Re: lockdep splat due to klist iteration from atomic context in Intel IOMMU driver

[Baolu Lu <baolu.lu@linux.intel.com>]

On 2022/8/17 14:09, Lennert Buytenhek wrote:
> On Wed, Aug 17, 2022 at 12:04:10PM +0800, Baolu Lu wrote:
> 
>>>>> On a build of 7ebfc85e2cd7 ("Merge tag 'net-6.0-rc1' of
>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net"), with
>>>>> CONFIG_INTEL_IOMMU_DEBUGFS enabled, I am seeing the lockdep splat
>>>>> below when an I/O page fault occurs on a machine with an Intel
>>>>> IOMMU in it.
>>>>>
>>>>> The issue seems to be the klist iterator functions using
>>>>> spin_*lock_irq*() but the klist insertion functions using
>>>>> spin_*lock(), combined with the Intel DMAR IOMMU driver iterating
>>>>> over klists from atomic (hardirq) context as of commit 8ac0b64b9735
>>>>> ("iommu/vt-d: Use pci_get_domain_bus_and_slot() in pgtable_walk()")
>>>>> when CONFIG_INTEL_IOMMU_DEBUGFS is enabled, where
>>>>> pci_get_domain_bus_and_slot() calls into bus_find_device() which
>>>>> iterates over klists.
>>>>>
>>>>> I found this commit from 2018:
>>>>>
>>>>> 	commit 624fa7790f80575a4ec28fbdb2034097dc18d051
>>>>> 	Author: Bart Van Assche<bvanassche@acm.org>
>>>>> 	Date:   Fri Jun 22 14:54:49 2018 -0700
>>>>>
>>>>> 	    scsi: klist: Make it safe to use klists in atomic context
>>>>>
>>>>> This commit switched lib/klist.c:klist_{prev,next} from
>>>>> spin_{,un}lock() to spin_{lock_irqsave,unlock_irqrestore}(), but left
>>>>> the spin_{,un}lock() calls in add_{head,tail}() untouched.
>>>>>
>>>>> The simplest fix for this would be to switch lib/klist.c:add_{head,tail}()
>>>>> over to use the IRQ-safe spinlock variants as well?
>>>> Another possibility would be to evaluate whether it is safe to revert commit
>>>> 624fa7790f80 ("scsi: klist: Make it safe to use klists in atomic context").
>>>> That commit is no longer needed by the SRP transport driver since the legacy
>>>> block layer has been removed from the kernel.
>>> And then to fix the 6.0-rc1 iommu/vt-d lockdep splat with
>>> CONFIG_INTEL_IOMMU_DEBUGFS enabled, we could convert the Intel DMAR
>>> IRQ handler to a threaded IRQ handler.  We (Arista) carry the patch
>>> below in our kernel tree, and the last two hunks of the patch do
>>> exactly that, for the same reason (having to call
>>> pci_get_domain_bus_and_slot() from the IRQ handler) but this is
>>> probably too big of a change for 6.0-rc.
>>>
>>>
>>>
>>> commit 90a8e7da0facf198692a641fcfe6f89c478608e0
>>> Author: Lennert Buytenhek<buytenh@wantstofly.org>
>>> Date:   Wed Jul 13 15:34:30 2022 +0300
>>>
>>>       iommu/vt-d: Use report_iommu_fault()
>>>       This patch makes iommu/vt-d call report_iommu_fault() when an I/O
>>>       page fault occurs, which has two effects:
>>>       1) It allows device drivers to register a callback to be notified
>>>          of I/O page faults, via the iommu_set_fault_handler() API.
>>>       2) It triggers the io_page_fault tracepoint in report_iommu_fault()
>>>          when an I/O page fault occurs.
>>>       The latter point is the main aim of this patch, as it allows
>>>       rasdaemon-like daemons to be notified of I/O page faults, and to
>>>       possibly initiate corrective action in response.
>>
>> The IOMMU subsystem already has a framework to handle I/O page faults:
>>
>>      commit fc36479db74e9 "iommu: Add a page fault handler"
>>
>> And below series,
>>
>> https://lore.kernel.org/linux-iommu/20220817012024.3251276-1-baolu.lu@linux.intel.com/
>>
>> is trying to make it more generic. It seems to be more suitable for your
>> case.
>>
>> The report_iommu_fault() probably will be replaced by
>> iommu_register_device_fault_handler() eventually. So I don't encourage
>> its usage in the VT-d driver.
> 
> We use the iommu/io_page_fault tracepoint from userspace to be notified
> of (non-ATS) I/O page faults so that we can detect malfunctioning PCIe
> devices, which in our systems are typically switch/router line cards,
> and take corrective action, such as restarting the offending line card.

Yes. Make sense.

> 
> Calling report_iommu_fault() causes the iommu/io_page_fault tracepoint
> to be invoked, which is why we made the AMD and Intel IOMMU drivers use
> report_iommu_fault() in our kernel tree.

Can iommu_register_device_fault_handler() also serve your case?
report_iommu_fault() is domain based, while the former is device based.

> 
> It seems that iommu_queue_iopf() is specific to the SVA use case, while
> we are not using SVA, in which case it would not address our use case.
> (We don't care about knowing about ATS translation faults, we just want
> to know when a non-ATS PCI device is malfunctioning.)

The iommu_queue_iopf() is for recoverable I/O page fault. Your case only
cares about unrecoverable DMA faults. So it's not suitable for you.
Sorry for the misunderstanding.

Best regards,
baolu