[RFC PATCH 6/8] rxrpc: Use selected call in recvmsg()

[David Howells]

Allow an AF_RXRPC socket to be limited temporarily such that recvmsg() will
only retreive messages and data pertaining to one particular call.  The
call to be read from must be preselected with setsockopt():

	setsockopt(client, SOL_RXRPC, RXRPC_SELECT_CALL_FOR_RECV,
		   &call_id, sizeof(call_id));
	ret = recvmsg(client, &msg, 0);

The preselected call can be cleared by giving it a call ID of 0.  The
preselection is also automatically cleared if recvmsg returns the
completion state of the call (MSG_EOR will be set).

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

 net/rxrpc/recvmsg.c |   59 ++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 56 insertions(+), 3 deletions(-)

diff --git a/net/rxrpc/recvmsg.c b/net/rxrpc/recvmsg.c
index 2b596e2172ce..3fc6bf8b1ff2 100644
--- a/net/rxrpc/recvmsg.c
+++ b/net/rxrpc/recvmsg.c
@@ -540,6 +540,51 @@ int rxrpc_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 		return -EAGAIN;
 	}
 
+	if (rx->selected_recv_call) {
+		/* The call to receive from was dictated by
+		 * setsockopt(RXRPC_SELECT_CALL_FOR_RECV).
+		 */
+		call = rx->selected_recv_call;
+		rxrpc_get_call(call, rxrpc_call_got);
+		if (!list_empty(&call->recvmsg_link)) {
+			write_lock_bh(&rx->recvmsg_lock);
+			goto use_this_call;
+		}
+
+		if (timeo == 0) {
+			ret = -EWOULDBLOCK;
+			call = NULL;
+			goto error_no_call;
+		}
+
+		release_sock(&rx->sk);
+
+		/* Wait for something to happen */
+		prepare_to_wait_exclusive(sk_sleep(&rx->sk), &wait,
+					  TASK_INTERRUPTIBLE);
+		for (;;) {
+			ret = sock_error(&rx->sk);
+			if (ret) {
+				rxrpc_put_call(call, rxrpc_call_put);
+				goto wait_error;
+			}
+
+			if (!list_empty(&call->recvmsg_link))
+				break;
+
+			if (signal_pending(current)) {
+				rxrpc_put_call(call, rxrpc_call_put);
+				goto wait_interrupted;
+			}
+			trace_rxrpc_recvmsg(NULL, rxrpc_recvmsg_wait,
+					    0, 0, 0, 0);
+			timeo = schedule_timeout(timeo);
+		}
+		finish_wait(sk_sleep(&rx->sk), &wait);
+		write_lock_bh(&rx->recvmsg_lock);
+		goto use_this_call;
+	}
+
 	if (list_empty(&rx->recvmsg_q)) {
 		ret = -EWOULDBLOCK;
 		if (timeo == 0) {
@@ -573,10 +618,11 @@ int rxrpc_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 	write_lock_bh(&rx->recvmsg_lock);
 	l = rx->recvmsg_q.next;
 	call = list_entry(l, struct rxrpc_call, recvmsg_link);
+	if (flags & MSG_PEEK)
+		rxrpc_get_call(call, rxrpc_call_got);
+use_this_call:
 	if (!(flags & MSG_PEEK))
 		list_del_init(&call->recvmsg_link);
-	else
-		rxrpc_get_call(call, rxrpc_call_got);
 	write_unlock_bh(&rx->recvmsg_lock);
 
 	trace_rxrpc_recvmsg(call, rxrpc_recvmsg_dequeue, 0, 0, 0, 0);
@@ -654,8 +700,15 @@ int rxrpc_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 		ret = rxrpc_recvmsg_term(call, msg);
 		if (ret < 0)
 			goto error_unlock_call;
-		if (!(flags & MSG_PEEK))
+		if (!(flags & MSG_PEEK)) {
+			struct rxrpc_call *old = call;
+			if (try_cmpxchg(&rx->selected_recv_call, &old, NULL))
+				rxrpc_put_call(call, rxrpc_call_put);
+			old = call;
+			if (try_cmpxchg(&rx->selected_send_call, &old, NULL))
+				rxrpc_put_call(call, rxrpc_call_put);
 			rxrpc_release_call(call);
+		}
 		msg->msg_flags |= MSG_EOR;
 		ret = 1;
 	}