Broken pci_block_user_cfg_access interface

[Jan Kiszka <jan.kiszka@siemens.com>]

Hi,

trying to port the generic device interrupt masking pattern of
uio_pci_generic to KVM's device assignment code, I stumbled over some
fundamental problem with the current pci_block/unblock_user_cfg_access
interface: it does not provide any synchronization between blocking
sides. This allows user space to trigger a kernel BUG, just run two

while true; do echo 1 > /sys/bus/pci/devices/<some-device>/reset; done

loops in parallel and watch the kernel oops.

Instead of some funky open-coded locking mechanism, we would rather need
a plain mutex across both the user space access (via sysfs) and the
sections guarded by pci_block/unblock_user_cfg_access so far. But I'm
not sure which of them already allow sleeping, specifically if the IPR
driver would be fine with such a change. Can someone in the CC list
comment on this?

uio_pci_generic would definitely not be able to sleep as it takes the
lock from (potentially hard) IRQ context. This particular use case, RMW
of command/status word, requires a separate mechanism. I'm considering
to introduce a dedicated raw spinlock with IRQ protection for that
words, maybe also a PCI core service to abstract INTx testing and masking.

Any further thoughts on how to resolve this issue?

Thanks,
Jan

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