linux-kernel - Re: PCI: Race condition in pci_create_sysfs_dev

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Date:   Wed, 7 Oct 2020 10:14:00 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Oliver O'Halloran <oohall@...il.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-pci <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Gregory Clement <gregory.clement@...tlin.com>,
        Andrew Lunn <andrew@...n.ch>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Yinghai Lu <yinghai@...nel.org>
Subject: Re: PCI: Race condition in pci_create_sysfs_dev_files

On Wednesday 07 October 2020 12:47:40 Oliver O'Halloran wrote:
> On Wed, Oct 7, 2020 at 10:26 AM Bjorn Helgaas <helgaas@...nel.org> 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?

> > 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?
> 
> Oliver