SUNRPC: Fixup socket wait for memory

We're seeing hangs in the NFS client code, with loops of the form:

 RPC: 30317 xmit incomplete (267368 left of 524448)
 RPC: 30317 call_status (status -11)
 RPC: 30317 call_transmit (status 0)
 RPC: 30317 xprt_prepare_transmit
 RPC: 30317 xprt_transmit(524448)
 RPC:       xs_tcp_send_request(267368) = -11
 RPC: 30317 xmit incomplete (267368 left of 524448)
 RPC: 30317 call_status (status -11)
 RPC: 30317 call_transmit (status 0)
 RPC: 30317 xprt_prepare_transmit
 RPC: 30317 xprt_transmit(524448)

Turns out commit ceb5d58b2170 ("net: fix sock_wake_async() rcu protection")
moved SOCKWQ_ASYNC_NOSPACE out of sock->flags and into sk->sk_wq->flags,
however it never tried to fix up the code in net/sunrpc.

The new idiom is to use the flags in the RCU protected struct socket_wq.
While we're at it, clear out the now redundant places where we set/clear
SOCKWQ_ASYNC_NOSPACE and SOCK_NOSPACE. In principle, sk_stream_wait_memory()
is supposed to set these for us, so we only need to clear them in the
particular case of our ->write_space() callback.

Fixes: ceb5d58b2170 ("net: fix sock_wake_async() rcu protection")
Cc: Eric Dumazet <edumazet@google.com>
Cc: stable@vger.kernel.org # 4.4
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

net/sunrpc/xprtsock.c

@@ -398,7 +398,6 @@ static int xs_sendpages(struct socket *sock, struct sockaddr *addr, int addrlen,
 	if (unlikely(!sock))
 		return -ENOTSOCK;
 
-	clear_bit(SOCKWQ_ASYNC_NOSPACE, &sock->flags);
 	if (base != 0) {
 		addr = NULL;
 		addrlen = 0;
@@ -442,7 +441,6 @@ static void xs_nospace_callback(struct rpc_task *task)
 	struct sock_xprt *transport = container_of(task->tk_rqstp->rq_xprt, struct sock_xprt, xprt);
 
 	transport->inet->sk_write_pending--;
-	clear_bit(SOCKWQ_ASYNC_NOSPACE, &transport->sock->flags);
 }
 
 /**
@@ -467,20 +465,11 @@ static int xs_nospace(struct rpc_task *task)
 
 	/* Don't race with disconnect */
 	if (xprt_connected(xprt)) {
-		if (test_bit(SOCKWQ_ASYNC_NOSPACE, &transport->sock->flags)) {
-			/*
-			 * Notify TCP that we're limited by the application
-			 * window size
-			 */
-			set_bit(SOCK_NOSPACE, &transport->sock->flags);
-			sk->sk_write_pending++;
-			/* ...and wait for more buffer space */
-			xprt_wait_for_buffer_space(task, xs_nospace_callback);
-		}
-	} else {
-		clear_bit(SOCKWQ_ASYNC_NOSPACE, &transport->sock->flags);
+		/* wait for more buffer space */
+		sk->sk_write_pending++;
+		xprt_wait_for_buffer_space(task, xs_nospace_callback);
+	} else
 		ret = -ENOTCONN;
-	}
 
 	spin_unlock_bh(&xprt->transport_lock);
 
@@ -616,9 +605,6 @@ process_status:
 	case -EAGAIN:
 		status = xs_nospace(task);
 		break;
-	default:
-		dprintk("RPC:       sendmsg returned unrecognized error %d\n",
-			-status);
 	case -ENETUNREACH:
 	case -ENOBUFS:
 	case -EPIPE:
@@ -626,7 +612,10 @@ process_status:
 	case -EPERM:
 		/* When the server has died, an ICMP port unreachable message
 		 * prompts ECONNREFUSED. */
-		clear_bit(SOCKWQ_ASYNC_NOSPACE, &transport->sock->flags);
+		break;
+	default:
+		dprintk("RPC:       sendmsg returned unrecognized error %d\n",
+			-status);
 	}
 
 	return status;
@@ -706,16 +695,16 @@ static int xs_tcp_send_request(struct rpc_task *task)
 	case -EAGAIN:
 		status = xs_nospace(task);
 		break;
-	default:
-		dprintk("RPC:       sendmsg returned unrecognized error %d\n",
-			-status);
 	case -ECONNRESET:
 	case -ECONNREFUSED:
 	case -ENOTCONN:
 	case -EADDRINUSE:
 	case -ENOBUFS:
 	case -EPIPE:
-		clear_bit(SOCKWQ_ASYNC_NOSPACE, &transport->sock->flags);
+		break;
+	default:
+		dprintk("RPC:       sendmsg returned unrecognized error %d\n",
+			-status);
 	}
 
 	return status;
@@ -1609,19 +1598,23 @@ static void xs_tcp_state_change(struct sock *sk)
 
 static void xs_write_space(struct sock *sk)
 {
-	struct socket *sock;
+	struct socket_wq *wq;
 	struct rpc_xprt *xprt;
 
-	if (unlikely(!(sock = sk->sk_socket)))
+	if (!sk->sk_socket)
 		return;
-	clear_bit(SOCK_NOSPACE, &sock->flags);
+	clear_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
 
 	if (unlikely(!(xprt = xprt_from_sock(sk))))
 		return;
-	if (test_and_clear_bit(SOCKWQ_ASYNC_NOSPACE, &sock->flags) == 0)
-		return;
+	rcu_read_lock();
+	wq = rcu_dereference(sk->sk_wq);
+	if (!wq || test_and_clear_bit(SOCKWQ_ASYNC_NOSPACE, &wq->flags) == 0)
+		goto out;
 
 	xprt_write_space(xprt);
+out:
+	rcu_read_unlock();
 }
 
 /**