int btrfs_get_dev_stats(struct btrfs_fs_info *fs_info,
			struct btrfs_ioctl_get_dev_stats *stats)
{
	struct btrfs_device *dev;
	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
	int i;

	mutex_lock(&fs_devices->device_list_mutex);
	dev = btrfs_find_device(fs_info->fs_devices, stats->devid, NULL, NULL,
				true);
	mutex_unlock(&fs_devices->device_list_mutex);

	if (!dev) {
		btrfs_warn(fs_info, "get dev_stats failed, device not found");
		return -ENODEV;
	} else if (!dev->dev_stats_valid) {
		btrfs_warn(fs_info, "get dev_stats failed, not yet valid");
		return -ENODEV;
	} else if (stats->flags & BTRFS_DEV_STATS_RESET) {
		for (i = 0; i < BTRFS_DEV_STAT_VALUES_MAX; i++) {
			if (stats->nr_items > i)
				stats->values[i] =
					btrfs_dev_stat_read_and_reset(dev, i);
			else
				btrfs_dev_stat_reset(dev, i);
		}
	} else {
		for (i = 0; i < BTRFS_DEV_STAT_VALUES_MAX; i++)
			if (stats->nr_items > i)
				stats->values[i] = btrfs_dev_stat_read(dev, i);
	}
	if (stats->nr_items > BTRFS_DEV_STAT_VALUES_MAX)
		stats->nr_items = BTRFS_DEV_STAT_VALUES_MAX;
	return 0;
}
A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit.