On Mon, 12 Feb 2018 14:20:57 -0700 Alex Williamson <alex.williamson@xxxxxxxxxx> wrote: > On Fri, 9 Feb 2018 11:27:16 +0100 > Cornelia Huck <cohuck@xxxxxxxxxx> wrote: > > > The registration code first registers the mdev device, and then > > proceeds to populate sysfs. An userspace application that listens > > for the ADD uevent is therefore likely to look for sysfs entries > > that have not yet been created. > > > > The canonical way to fix this is to use attribute groups that are > > registered by the driver core before it sends the ADD uevent; I > > unfortunately did not find a way to make this work in this case, > > though. > > > > An alternative approach is to suppress uevents before we register > > with the core and generate the ADD uevent ourselves after the > > sysfs infrastructure is in place. > > > > Signed-off-by: Cornelia Huck <cohuck@xxxxxxxxxx> > > --- > > > > This feels like a band-aid, but I can't figure out how to handle creating > > attribute groups when there's a callback in the parent involved. > > > > This should address the issue with libvirt's processing of mdevs raised in > > https://www.redhat.com/archives/libvir-list/2018-February/msg00023.html > > - although libvirt will still need to deal with older kernels, of course. > > > > Best to consider this an untested patch :) > > I agree, this feels like a band-aide. If every device in the kernel > needs to suppress udev events until until some key component is added, > that suggests that either udev is broken in general or not being used > as intended. I think udev is working exactly as designed - it's more a problem of when the kernel is sending what kind of notification to userspace, and the particular issue here is how the code sending the event (driver core) and the code assembling part of the user interface (mdev) interact. > Zongyong submitted a different proposal to fix this > here[1]. That proposal seems a bit more sound and has precedence > elsewhere in the kernel. What do you think of that approach? We > don't need both afaict. Thanks, > > Alex > > [1]https://patchwork.kernel.org/patch/10196197/ Zongyong's patch is sending an additional CHANGE uevent, and I agree that doing both does not make sense. However, I think the CHANGE uevent is not quite suitable in this case, and delaying the ADD uevent is better. [Warning, the following may be a bit rambling.] The Linux driver model works under the assumption that any device is represented as an in-kernel object that exposes information and knobs through sysfs. As long as the device exists, userspace can poke at the sysfs entries and retrieve information or configure things. The idea of the 'ADD' uevent is basically to let userspace know that there is now a new device with its related sysfs entries, and it may look at it and configure it. IOW, if I (as a userspace application) get the ADD uevent, I expect to be able to look at the device's sysfs entries and find all the files/directories that are usually there, without having to wait. This expectation is broken if a device is first registered with the driver core (generating the ADD uevent) and the driver adds sysfs attributes later. To fix this, the driver core added a way to specify default attribute groups for the device, which are registered by the driver core itself before it generates the ADD uevent. Unfortunately, I did not see a way to do this here (which does not mean there isn't). The alternative was to prevent the driver core from sending the ADD uevent and do it from the mdev code when it was ready. The 'CHANGE' uevent, on the other hand, tells userspace that something has changed for the device (that already existed). I (as a userspace application) would expect to see it if, for example, the information exposed via sysfs has changed, or maybe even if new, optional, entries have appeared and I might want to rescan. With Zongyong's patch, userspace gets the CHANGE uevent for something that was already expected to be there, and is now _really_ there. It does give userspace an indication that it can now work with the device (which certainly improves things), but I would prefer to get rid of the too-early uevent completely so that userspace does not get notified at all before the device is completely present in sysfs. So, in short, my patch does 'don't tell userspace until we're really done', and Zongyong's patch does 'tell userspace again when we're really done'.
