[PATCH] kNFSd: TCP nfsd connection hangs when partial record header is received

Below patch resolves a hang where a TCP nfsd connection will hang even
though new data is received on the socket. We've seen this a few times in
our lab, but it usually happened every few weeks.

If a short record header is received, the SK_BUSY flag is never cleared,
and even though new data arrives, it will not be handled. This in turn
leads to hangs of particular clients (while others will continue to work
without problem).

I also changed the return code for that condition to be the same as for a
(regular) short read.

net/sunrpc/svcsock.c

@@ -886,8 +886,12 @@ svc_tcp_recvfrom(struct svc_rqst *rqstp)
 			goto error;
 		svsk->sk_tcplen += len;
 
-		if (len < want)
-			return 0;
+		if (len < want) {
+			dprintk("svc: short recvfrom while reading record length (%d of %lu)\n",
+			        len, want);
+			svc_sock_received(svsk);
+			return -EAGAIN; /* record header not complete */
+		}
 
 		svsk->sk_reclen = ntohl(svsk->sk_reclen);
 		if (!(svsk->sk_reclen & 0x80000000)) {