On 2011-08-25 15:02, Brian King wrote: > On 08/25/2011 04:19 AM, Jan Kiszka wrote: >> 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)? > > The ipr driver can get an error interrupt which will trigger the driver > to reset the adapter. While the adapter is going through reset we need > to ensure user config accesses are blocked, since many ipr adapters > won't respond on the PCI bus during this time. What about offloading the reset to thread context (workqueue etc.)? > > >>> 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. > > I took a look at the sysfs triggered pci reset function and don't see any way > that the controlling device driver ever gets to be involved in this reset. > If code outside the ipr driver were to reset the adapter, the adapter firmware > would be left in an uninitialized state and until scsi core starts timing > out ops and driving EH, the card would be unusable. I can't imagine the > ipr driver is unique in this. Right, that's why a PCI core service is needed for coordination. 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
