linux-kernel - Re: [PATCHv6 2/7] pci: child bus alloc fix on constrained resource

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Date:	Thu, 17 Dec 2015 17:57:43 +0000
From:	Keith Busch <keith.busch@...el.com>
To:	Bjorn Helgaas <helgaas@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	linux-pci@...r.kernel.org, Jiang Liu <jiang.liu@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dan Williams <dan.j.williams@...el.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Bryan Veal <bryan.e.veal@...el.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Martin Mares <mj@....cz>,
	Jon Derrick <jonathan.derrick@...el.com>
Subject: Re: [PATCHv6 2/7] pci: child bus alloc fix on constrained resource

On Thu, Dec 17, 2015 at 11:27:18AM -0600, Bjorn Helgaas wrote:
> On Mon, Dec 07, 2015 at 02:32:24PM -0700, Keith Busch wrote:
> > +	if (busnr > parent->busn_res.end) {
> > +		dev_printk(KERN_DEBUG, &parent->dev,
> > +			  "can not alloc bus:%d under %pR\n", busnr,
> > +			  &parent->busn_res);
> > +		return NULL;
> 
> Can you take a look at 1820ffdccb9b ("PCI: Make sure bus number resources
> stay within their parents bounds") and 12d8706963f0 ("Revert "PCI: Make
> sure bus number resources stay within their parents bounds"")?
> 
> This is implemented differently, but it seems like it might expose the same
> problem we found with 1820ffdccb9b.
> 
> If you could take a look and confirm that "no, this does something
> differently than 1820ffdccb9b did" or "yes, this might expose that problem
> again," that would help.

Thank you for the references. I think 1820ffdccb9b had the check too
late, creating the child bus with resource conflicts. That'd probably
make it appear unavailable. But I believe this new proposal will still
break the same setup for a different reason.

We can live with this issue for now if we want to drop this patch from
the series. The only way we encounter a problem is when the config space
aperture is artificially constrained, so I can request the feature be
put on hold while a better fix is developed.
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