[libvirt] [RFC PATCH] udev: Remove udev handle from main loop when udev thread stops

[Marc Hartmayer]

On Tue, Feb 12, 2019 at 09:46 PM +0100, John Ferlan <jferlan at redhat.com> wrote:
> On 2/7/19 11:08 AM, Marc Hartmayer wrote:
>> Commit "nodedev: Move device enumumeration out of nodeStateInitialize"
>> (9f0ae0b18e3e620) has moved the heavy task of device enumeration into
>> a separate thread. The problem with this commit is that there is a
>> functionality change in the cleanup when udevEnumerateDevices
>> fails. Before commit 9f0ae0b18e3e620, the entire driver was cleaned up
>> by calling nodeStateCleanup (defined in node_device_udev.c) which
>> resulted in libvirtd stopping with the error message
>> 'daemonRunStateInit:800 : Driver state initialization failed'. With
>> the commit 9f0ae0b18e3e620 there is only a signal to the udev thread
>> that it must stop. This means that for example the watch handle isn't
>> removed from the main loop and this can result in the main loop
>> consuming 100% CPU time as soon as a new udev event occurs.
>>
>> This patch proposes a simple solution to the described problem. In
>> case the udev thread stops the watch handle is removed from the main
>> loop.
>>
>> Fixes: 9f0ae0b18e3e620 ("nodedev: Move device enumumeration out of nodeStateInitialize")
>> Signed-off-by: Marc Hartmayer <mhartmay at linux.ibm.com>
>> ---
>>
>> Note: I'm not sure whether we should stop libvirtd (as it would have
>>       been done before) or if this patch is already sufficient.
>>
>> ---
>>  src/node_device/node_device_udev.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>
> What you have seems reasonable - although I wonder if it would be better
> to remove the event handle in the error of nodeStateInitializeEnumerate.
>
> I think the assumption made was that by setting threadQuit that the next
> event have some sort of failure through udevEventMonitorSanityCheck
> which removes the EventHandle too. Although I cannot be sure it's been
> too long to remember for sure ;-)
>
> Furthermore, should nodeStateCleanup be altered to check for priv->watch
> == -1 and thus not worry about the code to set threadQuit, signal, and
> joining the thread.

A simple check for `priv->threadQuit` is probably even more useful since
it’s not safe that the watch callback is set.

I’ll send a v2 with the changes you suggested.

>
> John
>
> BTW: I'm subscribed to the list, no need to CC...
>
>> diff --git a/src/node_device/node_device_udev.c b/src/node_device/node_device_udev.c
>> index b1e5f00067e8..299f55260129 100644
>> --- a/src/node_device/node_device_udev.c
>> +++ b/src/node_device/node_device_udev.c
>> @@ -1628,6 +1628,13 @@ udevEventHandleThread(void *opaque ATTRIBUTE_UNUSED)
>>          }
>>
>>          if (priv->threadQuit) {
>> +            if (priv->watch != -1) {
>> +                /* Since the udev thread getting stopped remove the
>> +                 * watch handle from the main loop */
>> +                virEventRemoveHandle(priv->watch);
>> +                priv->watch = -1;
>> +            }
>> +
>>              virObjectUnlock(priv);
>>              return;
>>          }
>>
>
--
Kind regards / Beste Grüße
   Marc Hartmayer

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