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

Chuck Lever
2025-01-24T14:32:36Z

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?

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?

> 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);
>

--
Chuck Lever