linux-kernel - Re: git-latest: kernel oops in IOMMU setup

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Message-ID: <20090109070805.525c0de9@infradead.org>
Date:	Fri, 9 Jan 2009 07:08:05 -0800
From:	Dirk Hohndel <hohndel@...radead.org>
To:	"Han, Weidong" <weidong.han@...el.com>
Cc:	'Grant Grundler' <grundler@...isc-linux.org>,
	"'linux-pci@...r.kernel.org'" <linux-pci@...r.kernel.org>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
	'Jesse Barnes' <jbarnes@...tuousgeek.org>,
	"'iommu@...ts.linux-foundation.org'" 
	<iommu@...ts.linux-foundation.org>, 'Ingo Molnar' <mingo@...e.hu>,
	'Arjan van de Ven' <arjan@...radead.org>
Subject: Re: git-latest: kernel oops in IOMMU setup

On Fri, 9 Jan 2009 14:53:14 +0800
"Han, Weidong" <weidong.han@...el.com> wrote:

> Dirk Hohndel wrote:
> > On Thu, 8 Jan 2009 18:05:15 -0800
> > Dirk Hohndel <hohndel@...radead.org> wrote:
> > 
> >> On Fri, 9 Jan 2009 08:58:46 +0800 "Han, Weidong"
> >> 
> >> I updated to Linus' latest git (as your description made me wonder
> >> if the async stuff might play a role here). I still get an oops -
> >> but at a different spot and the system no longer hangs - it partly
> >> recovers (but things aren't too well - for example my USB
> >> keyboard / mouse don't work anymore).
> > 
> > Spoke too soon. Rebooted and had the same hard lockup again. This
> > time I had my camera within reach, so here's the trace:
> > 
> > device_to_iommu+0x33/0x73
> > domain_context_mapping_one+0x37/0x335
> > domain_context_mapping+0x25/0xa7
> > iommu_prepare_identity+0xd7/0xf3
> > intel_iommu_init+0x4e4/0x8f3
> > ? mutex_lock
> > ? sysctl_net_init
> > ? pci_iommu_init
> > pci_iommu_init
> > 
> > I also have stack, code and register values. Let me know if you need
> > them. Or I can just post the picture :-)
> > 
> > Again, very latest git tree, VT-d enabled.
> > 
> > /D
> 
> I tried latest git tree, it works for me. Above call trace looks
> right. 

Spent some more time reading the code. Can't quite claim to understand
all of it, yet, but I notice that most everywhere else drhd->devices[i]
is checked to be != NULL before it is accessed. Why is it safe not to
do that in device_to_iommu()?

Would the patch below be a valid fix? It stops my system from hanging at
boot. But I wonder if there is an assertion that if drhd->ignored is 0
then drhd->devices[0..drhd->device_cnt] is known to be != NULL and
therefore this test is just hiding a bug somewhere else...

/D

Signed-off-by: Dirk Hohndel <hohndel@...ux.intel.com>
---
 drivers/pci/intel-iommu.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/drivers/pci/intel-iommu.c b/drivers/pci/intel-iommu.c
index 235fb7a..3dfecb2 100644
--- a/drivers/pci/intel-iommu.c
+++ b/drivers/pci/intel-iommu.c
@@ -438,7 +438,8 @@ static struct intel_iommu *device_to_iommu(u8 bus,
u8 devfn) continue;
 
 		for (i = 0; i < drhd->devices_cnt; i++)
-			if (drhd->devices[i]->bus->number == bus &&
+			if (drhd->devices[i] &&
+			    drhd->devices[i]->bus->number == bus &&
 			    drhd->devices[i]->devfn == devfn)
 				return drhd->iommu;
 
-- 
1.6.0.6


-- 
Dirk Hohndel
Intel Open Source Technology Center
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