[Bug 2056461]

TJ, I'm happy to say that the hello package's DEP-8 tests all appears to
pass with your patched kernel. I'll just keep the .img around for my own
packages' testing.

Do you think your fix will make it into Ubuntu Noble in the near future?

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https://bugs.launchpad.net/bugs/2056461

Title:
autopkgtest-virt-qemu on noble images sometimes hangs doing copydown

Status in Linux:
Confirmed
Status in autopkgtest package in Ubuntu:
Confirmed
Status in linux package in Ubuntu:
In Progress
Status in autopkgtest package in Debian:
New

Bug description:
[Impact]

It seems that kernel 6.8 introduced a regression in the 9pfs related
to caching and netfslib, that can cause some user-space apps to read
content from files that is not up-to-date (when they are used in a
producer/consumer fashion).

It seems that the offending commit is this one:

80105ed2fd27 ("9p: Use netfslib read/write_iter")

Reverting the commit seems to fix the problem. However the actual bug
might be in netfslib or how netfslib is used in the 9p context.

The regression has been reported upstream and we are still
investigating (https://lore.kernel.org/lkml/Zj0ErxVBE3DYT2Ea@gpd/).

In the meantime it probably makes sense to temporarily revert the
commit as a SAUCE patch. Then we will drop the SAUCE patch once we'll
have a proper fix upstream.

[Test case]

The following test should complete correctly without any timeout:

  pull-lp-source -d hello
  autopkgtest-buildvm-ubuntu-cloud -r noble
  autopkgtest -U hello*.dsc -- qemu ./autopkgtest-noble-amd64.img


[Fix]

Revert the following commit (until we have a proper fix upstream):

80105ed2fd27 ("9p: Use netfslib read/write_iter")

[Regression potential]

We may experience other regressions related to 9pfs with this change,
however it's quite unlikely to happen since we are reverting a commit,
restoring the previous behavior.

[Original bug report]

autopkgtest-virt-qemu sometimes hangs when running tests on noble
images. Originally reported by schopin, who also provided a
reproducer:

  pull-lp-source -d hello
  autopkgtest-buildvm-ubuntu-cloud -r noble
  autopkgtest -U hello*.dsc -- qemu ./autopkgtest-noble-amd64.img

I've been able to reproduce it with debugging enabled:

  autopkgtest -ddd -U hello_2.10-3.dsc -- qemu --debug --show-boot
/path/to/image

It can get stuck during different stages, but AFAICT always during
"copydown" operations, log excerpts follow. It may be a coincidence,
but this started happening around the time linux-
image-6.8.0-11-generic (6.8.0-11.11) migrated to noble. The testbeds I
used booted 6.6 but then rebooted into that 6.8 kernel after being
upgraded by autopkgtest.

-- logs --

Removing autopkgtest-satdep (0) ...
[...]
autopkgtest-virt-qemu: DBG: executing copydown /tmp/autopkgtest.output.g8v75e8g/tests-tree/ /t/
autopkgtest-virt-qemu: DBG: ['cmdls', "(['tar', '--directory', '/tmp/autopkgtest.output.g8v75e]
autopkgtest-virt-qemu: DBG: ['srcstdin', "<_io.BufferedReader name='/dev/null'>", 'deststdout']
autopkgtest-virt-qemu: DBG: +< tar --directory /tmp/autopkgtest.output.g8v75e8g/tests-tree/ --
autopkgtest-virt-qemu: DBG: +> /tmp/autopkgtest-qemu.ztmr6f5k/runcmd sh -ec if ! test -d /tmp-
autopkgtest-virt-qemu: DBG: +>?

-- or --

autopkgtest: DBG: sending command to testbed: copydown /tmp/autopkgtest.output.c9utq3bx/tests-tree/ /tmp/autopkgtest.H8NDfW/build.DLR/src/
autopkgtest-virt-qemu: DBG: executing copydown /tmp/autopkgtest.output.c9utq3bx/tests-tree/ /tmp/autopkgtest.H8NDfW/build.DLR/src/
autopkgtest-virt-qemu: DBG: ['cmdls', "(['tar', '--directory', '/tmp/autopkgtest.output.c9utq3bx/tests-tree/', '--warning=none', '-c', '.', '-f', '-'], ['/tmp/autopkgtest-qemu.qtkcgg5l/runcm]
autopkgtest-virt-qemu: DBG: ['srcstdin', "<_io.BufferedReader name='/dev/null'>", 'deststdout', "<_io.BufferedReader name='/dev/null'>", 'devnull_read', <_io.BufferedReader name='/dev/null'>]
autopkgtest-virt-qemu: DBG: +< tar --directory /tmp/autopkgtest.output.c9utq3bx/tests-tree/ --warning=none -c . -f -
autopkgtest-virt-qemu: DBG: +> /tmp/autopkgtest-qemu.qtkcgg5l/runcmd sh -ec if ! test -d /tmp/autopkgtest.H8NDfW/build.DLR/src/; then mkdir -- /tmp/autopkgtest.H8NDfW/build.DLR/src/; fi; cd-
autopkgtest-virt-qemu: DBG: +>?

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