On Tue, Sep 7, 2021 at 10:21 PM Niklas Schnelle <schnelle@xxxxxxxxxxxxx> wrote: > > On Tue, 2021-09-07 at 10:45 +0200, Niklas Schnelle wrote: > > On Tue, 2021-09-07 at 12:04 +1000, Oliver O'Halloran wrote: > > > On Mon, Sep 6, 2021 at 7:49 PM Niklas Schnelle <schnelle@xxxxxxxxxxxxx> wrote: > > > > Patch 3 I already sent separately resulting in the discussion below but without > > > > a final conclusion. > > > > > > > > https://lore.kernel.org/lkml/20210720150145.640727-1-schnelle@xxxxxxxxxxxxx/ > > > > > > > > I believe even though there were some doubts about the use of > > > > pci_dev_is_added() by arch code the existing uses as well as the use in the > > > > final patch of this series warrant this export. > > > > > > The use of pci_dev_is_added() in arch/powerpc was because in the past > > > pci_bus_add_device() could be called before pci_device_add(). That was > > > fixed a while ago so It should be safe to remove those calls now. > > > > Hmm, ok that confirms Bjorns suspicion and explains how it came to be. > > I can certainly sent a patch for that. This would then leave only the > > existing use in s390 which I added because of a dead lock prevention > > and explained here: > > https://lore.kernel.org/lkml/87d15d5eead35c9eaa667958d057cf4a81a8bf13.camel@xxxxxxxxxxxxx/ > > > > Plus the need to use it in the recovery code of this series. I think in > > the EEH code the need for a similar check is alleviated by the checks > > in the beginning of > > arch/powerpc/kernel/eeh_driver.c:eeh_handle_normal_event() especially > > eeh_slot_presence_check() which checks presence via the hotplug slot. > > I guess we could use our own state tracking in a similar way but felt > > like pci_dev_is_added() is the more logical choice. The slot check is mainly there to prevent attempts to "recover" devices that have been surprise removed (i.e NVMe hot-unplug). The actual recovery process operates off the eeh_pe tree which is frozen in place when an error is detected. If a pci_dev is added or removed it's not really a problem since those are only ever looked at when notifying drivers which is done with the rescan_remove lock held. That said, I wouldn't really encourage anyone to follow the EEH model since it's pretty byzantine. > Looking into this again, I think we actually can't easily track this > state ourselves outside struct pci_dev. The reason for this is that > when e.g. arch/s390/pci/pci_sysfs.c:recover_store() removes the struct > pci_dev and scans it again the new struct pci_dev re-uses the same > struct zpci_dev because from a platform point of view the PCI device > was never removed but only disabled and re-enabled. Thus we can only > distinguish the stale struct pci_dev by looking at things stored in > struct pci_dev itself. IMO the real problem is removing and re-adding the pci_dev. I think it's something that's done largely because the PCI core doesn't really provide any better mechanism for getting a device back into a known-good state so it's abused to implement error recovery. This is something that's always annoyed me since it conflates recovery with hotplug. After a hot-(un)plug we might have a different device or no device. In the recovery case we expect to start and end with the same device. Why not apply the same logic to the pci_dev? Something I was tinkering with before I left IBM was re-working the way EEH handles recovering devices that don't have a driver with error handling callbacks to something like: 1. unbind the driver 2. pci_save_state() 3. do the reset 4. pci_restore_state() 5. re-bind the driver That would allow keeping the pci_dev around and let me delete a pile of confusing code which handles binding the eeh_dev to the new pci_dev. The obvious problem with that approach is the assumption the device is functional enough to allow saving the config space, but I don't think that's a deal breaker. We could stash a copy of the device state before we allow drivers to attach and use that to restore the device after the reset. The end result would be the same known-good state that we'd get after a re-scan. > That said, I think for the recovery case we might be able to drop the > pci_dev_is_added() and rely on pdev->driver != NULL which we check > anyway and that should catch any PCI device that was already removed. Would that work if there was an error on a device without a driver bound? If you're just trying to stop races between recovery and device removal then pci_dev_is_added() is probably the right tool for the job. Trying to substitute it with a proxy seems like a bad idea.
