nfscl: Fix oddball cases for session slot release

We have identified some cases where silent slot loss can occur
when operations on NFS mounts are aborted. We experience this
when using NFSv4.2, but it likely also occurs with NFSv4.1.

A slot is acquired for compound operations by nfsv4_setsequence()
and freed by newnfs_request(). Any call path that abandons the
compound before reaching newnfs_request() loses the slot permanently.

We identified four call sites where this happens, one of
which where it actually does happen for us in a semi-reproducible
way, which allowed us to develop a candidate patch, attached.

The patch adds one function, nfsv4_freeunsentslot(), to
nfs_clcomsubs.c. It is called from each of the four call
sites: nfsrpc_writerpc(), nfsrpc_writeds(), and two in
nfsrpc_setextattr().

nfsv4_freeunsentslot() calls nfsv4_freeslot() with resetseq
set to true because the aborted RPC will never be sent and
therefore the slot sequence ID should not advance.

With this patch and other diagnostics, we have confirmed
that slot loss does not occur under the conditions that
otherwise cause it in our environment.

PR:	298416
Reviewed by:	rmacklem
MFC after:	1 week

sys/fs/nfs/nfs_var.h

@@ -383,6 +383,8 @@ int nfscl_wcc_data(struct nfsrv_descript *, vnode_t,
 int nfsm_loadattr(struct nfsrv_descript *, struct nfsvattr *);
 int nfscl_request(struct nfsrv_descript *, vnode_t,
          NFSPROC_T *, struct ucred *);
+void nfsv4_freeunsentslot(struct nfsmount *, struct nfsclsession *,
+    struct nfsrv_descript *);
 
 /* nfs_nfsdsubs.c */
 void nfsd_fhtovp(struct nfsrv_descript *, struct nfsrvfh *, int,

sys/fs/nfsclient/nfs_clcomsubs.c

@@ -443,3 +443,21 @@ nfscl_lockderef(struct nfsv4lock *lckp)
 	}
 	NFSUNLOCKCLSTATE();
 }
+
+/*
+ * Free the session slot of a compound that was built but will not be sent
+ * to the server.
+ */
+void
+nfsv4_freeunsentslot(struct nfsmount *nmp, struct nfsclsession *sep,
+    struct nfsrv_descript *nd)
+{
+
+	if (nmp == NULL || (nd->nd_flag & (ND_NFSV41 | ND_HASSLOTID)) !=
+	    (ND_NFSV41 | ND_HASSLOTID))
+		return;
+	if (sep == NULL)
+		sep = nfsmnt_mdssession(nmp);
+	if (sep != NULL)
+		nfsv4_freeslot(sep, nd->nd_slotid, true);
+}

sys/fs/nfsclient/nfs_clrpcops.c

@@ -2138,6 +2138,7 @@ nfsrpc_writerpc(vnode_t vp, struct uio *uiop, vm_page_t *pgp, int *iomode,
 			error = nfsm_uiombuf(nd, uiop, len);
 		}
 		if (error != 0) {
+			nfsv4_freeunsentslot(nmp, NULL, nd);
 			m_freem(nd->nd_mreq);
 			free(nd, M_TEMP);
 			return (error);
@@ -7376,6 +7377,7 @@ nfsrpc_writeds(vnode_t vp, struct uio *uiop, int *iomode, int *must_commit,
 	*tl = txdr_unsigned(len);
 	error = nfsm_uiombuf(nd, uiop, len);
 	if (error != 0) {
+		nfsv4_freeunsentslot(nmp, &dsp->nfsclds_sess, nd);
 		m_freem(nd->nd_mreq);
 		return (error);
 	}
@@ -9740,6 +9742,7 @@ nfsrpc_setextattr(vnode_t vp, const char *name, struct uio *uiop,
 	NFSCL_REQSTART(nd, NFSPROC_SETEXTATTR, vp, cred);
 	if (uiop->uio_resid > nd->nd_maxreq) {
 		/* nd_maxreq is set by NFSCL_REQSTART(). */
+		nfsv4_freeunsentslot(VFSTONFS(vp->v_mount), NULL, nd);
 		m_freem(nd->nd_mreq);
 		return (EINVAL);
 	}
@@ -9750,6 +9753,7 @@ nfsrpc_setextattr(vnode_t vp, const char *name, struct uio *uiop,
 	*tl = txdr_unsigned(uiop->uio_resid);
 	error = nfsm_uiombuf(nd, uiop, uiop->uio_resid);
 	if (error != 0) {
+		nfsv4_freeunsentslot(VFSTONFS(vp->v_mount), NULL, nd);
 		m_freem(nd->nd_mreq);
 		return (error);
 	}