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

[Chuck Lever <chuck.lever@oracle.com>]

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