Drivers: hv: vmbus: Add timeout to vmbus_wait_for_unload

vmbus_wait_for_unload() looks for a CHANNELMSG_UNLOAD_RESPONSE message
coming from Hyper-V.  But if the message isn't found for some reason,
the panic path gets hung forever.  Add a timeout of 10 seconds to prevent
this.

Fixes: 41571916 ("Drivers: hv: vmbus: avoid scheduling in interrupt context in vmbus_initiate_unload()")
Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Dexuan Cui <decui@microsoft.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/1600026449-23651-1-git-send-email-mikelley@microsoft.comSigned-off-by: Wei Liu <wei.liu@kernel.org>

drivers/hv/channel_mgmt.c

@@ -750,7 +750,7 @@ static void vmbus_wait_for_unload(void)
 	void *page_addr;
 	struct hv_message *msg;
 	struct vmbus_channel_message_header *hdr;
-	u32 message_type;
+	u32 message_type, i;
 
 	/*
 	 * CHANNELMSG_UNLOAD_RESPONSE is always delivered to the CPU which was
@@ -760,8 +760,11 @@ static void vmbus_wait_for_unload(void)
 	 * functional and vmbus_unload_response() will complete
 	 * vmbus_connection.unload_event. If not, the last thing we can do is
 	 * read message pages for all CPUs directly.
+	 *
+	 * Wait no more than 10 seconds so that the panic path can't get
+	 * hung forever in case the response message isn't seen.
 	 */
-	while (1) {
+	for (i = 0; i < 1000; i++) {
 		if (completion_done(&vmbus_connection.unload_event))
 			break;