Messages in this thread | | | Date | Wed, 17 Aug 2022 15:20:46 +0800 | Subject | Re: lockdep splat due to klist iteration from atomic context in Intel IOMMU driver | From | Baolu Lu <> |
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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
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