[systemd-devel] Proper way for systemd service to wait mdev gvt device initialization

[Andrei Borzenkov]

On Mon, May 20, 2019 at 10:08 AM Mantas Mikulėnas <grawity at gmail.com> wrote:
>
> On Sun, May 19, 2019 at 9:50 PM Alex Ivanov <gnidorah at ya.ru> wrote:
>>
>> Hello.
>> What is the proper way to do that? I have a unit that creates gvt device in the system
>>
>> ExecStart = "sh -c 'echo a297db4a-f4c2-11e6-90f6-d3b88d6c9525 > /sys/bus/pci/devices/0000:00:02.0/mdev_supported_types/i915-GVTg_V5_8/create'";
>> ExecStop = "sh -c 'echo 1 > /sys/bus/pci/devices/0000:00:02.0/a297db4a-f4c2-11e6-90f6-d3b88d6c9525/remove'";
>
>
> Personally, I would use an udev rule:
>
> ACTION=="add", SUBSYSTEM=="pci", ENV{PCI_SLOT_NAME}=="0000:00:02.0", ATTR{mdev_supported_types/i915-GVTg_V5_8/create}="a297db4a-f4c2-11e6-90f6-d3b88d6c9525"
>

There is a race condition here, driver creates
.../mdev_supported_types after it has registered device so udev may
process event before directory is available.

> Though on the other hand, a service is a good choice if you want to `systemctl stop` it later on.
>
> ACTION=="add", SUBSYSTEM=="pci", ENV{PCI_SLOT_NAME}=="0000:00:02.0", ENV{SYSTEMD_WANTS}+="create-gvt.service"
>
>>
>>
>> Ideally I would to like to start this service when 0000:00:02.0 device appears in the system, but the problem is that /sys/bus/pci/devices/0000:00:02.0/mdev_supported_types/ tree is populated later, so my service will fail.
>>
>> So the question what is the proper way to fix that.
>
>
> If the driver doesn't populate its sysfs entries in time, maybe it at least generates 'change' uevents? (udevadm --monitor)
>

I would tentatively say this is driver bug. This directory is created
during initial device setup, not in response to some event later. From
https://github.com/torvalds/linux/blob/master/Documentation/driver-model/device.txt:

--><--
As explained in Documentation/kobject.txt, device attributes must be
created before the KOBJ_ADD uevent is generated.
--><--

Note that some drivers even disable KOBJ_ADD notification during
device_register() and trigger it manually later, after sysfs layout is
complete. I cannot evaluate whether this directory can be created and
populated before device_register().

> If there are no uevents either, well, there's nothing you can do from systemd's side. (Other than making a script that loops repeatedly checking "is it there yet? is it there yet?")

Should really be fixed on kernel side.