[PATCH v3 09/12] SUNRPC: Record last-request timestamp on svc_xprt

[Chuck Lever <cel@kernel.org>]

On a connection-oriented transport, the arrival of a later request
suggests the client received the prior reply. It is not proof: a
client with several requests outstanding sends the next one without
waiting for a reply.

Add xpt_last_recv to struct svc_xprt. svc_handle_xprt() samples
jiffies before calling xpo_recvfrom() and publishes the sample after
a successful receive. The DRC reads this value as a best-effort
signal that a completed reply has been delivered.

Signed-off-by: Chuck Lever <cel@kernel.org>
---
 include/linux/sunrpc/svc_xprt.h |  1 +
 net/sunrpc/svc_xprt.c           | 14 ++++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/include/linux/sunrpc/svc_xprt.h b/include/linux/sunrpc/svc_xprt.h
index ec1dcb51c2a2..da234a078c5d 100644
--- a/include/linux/sunrpc/svc_xprt.h
+++ b/include/linux/sunrpc/svc_xprt.h
@@ -81,6 +81,7 @@ struct svc_xprt {
 	struct net		*xpt_net;
 	netns_tracker		ns_tracker;
 	const struct cred	*xpt_cred;
+	unsigned long		xpt_last_recv;	/* jiffies of last request */
 	struct rpc_xprt		*xpt_bc_xprt;	/* NFSv4.1 backchannel */
 	struct rpc_xprt_switch	*xpt_bc_xps;	/* NFSv4.1 backchannel */
 };
diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index 7e8ef4832421..f57dac54c8dc 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -230,6 +230,7 @@ bool svc_xprt_init(struct net *net, struct svc_xprt_class *xcl,
 	set_bit(XPT_BUSY, &xprt->xpt_flags);
 	xprt->xpt_net = get_net_track(net, &xprt->ns_tracker, GFP_KERNEL);
 	strcpy(xprt->xpt_remotebuf, "uninitialized");
+	xprt->xpt_last_recv = jiffies;
 
 	if (xa_alloc_cyclic(&sn->svc_xprt_ids, &id, xprt,
 			    XA_LIMIT(1, UINT_MAX), &sn->svc_xprt_id_next,
@@ -901,6 +902,8 @@ static void svc_handle_xprt(struct svc_rqst *rqstp, struct svc_xprt *xprt)
 		svc_xprt_received(xprt);
 	} else if (svc_xprt_reserve_slot(rqstp, xprt)) {
 		/* XPT_DATA|XPT_DEFERRED case: */
+		unsigned long recv_time = jiffies;
+
 		rqstp->rq_deferred = svc_deferred_dequeue(xprt);
 		if (rqstp->rq_deferred)
 			len = svc_deferred_recv(rqstp);
@@ -916,6 +919,17 @@ static void svc_handle_xprt(struct svc_rqst *rqstp, struct svc_xprt *xprt)
 
 		clear_bit(XPT_OLD, &xprt->xpt_flags);
 
+		/*
+		 * A deferred request arrived before its deferral, so its
+		 * replay does not advance the timestamp. recv_time was
+		 * sampled before xpo_recvfrom() released XPT_BUSY.
+		 * time_after() keeps a slow thread from pushing the
+		 * timestamp past requests received since then.
+		 */
+		if (!rqstp->rq_deferred &&
+		    time_after(recv_time, READ_ONCE(xprt->xpt_last_recv)))
+			WRITE_ONCE(xprt->xpt_last_recv, recv_time);
+
 		rqstp->rq_chandle.defer = svc_defer;
 
 		if (serv->sv_stats)

-- 
2.55.0