adb remount of system failed: Permission denied

[fulinux@ubuntu rk3288-5.1]$ adb remount 
* daemon not running. starting it now on port 5037 *
* daemon started successfully *
remount of system failed: Permission denied
remount failed

[fulinux@ubuntu rk3288-5.1]$ adb root
* daemon not running. starting it now on port 5037 *
* daemon started successfully *
restarting adbd as root
[fulinux@ubuntu rk3288-5.1]$ adb remount
remount succeeded