[Bug 1933969] Re: test_pmtu_vti4_link_add_mtu() test from net/pmtu.sh in ubuntu_kernel_selftests cannot finish properly on Groovy

@Sam, I will be uploading a fixup Hirsute kernel (with the 4 additional
patches picked) to

https://launchpad.net/~smb/+archive/ubuntu/temp01

That showed the same regression in ADT logs as Groovy did. So if it
works there it would help Groovy as well. If you could rerun the linux
regression tests on that first. Thanks.

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https://bugs.launchpad.net/bugs/1933969

Title:
test_pmtu_vti4_link_add_mtu() test from net/pmtu.sh in
ubuntu_kernel_selftests cannot finish properly on Groovy

Status in ubuntu-kernel-tests:
New
Status in linux package in Ubuntu:
New
Status in linux source package in Groovy:
New

Bug description:
Issue found with Groovy 5.8.0-60.67 on different amd64 / arm64 / s390x
instances

Test case:
* tools/testing/selftests/net/pmtu.sh

The pmtu.sh test will hang, and gets killed:
# TEST: IPv6 over IPv6: PMTU exceptions - nexthop objects [ OK ]
# TEST: vti6: PMTU exceptions [ OK ]
Timer expired (1800 sec.), nuking pid 13090

If you take a closer look on this node, you will see it's complaining about unregister_netdevice and modprobe command hung:
  unregister_netdevice: waiting for ip6_vti0 to become free. Usage count = 1
  unregister_netdevice: waiting for ip6_vti0 to become free. Usage count = 1
  INFO: task modprobe:40237 blocked for more than 122 seconds.
        Tainted: P O 5.8.0-60-generic #67-Ubuntu
  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  modprobe D 0 40237 1265 0x00000000
  Call Trace:
   [<0000000036dc7860>] __schedule+0x300/0x840
   [<0000000036dc7e02>] schedule+0x62/0x100
   [<00000000362c3bce>] rwsem_down_write_slowpath+0x23e/0x6c0
   [<0000000036b3716c>] register_pernet_device+0x2c/0xa0
   [<000003ff80a16034>] vti_init+0x34/0x1000 [ip_vti]
   [<00000000361a88b0>] do_one_initcall+0x40/0x200
   [<000000003632b34e>] do_init_module+0x6e/0x530
   [<000000003632e06a>] __do_sys_finit_module+0xaa/0xf0
   [<0000000036dcefbc>] system_call+0xe0/0x2b8
  unregister_netdevice: waiting for ip6_vti0 to become free. Usage count = 1

Running process shows:
  root 40236 0.0 0.0 3748 1040 ? D 10:39 0:00 ip link add vti4_a type vti local 192.168.1.1 remote 192.168.1.2 key 10
  root 40237 0.0 0.0 6092 1560 ? D 10:39 0:00 /sbin/modprobe -q -- rtnl-link-vti

This command "ip link add vti4_a type vti local" belongs to the
test_pmtu_vti4_link_add_mtu() test in pmtu.sh

This will prevent the whole ubuntu_kernel_selftests to finish

On Jenkins, the test will hang and killed by the timeout:
10:08:18 INFO | ========== Summary ===========
10:08:18 INFO | No failed cases reported
10:08:18 INFO | GOOD ubuntu_kernel_selftests.ftrace ubuntu_kernel_selftests.ftrace timestamp=1624439298 localtime=Jun 23 10:08:18 completed successfully
10:08:18 INFO | END GOOD ubuntu_kernel_selftests.ftrace ubuntu_kernel_selftests.ftrace timestamp=1624439298 localtime=Jun 23 10:08:18
10:08:18 DEBUG| Persistent state client._record_indent now set to 1
10:08:18 DEBUG| Persistent state client.unexpected_reboot deleted
10:08:18 INFO | END GOOD ---- ---- timestamp=1624439298 localtime=Jun 23 10:08:18
10:08:18 DEBUG| Persistent state client._record_indent now set to 0
10:08:18 INFO | Report successfully generated at /home/ubuntu/autotest/client/results/default/job_report.html
Build timed out (after 240 minutes). Marking the build as aborted.
Build was aborted
Recording test results
ERROR: Step 'Publish JUnit test result report' failed: No test report files were found. Configuration error?

Since we don't have any change to the pmtu.sh in this cycle. This
looks like a potential regression to me.

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