SUNRPC: Move xpt_mutex into socket xpo_sendto methods

It appears that the RPC/RDMA transport does not need serialization
of calls to its xpo_sendto method. Move the mutex into the socket
methods that still need that serialization.

Tail latencies are unambiguously better with this patch applied.
fio randrw 8KB 70/30 on NFSv3, smaller numbers are better:

    clat percentiles (usec):

With xpt_mutex:
r    | 99.99th=[ 8848]
w    | 99.99th=[ 9634]

Without xpt_mutex:
r    | 99.99th=[ 8586]
w    | 99.99th=[ 8979]

Serializing the construction of RPC/RDMA transport headers is not
really necessary at this point, because the Linux NFS server
implementation never changes its credit grant on a connection. If
that should change, then svc_rdma_sendto will need to serialize
access to the transport's credit grant fields.

Reported-by: kbuild test robot <lkp@intel.com>
[ cel: fix uninitialized variable warning ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

2 files changed, 20 insertions, 25 deletions

diff --git a/net/sunrpc/xprtrdma/svc_rdma_backchannel.c b/net/sunrpc/xprtrdma/svc_rdma_backchannel.c
index af7eb8d202ae..d9aab4504f2c 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_backchannel.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_backchannel.c
@@ -210,34 +210,31 @@ drop_connection:
 	return -ENOTCONN;
 }
 
-/* Send an RPC call on the passive end of a transport
- * connection.
+/**
+ * xprt_rdma_bc_send_request - Send a reverse-direction Call
+ * @rqst: rpc_rqst containing Call message to be sent
+ *
+ * Return values:
+ *   %0 if the message was sent successfully
+ *   %ENOTCONN if the message was not sent
  */
-static int
-xprt_rdma_bc_send_request(struct rpc_rqst *rqst)
+static int xprt_rdma_bc_send_request(struct rpc_rqst *rqst)
 {
 	struct svc_xprt *sxprt = rqst->rq_xprt->bc_xprt;
-	struct svcxprt_rdma *rdma;
+	struct svcxprt_rdma *rdma =
+		container_of(sxprt, struct svcxprt_rdma, sc_xprt);
 	int ret;
 
 	dprintk("svcrdma: sending bc call with xid: %08x\n",
 		be32_to_cpu(rqst->rq_xid));
 
-	mutex_lock(&sxprt->xpt_mutex);
-
-	ret = -ENOTCONN;
-	rdma = container_of(sxprt, struct svcxprt_rdma, sc_xprt);
-	if (!test_bit(XPT_DEAD, &sxprt->xpt_flags)) {
-		ret = rpcrdma_bc_send_request(rdma, rqst);
-		if (ret == -ENOTCONN)
-			svc_close_xprt(sxprt);
-	}
+	if (test_bit(XPT_DEAD, &sxprt->xpt_flags))
+		return -ENOTCONN;
 
-	mutex_unlock(&sxprt->xpt_mutex);
-
-	if (ret < 0)
-		return ret;
-	return 0;
+	ret = rpcrdma_bc_send_request(rdma, rqst);
+	if (ret == -ENOTCONN)
+		svc_close_xprt(sxprt);
+	return ret;
 }
 
 static void
diff --git a/net/sunrpc/xprtrdma/svc_rdma_sendto.c b/net/sunrpc/xprtrdma/svc_rdma_sendto.c
index b6c8643867f2..38e7c3c8c4a9 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_sendto.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_sendto.c
@@ -868,12 +868,10 @@ int svc_rdma_sendto(struct svc_rqst *rqstp)
 	__be32 *p;
 	int ret;
 
-	/* Create the RDMA response header. xprt->xpt_mutex,
-	 * acquired in svc_send(), serializes RPC replies. The
-	 * code path below that inserts the credit grant value
-	 * into each transport header runs only inside this
-	 * critical section.
-	 */
+	ret = -ENOTCONN;
+	if (svc_xprt_is_dead(xprt))
+		goto err0;
+
 	ret = -ENOMEM;
 	sctxt = svc_rdma_send_ctxt_get(rdma);
 	if (!sctxt)