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? > > -- > 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
