On 2011-08-25 11:40, Michael S. Tsirkin wrote: > On Thu, Aug 25, 2011 at 11:19:54AM +0200, 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)? >> >>> 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 > > It does look like all of the problems are actually around reset. > So maybe all we need to do is synchronize the sysfs-triggered > pci_reset_function with pci_block/unblock_user_cfg_access? > > In other words, when reset is triggered from sysfs, it > should obey pci_block/unblock_user_cfg_access > restrictions? > > It does not look like reset needs to sleep, so fixing > that should not be hard, right? It obviously does need to sleep (wait). I just don't know if blocking config space access is required during that phase, but I wouldn't be surprised if it was. 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
