On 2011-08-24 17:02, Brian King wrote: > On 08/24/2011 05:43 AM, Jan Kiszka wrote: >> 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? > > The ipr driver calls pci_block/unblock_user_cfg_access from interrupt > context, so a mutex won't work. Ugh. What precisely does it have to do with the config space while running inside an IRQ handler (or holding a lock that synchronizes it with such a handler)? > When the pci_block/unblock API was > originally added, it did not have the checking it has today to detect > if it is being called nested. This was added some time later. The For a reason... > API that works best for the ipr driver is to allow for many block calls, > but a single unblock call unblocks access. It seems like what might > work well in the case above is a block count. Each call to pci_block > increments a count. Each pci_unblock decrements the count and only > actually do the unblock if the count drops to zero. It should be reasonably > simple for ipr to use that sort of an API as well. That will just paper over the underlying bug: multiple kernel users (!= sysfs access) fiddle with the config space in an unsynchronized fashion. Think of sysfs-triggered pci_reset_function while your ipr driver does its accesses. So it's pointless to tweak the current pci_block semantics, we rather need to establish a new mechanism that synchronizes *all* users of the config space. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
