Re: [BUG] WARN_ON(!context) in drivers/pci/hotplug/acpiphp_glue.c

Rafael J. Wysocki
2013-10-11T23:56:31Z

On Friday, October 11, 2013 11:58:48 PM Rafael J. Wysocki wrote:
> On Friday, October 11, 2013 10:21:35 AM Linus Torvalds wrote:
> > On Fri, Oct 11, 2013 at 4:13 AM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:

[...]

> > Or am I misreading the code? It's more readable, and no longer makes
> > me homicidal, but I don't actually know the code itself.
>
> I think you're reading it correctly, it really makes acpiphp see all slots
> even if pciehp sees them too. So the change is somewhat risky.
>
> That said the risk doesn't seem to be huge and there seem to be cases in
> which it actually would be useful to have both acpiphp and pciehp signaling
> available for the same device. For example, even if the BIOS told us that
> we could use the native mechanism (pciehp), it may not actually work. That is,
> we may not get any hotplug interrupts from PCIe ports due to platform bugs of
> some sort and we may get ACPI notifications instead (because the platform
> designer knew about those bugs and thought it would be smart to use ACPI to
> work around them).
>
> There are bug reports indicating thinks like that, so we were going to allow
> acpiphp and pciehp to handle the same devices anyway at one point. I thought
> we might as well try to do it now and see how it goes. Still, if you think
> it's too risky for this stage of the cycle, I'll just send a patch removing
> the WARN_ON() and we'll revisit that thing in 3.13.

Having reconsidered this I think it's best to just drop the WARN_ON() for now
after all and sort out the coexistence between acpiphp and pciehp later, so
that we don't run into a weird corner case late in the cycle.

So the one below is what I'm going to do for 3.12.

Rafael

---
From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Subject: ACPI / hotplug / PCI: Drop WARN_ON() from acpiphp_enumerate_slots()

The WARN_ON() in acpiphp_enumerate_slots() triggers unnecessarily for
devices whose bridges are going to be handled by native PCIe hotplug
(pciehp) and the simplest way to prevent that from happening is to
drop the WARN_ON().

References: https://bugzilla.kernel.org/show_bug.cgi?id=62831
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
drivers/pci/hotplug/acpiphp_glue.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

Index: linux-pm/drivers/pci/hotplug/acpiphp_glue.c
===================================================================
--- linux-pm.orig/drivers/pci/hotplug/acpiphp_glue.c
+++ linux-pm/drivers/pci/hotplug/acpiphp_glue.c
@@ -999,12 +999,13 @@ void acpiphp_enumerate_slots(struct pci_

/*
* This bridge should have been registered as a hotplug function
- * under its parent, so the context has to be there. If not, we
- * are in deep goo.
+ * under its parent, so the context should be there, unless the
+ * parent is going to be handled by pciehp, in which case this
+ * bridge is not interesting to us either.
*/
mutex_lock(&acpiphp_context_lock);
context = acpiphp_get_context(handle);
- if (WARN_ON(!context)) {
+ if (!context) {
mutex_unlock(&acpiphp_context_lock);
put_device(&bus->dev);
kfree(bridge);