the checks but still return 1 so we don't set the
SEQ4_STATUS_CB_PATH_DOWN flag?

> and let the client deal with the ensuing
> RECALLABLE_STATE_REVOKED flag.

The client's already dealing with the RECALLABLE_STATE_REVOKED flag,
that's why it sent a TEST_STATEID and FREE_STATEID before it got this
particular CB_RECALL.  The idea behind the patch is to not give the
state manager on the client additional work by setting CB_PATH_DOWN when
the callback channel is clearly working...

-Scott
> 
> --b.
> 
> > 
> > Signed-off-by: Scott Mayhew <smayhew@redhat.com>
> > ---
> >  fs/nfsd/nfs4state.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> > index 6a45fb00c5fc..e88e429133a8 100644
> > --- a/fs/nfsd/nfs4state.c
> > +++ b/fs/nfsd/nfs4state.c
> > @@ -3958,6 +3958,14 @@ static int nfsd4_cb_recall_done(struct nfsd4_callback *cb,
> >  			rpc_delay(task, 2 * HZ);
> >  			return 0;
> >  		}
> > +		/*
> > +		 * Race: client may have done a FREE_STATEID before
> > +		 * receiving the CB_RECALL.
> > +		 */
> > +		if (dp->dl_stid.sc_type == NFS4_REVOKED_DELEG_STID &&
> > +				refcount_read(&dp->dl_stid.sc_count) == 1 &&
> > +				list_empty(&dp->dl_recall_lru))
> > +			return 1;
> >  		/*FALLTHRU*/
> >  	default:
> >  		return -1;
> > -- 
> > 2.17.2