Re: [PATCH 2/8] nfsd: fix CB_SEQUENCE error handling of NFS4ERR_{BADSLOT,BADSESSION,SEQ_MISORDERED}

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2025-01-24 at 09:32 -0500, Chuck Lever wrote:
> On 1/23/25 3:25 PM, Jeff Layton wrote:
> > The current error handling has some problems:
> > 
> > BADSLOT and BADSESSION: don't release the slot before retrying the call
> > 
> > SEQ_MISORDERED: does some sketchy resetting of the seqid? I can't find any
> > recommendation about doing that in the spec, and it seems wrong.
> 
> Random thought: You might use the Linux NFS client's forechannel session
> implementation as a code reference.
> 
> 
> > Handle all three errors the same way: release the slot, but then handle
> > it just like we would as if we hadn't gotten a reply; mark the session
> > as faulty, and retry the call.
> 
> Some questions:
> 
> Why does it matter whether NFSD keeps the slot if both sides plan to
> destroy the session?
> 

It may not be required, but there is no reason to hold onto the slot in
these cases. Also, at this point, only nfsd has declared that it needs
a new session (see below).

> Also, AFAICT marking CB_FAULT does not destroy the session, it simply
> tries to recreate backchannel's rpc_clnt. Perhaps NFSD's callback code
> should actively destroy the session and let the client drive a fresh
> CREATE_SESSION to recover?
> 

Marking it with a fault just sets the cl_cb_state to NFSD4_CB_FAULT.
Then, on the next SEQUENCE call, that makes nfsd set
SEQ4_STATUS_BACKCHANNEL_FAULT, which should make the client recreate
the session. Obviously, there is some delay involved there since we
might have to wait for the client to do a lease renewal before this
happens.

> 
> > Fixes: 7ba6cad6c88f ("nfsd: New helper nfsd4_cb_sequence_done() for processing more cb errors")
> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > ---
> >   fs/nfsd/nfs4callback.c | 27 +++++++++++----------------
> >   1 file changed, 11 insertions(+), 16 deletions(-)
> > 
> > diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
> > index e12205ef16ca932ffbcc86d67b0817aec2436c89..bfc9de1fcb67b4f05ed2f7a28038cd8290809c17 100644
> > --- a/fs/nfsd/nfs4callback.c
> > +++ b/fs/nfsd/nfs4callback.c
> > @@ -1371,17 +1371,24 @@ static bool nfsd4_cb_sequence_done(struct rpc_task *task, struct nfsd4_callback
> >   		nfsd4_mark_cb_fault(cb->cb_clp);
> >   		ret = false;
> >   		break;
> > +	case -NFS4ERR_BADSESSION:
> > +	case -NFS4ERR_BADSLOT:
> > +	case -NFS4ERR_SEQ_MISORDERED:
> > +		/*
> > +		 * These errors indicate that something has gone wrong
> > +		 * with the server and client's synchronization. Release
> > +		 * the slot, but handle it as if we hadn't gotten a reply.
> > +		 */
> > +		nfsd41_cb_release_slot(cb);
> > +		fallthrough;
> >   	case 1:
> >   		/*
> >   		 * cb_seq_status remains 1 if an RPC Reply was never
> >   		 * received. NFSD can't know if the client processed
> >   		 * the CB_SEQUENCE operation. Ask the client to send a
> > -		 * DESTROY_SESSION to recover.
> > +		 * DESTROY_SESSION to recover, but keep the slot.
> >   		 */
> > -		fallthrough;
> > -	case -NFS4ERR_BADSESSION:
> >   		nfsd4_mark_cb_fault(cb->cb_clp);
> > -		ret = false;
> >   		goto need_restart;
> >   	case -NFS4ERR_DELAY:
> >   		cb->cb_seq_status = 1;
> > @@ -1390,14 +1397,6 @@ static bool nfsd4_cb_sequence_done(struct rpc_task *task, struct nfsd4_callback
> >   
> >   		rpc_delay(task, 2 * HZ);
> >   		return false;
> > -	case -NFS4ERR_BADSLOT:
> > -		goto retry_nowait;
> > -	case -NFS4ERR_SEQ_MISORDERED:
> > -		if (session->se_cb_seq_nr[cb->cb_held_slot] != 1) {
> > -			session->se_cb_seq_nr[cb->cb_held_slot] = 1;
> > -			goto retry_nowait;
> > -		}
> > -		break;
> >   	default:
> >   		nfsd4_mark_cb_fault(cb->cb_clp);
> >   	}
> > @@ -1405,10 +1404,6 @@ static bool nfsd4_cb_sequence_done(struct rpc_task *task, struct nfsd4_callback
> >   	nfsd41_cb_release_slot(cb);
> >   out:
> >   	return ret;
> > -retry_nowait:
> > -	if (rpc_restart_call_prepare(task))
> > -		ret = false;
> > -	goto out;
> >   need_restart:
> >   	if (!test_bit(NFSD4_CLIENT_CB_KILL, &clp->cl_flags)) {
> >   		trace_nfsd_cb_restart(clp, cb);
> > 
> 
> 

-- 
Jeff Layton <jlayton@kernel.org>