Hello Krzysztof! On Friday 09 October 2020 10:08:53 Pali Rohár wrote: > On Thursday 08 October 2020 14:59:07 Bjorn Helgaas wrote: > > On Wed, Oct 07, 2020 at 11:14:34AM -0500, Bjorn Helgaas wrote: > > > On Wed, Oct 07, 2020 at 10:14:00AM +0200, Pali Rohár wrote: > > > > On Wednesday 07 October 2020 12:47:40 Oliver O'Halloran wrote: > > > > > On Wed, Oct 7, 2020 at 10:26 AM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: > > > > > > > > > > > > I'm not really a fan of this because pci_sysfs_init() is a bit of a > > > > > > hack to begin with, and this makes it even more complicated. > > > > > > > > > > > > It's not obvious from the code why we need pci_sysfs_init(), but > > > > > > Yinghai hinted [1] that we need to create sysfs after assigning > > > > > > resources. I experimented by removing pci_sysfs_init() and skipping > > > > > > the ROM BAR sizing. In that case, we create sysfs files in > > > > > > pci_bus_add_device() and later assign space for the ROM BAR, so we > > > > > > fail to create the "rom" sysfs file. > > > > > > > > > > > > The current solution to that is to delay the sysfs files until > > > > > > pci_sysfs_init(), a late_initcall(), which runs after resource > > > > > > assignments. But I think it would be better if we could create the > > > > > > sysfs file when we assign the BAR. Then we could get rid of the > > > > > > late_initcall() and that implicit ordering requirement. > > > > > > > > > > You could probably fix that by using an attribute_group to control > > > > > whether the attribute shows up in sysfs or not. The .is_visible() for > > > > > the group can look at the current state of the device and hide the rom > > > > > attribute if the BAR isn't assigned or doesn't exist. That way we > > > > > don't need to care when the actual assignment occurs. > > > > > > > > And cannot we just return e.g. -ENODATA (or other error code) for those > > > > problematic sysfs nodes until late_initcall() is called? > > > > > > I really like Oliver's idea and I think we should push on that to see > > > if it can be made to work. If so, we can remove the late_initcall() > > > completely. > > > > > > > > > But I haven't tried to code it up, so it's probably more complicated > > > > > > than this. I guess ideally we would assign all the resources before > > > > > > pci_bus_add_device(). If we could do that, we could just remove > > > > > > pci_sysfs_init() and everything would just work, but I think that's a > > > > > > HUGE can of worms. > > > > > > > > > > I was under the impression the whole point of pci_bus_add_device() was > > > > > to handle any initialisation that needed to be done after resources > > > > > were assigned. Is the ROM BAR being potentially unassigned an x86ism > > > > > or is there some bigger point I'm missing? > > > > > > We can't assign resources for each device as we enumerate it because > > > we don't know what's in use by other devices yet to be enumerated. > > > That part is generic, not x86-specific. > > > > > > The part that is x86-specific (or at least specific to systems using > > > ACPI) is that the ACPI core doesn't reserve resources used by ACPI > > > devices. Sometimes those resources are included in the PCI host > > > bridge windows, and we don't want to assign them to PCI devices. > > > > > > I didn't trace this all the way, but the pcibios_assign_resources() > > > and pnp_system_init() comments look relevant. It's a little concerning > > > that they're both fs_initcalls() and the ordering looks important, but > > > it would only be by accident of link ordering that pnp_system_init() > > > happens first. > > > > Pali, what's your thought on this? Do you plan to work on this > > yourself? If not and if you can live with your workaround a while > > longer, I think Krzysztof might be interested in taking a crack at it. > > I would just hate to see you guys duplicate each others' work :) > > Hello Bjorn! > > If Krzysztof wants and would be working on this issue I can let it as is > for now. Krzysztof, as Bjorn wrote, do you want to take this issue? > But we should think how to deliver fix for this issue also into stable > kernels where this race condition is happening. > > I think that my workaround avoid those two race conditions and if proper > fix (= removal of pci_sysfs_init function) would take a long, what about > trying to workaround that race condition for now? > > My "fix" is relatively small and simple, so it should not be much hard > to review it. > > Krzysztof, what do you think?
