nfsd: fix the slot sequence# when a callback fails

Commit 4281bfec3628 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

1 files changed, 1 insertions, 1 deletions

diff --git a/sys/fs/nfs/nfs_var.h b/sys/fs/nfs/nfs_var.h
index 201c5a5fc2b5..f23d56050449 100644
--- a/sys/fs/nfs/nfs_var.h
+++ b/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);