[libvirt PATCH] nodedev: wait a bit longer for new node devices

[Jonathon Jongsma]

On 8/24/22 2:09 AM, Erik Skultety wrote:
> On Tue, Aug 23, 2022 at 12:43:03PM -0500, Jonathon Jongsma wrote:
>> Openstack developers reported that newly-created mdevs were not
>> recognized by libvirt until after a libvirt daemon restart. The source
>> of the problem appears to be that when libvirt gets the udev 'add'
>> event, the sysfs tree for that device might not be ready and so libvirt
>> waits 100ms for it to appear (max 100 waits of 1ms each). But in the
>> OpenStack environment, the sysfs tree for new mediated devices was
>> taking closer to 250ms to appear and therefore libvirt gave up waiting
>> and didn't add these new devices to its list of nodedevs.
>>
>> By changing the wait time to 1 second (max 100 waits of 10ms each), this
>> should provide enough time to enable these deployments to recognize
>> newly-created mediated devices, but it shouldn't increase the delay for
>> more traditional deployments too much.
>>
>> Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2109450
>>
>> Signed-off-by: Jonathon Jongsma <jjongsma at redhat.com>
>> ---
>>
>> Alternatively, we could switch to triggering off of the udev 'bind' event
>> rather than the 'add' event, but I wasn't able to convince myself that this
>> would result in 100% compatible behavior, so this felt like the safest
>> solution. If others can convince me that switching to 'bind' is safe, I can
>> re-submit this patch.
> 
> Is there a guarantee that the filesystem tree is ready by the time the event
> arrives? I remember back in the day when I implemented this, this was even
> discussed on the kernel list and the outcome was that each application needs to
> sort this out on its own hinting that at least at that time there wasn't
> any other way to do this reliably? Has something changed in the meantime?
> 
> Erik
> 

I'm afraid I don't actually know if anything has changed in the kernel 
in this area. That's basically the reason that I proposed the approach 
that I did. But I do know that in the bug referenced, the 'bind' event 
comes about 250ms later than the 'add' event. I'm not sure if the 
filesystem tree is necessarily ready on 'bind', but the fact that it is 
250ms later means that, at minimum, there's a significantly better 
chance that it is ready by that point than at the time of 'add'.

Jonathon