xen/acpi: xen memory hotplug minor updates

Dan Carpenter found current xen memory hotplug logic
has potential issue: at func acpi_memory_get_device()
*mem_device = acpi_driver_data(device);
while the device may be NULL and then dereference.

At native side, Rafael recently updated acpi_memory_get_device(),
dropping acpi_bus_add, adding lock, and avoiding above issue.

This patch updates xen memory hotplug logic accordingly, removing
redundant logic, adding lock, and avoiding dereference.
Signed-off-by: Liu Jinsong <jinsong.liu@intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xen-acpi-memhotplug.c

@@ -158,31 +158,17 @@ acpi_memory_get_device_resources(struct acpi_memory_device *mem_device)
 	return 0;
 }
 
-static int
-acpi_memory_get_device(acpi_handle handle,
-		       struct acpi_memory_device **mem_device)
+static int acpi_memory_get_device(acpi_handle handle,
+				  struct acpi_memory_device **mem_device)
 {
-	acpi_status status;
-	acpi_handle phandle;
 	struct acpi_device *device = NULL;
-	struct acpi_device *pdevice = NULL;
-	int result;
+	int result = 0;
 
-	if (!acpi_bus_get_device(handle, &device) && device)
-		goto end;
+	acpi_scan_lock_acquire();
 
-	status = acpi_get_parent(handle, &phandle);
-	if (ACPI_FAILURE(status)) {
-		pr_warn(PREFIX "Cannot find acpi parent\n");
-		return -EINVAL;
-	}
-
-	/* Get the parent device */
-	result = acpi_bus_get_device(phandle, &pdevice);
-	if (result) {
-		pr_warn(PREFIX "Cannot get acpi bus device\n");
-		return -EINVAL;
-	}
+	acpi_bus_get_device(handle, &device);
+	if (device)
+		goto end;
 
 	/*
 	 * Now add the notified device.  This creates the acpi_device
@@ -190,18 +176,28 @@ acpi_memory_get_device(acpi_handle handle,
 	 */
 	result = acpi_bus_scan(handle);
 	if (result) {
-		pr_warn(PREFIX "Cannot add acpi bus\n");
-		return -EINVAL;
+		pr_warn(PREFIX "ACPI namespace scan failed\n");
+		result = -EINVAL;
+		goto out;
+	}
+	result = acpi_bus_get_device(handle, &device);
+	if (result) {
+		pr_warn(PREFIX "Missing device object\n");
+		result = -EINVAL;
+		goto out;
 	}
 
 end:
 	*mem_device = acpi_driver_data(device);
 	if (!(*mem_device)) {
-		pr_err(PREFIX "Driver data not found\n");
-		return -ENODEV;
+		pr_err(PREFIX "driver data not found\n");
+		result = -ENODEV;
+		goto out;
 	}
 
-	return 0;
+out:
+	acpi_scan_lock_release();
+	return result;
 }
 
 static int acpi_memory_check_device(struct acpi_memory_device *mem_device)
@@ -259,12 +255,15 @@ static void acpi_memory_device_notify(acpi_handle handle, u32 event, void *data)
 		ACPI_DEBUG_PRINT((ACPI_DB_INFO,
 			"\nReceived EJECT REQUEST notification for device\n"));
 
+		acpi_scan_lock_acquire();
 		if (acpi_bus_get_device(handle, &device)) {
+			acpi_scan_lock_release();
 			pr_err(PREFIX "Device doesn't exist\n");
 			break;
 		}
 		mem_device = acpi_driver_data(device);
 		if (!mem_device) {
+			acpi_scan_lock_release();
 			pr_err(PREFIX "Driver Data is NULL\n");
 			break;
 		}
@@ -274,6 +273,7 @@ static void acpi_memory_device_notify(acpi_handle handle, u32 event, void *data)
 		 * acpi_bus_remove if Xen support hotremove in the future
 		 */
 		acpi_memory_disable_device(mem_device);
+		acpi_scan_lock_release();
 		break;
 
 	default: