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The systemd System and Service Manager
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If /sys/class/OOO node is created and destroyed during booting (kernle driver initialization fails),
systemd-udev-trigger.service fails due to race condition.
***** race condition ***********************************************************************************
1. kernel driver create /sys/class/OOO
2. systemd-udev-trigger.service execues "/usr/bin/udevadm trigger --type=devices --action=add"
3. device_enumerator_scan_devices() => enumerator_scan_devices_all() => enumerator_scan_dir("class") =>
opendir("/sys/class") and iterate all subdirs ==> enumerator_scan_dir_and_add_devices("/sys/class/OOO")
4. kernel driver fails and destroy /sys/class/OOO
5. enumerator_scan_dir_and_add_devices("/sys/class/OOO") fails in opendir("/sys/class/OOO")
6. "systemd-udev-trigger.service" fails
7. udev coldplug fails and some device units not ready
8. mount units asociated with device units fail
9. local-fs.target fails
10. enters emergency mode
********************************************************************************************************
***** status of systemd-udev-trigger.service unit ******************************************************
$ systemctl status systemd-udev-trigger.service
systemd-udev-trigger.service - udev Coldplug all Devices
Loaded: loaded (/usr/lib/systemd/system/systemd-udev-trigger.service; static; vendor preset: enabled)
Active: failed (Result: exit-code) since Thu 2020-01-02 13:16:54 KST; 22min ago
Docs: man:udev(7)
man:systemd-udevd.service(8)
Process: 2162 ExecStart=/usr/bin/udevadm trigger --type=subsystems --action=add (code=exited, status=0/SUCCESS)
Process: 2554 ExecStart=/usr/bin/udevadm trigger --type=devices --action=add (code=exited, status=1/FAILURE)
Main PID: 2554 (code=exited, status=1/FAILURE)
Jan 02 13:16:54 localhost udevadm[2554]: Failed to scan devices: No such file or directory
Jan 02 13:16:54 localhost systemd[1]: systemd-udev-trigger.service: Main process exited, code=exited, status=1/FAILURE
Jan 02 13:16:54 localhost systemd[1]: systemd-udev-trigger.service: Failed with result 'exit-code'.
Jan 02 13:16:54 localhost systemd[1]: Failed to start udev Coldplug all Devices.
*******************************************************************************************************
***** journal log with Environment=SYSTEMD_LOG_LEVEL=debug in systemd-udev-trigger.service ***********
Jan 01 21:57:20 localhost udevadm[2039]: sd-device-enumerator: Scanning /sys/bus
Jan 01 21:57:20 localhost udevadm[2522]: sd-device-enumerator: Scan all dirs
Jan 01 21:57:20 localhost udevadm[2522]: sd-device-enumerator: Scanning /sys/bus
Jan 01 21:57:21 localhost udevadm[2522]: sd-device-enumerator: Scanning /sys/class
Jan 01 21:57:21 localhost udevadm[2522]: sd-device-enumerator: Failed to scan /sys/class: No such file or directory
Jan 01 21:57:21 localhost udevadm[2522]: Failed to scan devices: No such file or directory
*******************************************************************************************************
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System and Service Manager
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