SUNRPC: change locking for xs_swap_enable/disable

It is not in general safe to wait for XPRT_LOCKED to clear.
A wakeup is only sent when
 - connection completes
 - sock close completes
so during normal operations, this can wait indefinitely.

The event we need to protect against is ->inet being set to NULL, and
that happens under the recv_mutex lock.

So drop the handlign of XPRT_LOCKED and use recv_mutex instead.
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

net/sunrpc/xprtsock.c

@@ -1936,9 +1936,9 @@ static void xs_local_connect(struct rpc_xprt *xprt, struct rpc_task *task)
 
 #if IS_ENABLED(CONFIG_SUNRPC_SWAP)
 /*
- * Note that this should be called with XPRT_LOCKED held (or when we otherwise
- * know that we have exclusive access to the socket), to guard against
- * races with xs_reset_transport.
+ * Note that this should be called with XPRT_LOCKED held, or recv_mutex
+ * held, or when we otherwise know that we have exclusive access to the
+ * socket, to guard against races with xs_reset_transport.
  */
 static void xs_set_memalloc(struct rpc_xprt *xprt)
 {
@@ -1967,13 +1967,11 @@ xs_enable_swap(struct rpc_xprt *xprt)
 {
 	struct sock_xprt *xs = container_of(xprt, struct sock_xprt, xprt);
 
-	if (atomic_inc_return(&xprt->swapper) != 1)
-		return 0;
-	if (wait_on_bit_lock(&xprt->state, XPRT_LOCKED, TASK_KILLABLE))
-		return -ERESTARTSYS;
-	if (xs->inet)
+	mutex_lock(&xs->recv_mutex);
+	if (atomic_inc_return(&xprt->swapper) == 1 &&
+	    xs->inet)
 		sk_set_memalloc(xs->inet);
-	xprt_release_xprt(xprt, NULL);
+	mutex_unlock(&xs->recv_mutex);
 	return 0;
 }
 
@@ -1989,13 +1987,11 @@ xs_disable_swap(struct rpc_xprt *xprt)
 {
 	struct sock_xprt *xs = container_of(xprt, struct sock_xprt, xprt);
 
-	if (!atomic_dec_and_test(&xprt->swapper))
-		return;
-	if (wait_on_bit_lock(&xprt->state, XPRT_LOCKED, TASK_KILLABLE))
-		return;
-	if (xs->inet)
+	mutex_lock(&xs->recv_mutex);
+	if (atomic_dec_and_test(&xprt->swapper) &&
+	    xs->inet)
 		sk_clear_memalloc(xs->inet);
-	xprt_release_xprt(xprt, NULL);
+	mutex_unlock(&xs->recv_mutex);
 }
 #else
 static void xs_set_memalloc(struct rpc_xprt *xprt)