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

Thanks for the report!! We may be missing this patch:

https://git.kernel.org/linus/0d7a7b2014b1 ("ipv6: remove extra
dev_hold() for fallback tunnels")

It has a fixes tag for upstream 40cb881b5aaa, which was merged in our newer 5.8 kernels - can you test with the fix @cypressyew and let us know?
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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