[PATCH v6 08/12] SUNRPC: Publish reply positions for upper-layer consumers

[Chuck Lever <cel@kernel.org>]

An RPC server's duplicate reply cache has to hold a cached reply
until the client has received it. SUNRPC gives an upper layer no way
to learn that, so a cache entry lives for the full retention
interval even when the reply was delivered at once.

Add two transport-neutral values that an upper layer can compare:
the position of each reply on its transport, recorded in the svc_rqst
by xpo_sendto(), and the position up to which the peer has
acknowledged, recorded on the svc_xprt. The units are private to the
transport (bytes for a stream, Send sequence for RDMA); a consumer
learns only whether one is less than or equal to the other. A reply
that xpo_sendto() does not hand to the wire keeps position zero and
is never reported as acknowledged.

The acknowledged position is an atomic64_t. A transport publishes it
outside any lock the consumer holds, and a torn 64-bit read on a
32-bit host could run ahead of the peer.

Add a per-service hook that runs once per request when its reply
phase ends, whether svc_send() sent the reply or svc_process()
dropped it, so an upper layer can record the reply's position
against its own state either way. On the drop path the position is
zero.

Assisted-by: LLM
Signed-off-by: Chuck Lever <cel@kernel.org>
---
 include/linux/sunrpc/svc.h      | 16 ++++++++++++++++
 include/linux/sunrpc/svc_xprt.h |  2 ++
 net/sunrpc/svc.c                |  3 +++
 net/sunrpc/svc_xprt.c           |  2 ++
 4 files changed, 23 insertions(+)

diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index 24698856eb40..01036e093012 100644
--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -59,6 +59,8 @@ enum {
 };
 
 
+struct svc_rqst;
+
 /*
  * RPC service.
  *
@@ -96,6 +98,13 @@ struct svc_serv {
 						 * connection */
 	bool			sv_bc_enabled;	/* service uses backchannel */
 #endif /* CONFIG_SUNRPC_BACKCHANNEL */
+
+	/*
+	 * Called once per request after its reply phase, whether
+	 * xpo_sendto() succeeded, failed, or was never reached because
+	 * svc_process() dropped the reply. See rq_reply_pos.
+	 */
+	void			(*sv_reply_sent)(struct svc_rqst *rqstp);
 };
 
 /* This is used by pool_stats to find and lock an svc */
@@ -267,6 +276,13 @@ struct svc_rqst {
 	unsigned int		bc_to_retries;
 	unsigned int		rq_status_counter; /* RPC processing counter */
 	void			*rq_private;	/* For use by the service thread */
+
+	/*
+	 * Transport position of this request's reply, set by
+	 * xpo_sendto(); zero when the reply did not reach the wire.
+	 * Comparable only with the xpt_acked_pos of the same svc_xprt.
+	 */
+	u64			rq_reply_pos;
 };
 
 /* bits for rq_flags */
diff --git a/include/linux/sunrpc/svc_xprt.h b/include/linux/sunrpc/svc_xprt.h
index 52082d8ea6dd..7176c42f19d7 100644
--- a/include/linux/sunrpc/svc_xprt.h
+++ b/include/linux/sunrpc/svc_xprt.h
@@ -66,6 +66,8 @@ struct svc_xprt {
 	atomic_t		xpt_reserved;	/* outq space rsvd, UDP only */
 	atomic_t		xpt_nr_rqsts;	/* Number of requests */
 	struct mutex		xpt_mutex;	/* to serialize sending data */
+	atomic64_t		xpt_acked_pos;	/* replies up to this position
+						 * are acknowledged; 0 = none */
 	spinlock_t		xpt_lock;	/* protects sk_deferred
 						 * and xpt_auth_cache */
 	void			*xpt_auth_cache;/* auth cache */
diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index f73412e123a1..9ef0661bb422 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -1488,6 +1488,7 @@ svc_process_common(struct svc_rqst *rqstp)
 
 	/* Reset the accept_stat for the RPC */
 	rqstp->rq_accept_statp = NULL;
+	rqstp->rq_reply_pos = 0;
 
 	/* Will be turned off only when NFSv4 Sessions are used */
 	set_bit(RQ_USEDEFERRAL, &rqstp->rq_flags);
@@ -1733,6 +1734,8 @@ void svc_process(struct svc_rqst *rqstp)
 		goto out_baddir;
 
 	if (!svc_process_common(rqstp)) {
+		if (rqstp->rq_server->sv_reply_sent)
+			rqstp->rq_server->sv_reply_sent(rqstp);
 		svc_release_rqst(rqstp);
 		goto out_drop;
 	}
diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index 463ba8d77f0c..1668d96ac9ff 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -1028,6 +1028,8 @@ void svc_send(struct svc_rqst *rqstp)
 	trace_svc_stats_latency(rqstp);
 
 	status = xprt->xpt_ops->xpo_sendto(rqstp);
+	if (xprt->xpt_server->sv_reply_sent)
+		xprt->xpt_server->sv_reply_sent(rqstp);
 
 	trace_svc_send(rqstp, status);
 }

-- 
2.55.0