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

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

On 2022/8/15 21:57, Lennert Buytenhek wrote:
> On Mon, Aug 15, 2022 at 06:32:24AM -0700, Bart Van Assche 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.

Best regards,
baolu