Hey, Po-Hsu Lin.

I marked this as a duplicate of #1999554, as I did some research on it
before. So, many of these tests use a loop device and then try to clear
it. The clear function will retry multiple times because udev may have a
hold on the device. After a number of tries with some timeout, it will
emit that WARNING because the loop device failed to clear. That is
because udev still has a hold on it.

I debugged this and the reason for that is because udev will read
efivars every once in a while (instead of caching the results), and
reading efivar on those gcp instance types takes multiple seconds. That
has been causing other issues, like nbd being busy as well and that test
failing. I believe this should be fixed on udev.

Cascardo.

** Also affects: udev (Ubuntu)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1999554

Title:
  LTP uevent01 will fail on GCP n2d-standard-64

Status in linux package in Ubuntu:
  Confirmed
Status in udev package in Ubuntu:
  New

Bug description:
  Because udev takes too long to release the loop device, LTP test
  uevent01 (and some other tests) may warn about not being able to
  detach the loop device. This is identified as a failure.

  
  ubuntu@selfprovisioned-cascardo-n2d:~$ sudo ./uevent01 
  tst_test.c:1423: TINFO: Timeout per run is 0h 05m 00s
  tst_device.c:88: TINFO: Found free device 6 '/dev/loop6'
  uevent01.c:24: TINFO: Attaching device /dev/loop6
  uevent.h:49: TINFO: Got uevent:
  uevent.h:52: TINFO: change@/devices/virtual/block/loop6
  uevent.h:52: TINFO: ACTION=change
  uevent.h:52: TINFO: DEVPATH=/devices/virtual/block/loop6
  uevent.h:52: TINFO: SUBSYSTEM=block
  uevent.h:52: TINFO: DISK_MEDIA_CHANGE=1
  uevent.h:52: TINFO: MAJOR=7
  uevent.h:52: TINFO: MINOR=6
  uevent.h:52: TINFO: DEVNAME=loop6
  uevent.h:52: TINFO: DEVTYPE=disk
  uevent.h:52: TINFO: DISKSEQ=7
  uevent.h:52: TINFO: SEQNUM=2959
  uevent.h:140: TPASS: Got expected UEVENT
  uevent.h:49: TINFO: Got uevent:
  uevent.h:52: TINFO: change@/devices/virtual/block/loop6
  uevent.h:52: TINFO: ACTION=change
  uevent.h:52: TINFO: DEVPATH=/devices/virtual/block/loop6
  uevent.h:52: TINFO: SUBSYSTEM=block
  uevent.h:52: TINFO: MAJOR=7
  uevent.h:52: TINFO: MINOR=6
  uevent.h:52: TINFO: DEVNAME=loop6
  uevent.h:52: TINFO: DEVTYPE=disk
  uevent.h:52: TINFO: DISKSEQ=7
  uevent.h:52: TINFO: SEQNUM=2960
  uevent.h:140: TPASS: Got expected UEVENT
  uevent01.c:26: TINFO: Detaching device /dev/loop6
  tst_device.c:254: TWARN: ioctl(/dev/loop6, LOOP_CLR_FD, 0) no ENXIO for too 
long

  Summary:
  passed   2
  failed   0
  broken   0
  skipped  0
  warnings 1

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