Messages in this thread | | | Date | Wed, 17 Aug 2022 08:49:40 +0300 | From | Lennert Buytenhek <> | Subject | Re: lockdep splat due to klist iteration from atomic context in Intel IOMMU driver |
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On Wed, Aug 17, 2022 at 12:45:26PM +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. > > If so, pci_get_domain_bus_and_slot() can not be used in this interrupt > context, right?
The 624fa7790f80 commit from 2018 tried to make klist use safe from atomic context, but since it missed a few of the klist accessors, it didn't actually manage to make it safe, so it's already not safe to use pci_get_domain_bus_and_slot() from interrupt context right now, even without reverting this commit. Reverting the commit would just be a declaration that klist use from atomic context isn't safe and never was.
A quick check doesn't turn up any other cases where people have run into this issue with code in mainline, so it would seem that the newly-added use of pci_get_domain_bus_and_slot() to the iommu/vt-d fault reporting interrupt handler is one of very few (if not only) cases of mainline code wanting to access klists from atomic context.
Kind regards, Lennert
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