SUNRPC: Lock the transport layer on shutdown

Avoid all races with the connect/disconnect handlers by taking the
transport lock.
Reported-by: N"Suzuki K. Poulose" <suzuki.poulose@arm.com>
Acked-by: NJeff Layton <jlayton@poochiereds.net>
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

net/sunrpc/xprt.c

@@ -614,6 +614,7 @@ static void xprt_autoclose(struct work_struct *work)
 	clear_bit(XPRT_CLOSE_WAIT, &xprt->state);
 	xprt->ops->close(xprt);
 	xprt_release_write(xprt, NULL);
+	wake_up_bit(&xprt->state, XPRT_LOCKED);
 }
 
 /**
@@ -723,6 +724,7 @@ void xprt_unlock_connect(struct rpc_xprt *xprt, void *cookie)
 	xprt->ops->release_xprt(xprt, NULL);
 out:
 	spin_unlock_bh(&xprt->transport_lock);
+	wake_up_bit(&xprt->state, XPRT_LOCKED);
 }
 
 /**
@@ -1394,6 +1396,10 @@ struct rpc_xprt *xprt_create_transport(struct xprt_create *args)
 static void xprt_destroy(struct rpc_xprt *xprt)
 {
 	dprintk("RPC:       destroying transport %p\n", xprt);
+
+	/* Exclude transport connect/disconnect handlers */
+	wait_on_bit_lock(&xprt->state, XPRT_LOCKED, TASK_UNINTERRUPTIBLE);
+
 	del_timer_sync(&xprt->timer);
 
 	rpc_xprt_debugfs_unregister(xprt);