nfsd: fix the slot sequence# when a callback fails

Commit 4281bfec36 patched the server so that the
callback session slot would be free'd for reuse when
a callback attempt fails.
However, this can often result in the sequence# for
the session slot to be advanced such that the client
end will reply NFSERR_SEQMISORDERED.

To avoid the NFSERR_SEQMISORDERED client reply,
this patch negates the sequence# advance for the
case where the callback has failed.
The common case is a failed back channel, where
the callback cannot be sent to the client, and
not advancing the sequence# is correct for this
case.  For the uncommon case where the client's
reply to the callback is lost, not advancing the
sequence# will indicate to the client that the
next callback is a retry and not a new callback.
But, since the FreeBSD server always sets "csa_cachethis"
false in the callback sequence operation, a retry
and a new callback should be handled the same way
by the client, so this should not matter.

Until you have this patch in your NFSv4.1/4.2 server,
you should consider avoiding the use of delegations.
Even with this patch, interoperation with the
Linux NFSv4.1/4.2 client in kernel versions prior
to 5.3 can result in frequent 15second delays if
delegations are enabled.  This occurs because, for
kernels prior to 5.3, the Linux client does a TCP
reconnect every time it sees multiple concurrent
callbacks and then it takes 15seconds to recover
the back channel after doing so.

MFC after:	2 weeks

sys/fs/nfs/nfs_commonkrpc.c

@@ -871,7 +871,7 @@ newnfs_request(struct nfsrv_descript *nd, struct nfsmount *nmp,
 			sep->nfsess_slotseq[nd->nd_slotid] += 10;
 			mtx_unlock(&sep->nfsess_mtx);
 			/* And free the slot. */
-			nfsv4_freeslot(sep, nd->nd_slotid);
+			nfsv4_freeslot(sep, nd->nd_slotid, false);
 		}
 		NFSINCRGLOBAL(nfsstatsv1.rpcinvalid);
 		error = ENXIO;
@@ -1108,7 +1108,7 @@ newnfs_request(struct nfsrv_descript *nd, struct nfsmount *nmp,
 		if ((nd->nd_flag & ND_NFSV4) != 0) {
 			/* Free the slot, as required. */
 			if (freeslot != -1)
-				nfsv4_freeslot(sep, freeslot);
+				nfsv4_freeslot(sep, freeslot, false);
 			/*
 			 * If this op is Putfh, throw its results away.
 			 */

sys/fs/nfs/nfs_commonsubs.c

@@ -4811,7 +4811,7 @@ nfsv4_sequencelookup(struct nfsmount *nmp, struct nfsclsession *sep,
  * Free a session slot.
  */
 void
-nfsv4_freeslot(struct nfsclsession *sep, int slot)
+nfsv4_freeslot(struct nfsclsession *sep, int slot, bool resetseq)
 {
 	uint64_t bitval;
 
@@ -4819,6 +4819,8 @@ nfsv4_freeslot(struct nfsclsession *sep, int slot)
 	if (slot > 0)
 		bitval <<= slot;
 	mtx_lock(&sep->nfsess_mtx);
+	if (resetseq)
+		sep->nfsess_slotseq[slot]--;
 	if ((bitval & sep->nfsess_slots) == 0)
 		printf("freeing free slot!!\n");
 	sep->nfsess_slots &= ~bitval;

sys/fs/nfs/nfs_var.h

@@ -355,7 +355,7 @@ void nfsv4_setsequence(struct nfsmount *, struct nfsrv_descript *,
     struct nfsclsession *, int);
 int nfsv4_sequencelookup(struct nfsmount *, struct nfsclsession *, int *,
     int *, uint32_t *, uint8_t *);
-void nfsv4_freeslot(struct nfsclsession *, int);
+void nfsv4_freeslot(struct nfsclsession *, int, bool);
 struct ucred *nfsrv_getgrpscred(struct ucred *);
 struct nfsdevice *nfsv4_findmirror(struct nfsmount *);
 void nfsm_set(struct nfsrv_descript *, u_int);

sys/fs/nfsserver/nfs_nfsdstate.c

@@ -4572,7 +4572,8 @@ nfsrv_docallback(struct nfsclient *clp, int procnum, nfsv4stateid_t *stateidp,
 		if ((clp->lc_flags & LCL_NFSV41) != 0) {
 			error = ECONNREFUSED;
 			if (procnum != NFSV4PROC_CBNULL)
-				nfsv4_freeslot(&sep->sess_cbsess, slotpos);
+				nfsv4_freeslot(&sep->sess_cbsess, slotpos,
+				    true);
 			nfsrv_freesession(sep, NULL);
 		} else if (nd->nd_procnum == NFSV4PROC_CBNULL)
 			error = newnfs_connect(NULL, &clp->lc_req, cred,
@@ -4604,8 +4605,24 @@ nfsrv_docallback(struct nfsclient *clp, int procnum, nfsv4stateid_t *stateidp,
 				error = ECONNREFUSED;
 			}
 			NFSD_DEBUG(4, "aft newnfs_request=%d\n", error);
-			if (error != 0 && procnum != NFSV4PROC_CBNULL)
+			if (error != 0 && procnum != NFSV4PROC_CBNULL) {
-				nfsv4_freeslot(&sep->sess_cbsess, slotpos);
+				/*
+				 * It is likely that the callback was never
+				 * processed by the client and, as such,
+				 * the sequence# for the session slot needs
+				 * to be backed up by one to avoid a
+				 * NFSERR_SEQMISORDERED error reply.
+				 * For the unlikely case where the callback
+				 * was processed by the client, this will
+				 * make the next callback on the slot
+				 * appear to be a retry.
+				 * Since callbacks never specify that the
+				 * reply be cached, this "apparent retry"
+				 * should not be a problem.
+				 */
+				nfsv4_freeslot(&sep->sess_cbsess, slotpos,
+				    true);
+			}
 			nfsrv_freesession(sep, NULL);
 		} else
 			error = newnfs_request(nd, NULL, clp, &clp->lc_req,