[libvirt] [PATCH 3/3] nodedev: Move the udevPCITranslateInit call

John Ferlan jferlan at redhat.com
Thu Dec 14 13:36:30 UTC 2017

On 12/14/2017 08:19 AM, Erik Skultety wrote:
> On Sat, Dec 09, 2017 at 12:29:14PM -0500, John Ferlan wrote:
>> If the timing is "just right", there is a possibility that the
>> udev nodeStateInitialize conflicts with another systemd thread
>> running an lspci command leaving both waiting for "something",
>> but resulting in a hung libvirtd (and hung lspci thread) from
>> which the only recovery is a reboot because killing either thread
>> is impossible and results in a defunct libvirtd process if a
>> SIGKILL is performed.
>> In order to avoid this let's move where the PCI initialization
>> is done to be where it's actually needed. Ensure we only perform
>> the initialization once via a driver bool.  Likewise, during
>> cleanup ensure we only call udevPCITranslateDeinit once the
>> initialization is successful.
>> At least a failure for this driver won't hang out the rest of the
>> the libvirt event loop. May not make certain things usable though.
>> Still a libvirtd restart is far easier than a host reboot.
> Is there a BZ for this or can you at least share what steps are necessary to
> have a chance of hitting this issue? I'm asking because it sounds like we
> should file a BZ against udev as well (possibly kernel) and a thorough
> investigation of where the deadlock happens is necessary because I don't see a
> any guarantee that just with a simple logic movement (and adding a trigger
> condition) we can make disappear a race outside of our scope for good. On the
> other hand, having to choose between a hung process requiring a host restart and
> a hung worker thread requiring a service restart, I'd obviously opt for the
> latter. So I'd say the next steps depend on how frequently and under what
> circumstances (specific host devices, kernel version, etc.) this happens,
> because to me it sounds odd how systemd and libpciaccess clash here.
> Erik

w/r/t: reproducing

Have you ever set up virt-test or avocado-vt? Have a bit of patience to
retry the same test multiple times only to have it trigger once when the
moon, stars, and sun align in the perfect order during the 14th hour of
the 3rd day of the 11th month? ;-)

Seriously though, it's not very reproducible - I tried multiple ways
without libvirtd involved. However, when it does reproduce, then things
are hosed w/r/t a defunct libvirtd from which only a reboot resolves. By
moving to a separate thread at least I can restart libvirtd and have a
somewhat crippled environment that doesn't include nodedev.

w/r/t: bug report

Well writing that report could be a challenge say nothing about the
indeterminable wait for a resolution to said bug. At this point, I'm not
sure if it's Fedora related, udev related, systemd related, or libvirtd
related. I'd have to get back into that state without this patch in
order to attempt to gather/recall more information that could be even
useful for said bug report.

I don't mind holding off on these last 2 patches, but by posting them I
was kind of hoping there might be someone out there who saw the same
thing and might be able to give me some ideas related to helping debug
or resolve.


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