Drivers: hv: kvp,vss: Fast propagation of userspace communication failure

If we fail to send a message to userspace daemon with cn_netlink_send()
there is no need to wait for userspace to reply as it is not going to
happen. This happens when kvp or vss daemon is stopped after a successful
handshake. Report HV_E_FAIL immediately and cancel the timeout job so
host won't receive two failures.
Use pr_warn() for VSS and pr_debug() for KVP deliberately as VSS request
are rare and result in a failed backup. KVP requests are much more frequent
after a successful handshake so avoid flooding logs. It would be nice to
have an ability to de-negotiate with the host in case userspace daemon gets
disconnected so we won't receive new requests. But I'm not sure it is
possible.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hv/hv_kvp.c

@@ -350,6 +350,7 @@ kvp_send_key(struct work_struct *dummy)
 	__u8 pool = kvp_transaction.kvp_msg->kvp_hdr.pool;
 	__u32 val32;
 	__u64 val64;
+	int rc;
 
 	msg = kzalloc(sizeof(*msg) + sizeof(struct hv_kvp_msg) , GFP_ATOMIC);
 	if (!msg)
@@ -446,7 +447,13 @@ kvp_send_key(struct work_struct *dummy)
 	}
 
 	msg->len = sizeof(struct hv_kvp_msg);
-	cn_netlink_send(msg, 0, 0, GFP_ATOMIC);
+	rc = cn_netlink_send(msg, 0, 0, GFP_ATOMIC);
+	if (rc) {
+		pr_debug("KVP: failed to communicate to the daemon: %d\n", rc);
+		if (cancel_delayed_work_sync(&kvp_work))
+			kvp_respond_to_host(message, HV_E_FAIL);
+	}
+
 	kfree(msg);
 
 	return;

drivers/hv/hv_snapshot.c

@@ -96,6 +96,7 @@ vss_cn_callback(struct cn_msg *msg, struct netlink_skb_parms *nsp)
 static void vss_send_op(struct work_struct *dummy)
 {
 	int op = vss_transaction.msg->vss_hdr.operation;
+	int rc;
 	struct cn_msg *msg;
 	struct hv_vss_msg *vss_msg;
 
@@ -111,7 +112,12 @@ static void vss_send_op(struct work_struct *dummy)
 	vss_msg->vss_hdr.operation = op;
 	msg->len = sizeof(struct hv_vss_msg);
 
-	cn_netlink_send(msg, 0, 0, GFP_ATOMIC);
+	rc = cn_netlink_send(msg, 0, 0, GFP_ATOMIC);
+	if (rc) {
+		pr_warn("VSS: failed to communicate to the daemon: %d\n", rc);
+		if (cancel_delayed_work_sync(&vss_timeout_work))
+			vss_respond_to_host(HV_E_FAIL);
+	}
 	kfree(msg);
 
 	return;