linux-kernel - Re: [PATCH v2] PCI: pciehp: Check link state before accessing device during removal

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Date:	Thu, 11 Dec 2014 13:39:50 -0800
From:	Guenter Roeck <groeck@...iper.net>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
CC:	Rajat Jain <rajatxjain@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Rajat Jain <rajatjain@...iper.net>
Subject: Re: [PATCH v2] PCI: pciehp: Check link state before accessing device
 during removal

On Thu, Dec 11, 2014 at 02:00:06PM -0700, Bjorn Helgaas wrote:
> On Thu, Dec 11, 2014 at 1:45 PM, Guenter Roeck <groeck@...iper.net> wrote:
> > On Thu, Dec 11, 2014 at 01:26:47PM -0700, Bjorn Helgaas wrote:
> >> On Thu, Dec 11, 2014 at 9:38 AM, Guenter Roeck <groeck@...iper.net> wrote:
> >> > On Wed, Dec 10, 2014 at 05:26:30PM -0700, Bjorn Helgaas wrote:
> >> >> On Thu, Nov 20, 2014 at 02:54:24PM -0800, Rajat Jain wrote:
> 
> >> >> If we do need it (and it looks like most or all hotplug drivers copied it),
> >> >> isn't there still a race?  Can't we have the following sequence?
> >> >>
> >> >>   - pciehp_check_link_active()                # returns true
> >> >>   - Link goes down
> >> >>   - pci_read_config_byte()            # fails because link is down
> >> >>
> >> > I would guess so. Question is how to address it. Read the configuration byte
> >> > first, then check if the link is down ? Check if link is still up after reading
> >> > the configuration byte ? Add a note that there may be a potential race condition
> >> > and do nothing until it is actually seen ?
> >>
> >> I think we should just read PCI_BRIDGE_CONTROL and look for a 0xff
> >> value.  That's not a legal value for the register, so if we see it, it
> >> should be pretty safe to assume the link is down or the device is not
> >> present at all.
> >>
> > Something like
> >         if (bctl != 0xff && (bctl & PCI_BRIDGE_CTL_VGA)) {
> > in addition to Rajat's changes ?
> >
> > I think it would be good to keep the change Rajat proposed, ie to check
> > the link state instead of presence. Question then is if you'd want a new
> > revision of Rajat's patch or another patch on top of it with the bctl
> > related change.
> 
> Why do we need the link state or the presence check?  It seems like
> those are sort of a 90% solution, and doing them provides the illusion
> of value but without real value.  If we think that checking for 0xff
> is a 100% solution, we should rely on that and not bother with
> anything else.
> 
Interesting thought. Let me play with that.

Guenter
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