SUNRPC: Use poll() to fix up the socket requeue races

Because we clear XPRT_SOCK_DATA_READY before reading, we can end up
with a situation where new data arrives, causing xs_data_ready() to
queue up a second receive worker job for the same socket, which then
immediately gets stuck waiting on the transport receive mutex.
The fix is to only clear XPRT_SOCK_DATA_READY once we're done reading,
and then to use poll() to check if we might need to queue up a new
job in order to deal with any new data.
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

net/sunrpc/xprtsock.c

@@ -656,13 +656,34 @@ xs_read_stream(struct sock_xprt *transport, int flags)
 	return ret != 0 ? ret : -ESHUTDOWN;
 }
 
+static __poll_t xs_poll_socket(struct sock_xprt *transport)
+{
+	return transport->sock->ops->poll(NULL, transport->sock, NULL);
+}
+
+static bool xs_poll_socket_readable(struct sock_xprt *transport)
+{
+	__poll_t events = xs_poll_socket(transport);
+
+	return (events & (EPOLLIN | EPOLLRDNORM)) && !(events & EPOLLRDHUP);
+}
+
+static void xs_poll_check_readable(struct sock_xprt *transport)
+{
+
+	clear_bit(XPRT_SOCK_DATA_READY, &transport->sock_state);
+	if (!xs_poll_socket_readable(transport))
+		return;
+	if (!test_and_set_bit(XPRT_SOCK_DATA_READY, &transport->sock_state))
+		queue_work(xprtiod_workqueue, &transport->recv_worker);
+}
+
 static void xs_stream_data_receive(struct sock_xprt *transport)
 {
 	size_t read = 0;
 	ssize_t ret = 0;
 
 	mutex_lock(&transport->recv_mutex);
-	clear_bit(XPRT_SOCK_DATA_READY, &transport->sock_state);
 	if (transport->sock == NULL)
 		goto out;
 	for (;;) {
@@ -672,6 +693,7 @@ static void xs_stream_data_receive(struct sock_xprt *transport)
 		read += ret;
 		cond_resched();
 	}
+	xs_poll_check_readable(transport);
 out:
 	mutex_unlock(&transport->recv_mutex);
 	trace_xs_stream_read_data(&transport->xprt, ret, read);
@@ -1362,7 +1384,6 @@ static void xs_udp_data_receive(struct sock_xprt *transport)
 	int err;
 
 	mutex_lock(&transport->recv_mutex);
-	clear_bit(XPRT_SOCK_DATA_READY, &transport->sock_state);
 	sk = transport->inet;
 	if (sk == NULL)
 		goto out;
@@ -1374,6 +1395,7 @@ static void xs_udp_data_receive(struct sock_xprt *transport)
 		consume_skb(skb);
 		cond_resched();
 	}
+	xs_poll_check_readable(transport);
 out:
 	mutex_unlock(&transport->recv_mutex);
 }