On Thu, 19 Jan 2023 03:43:36 +0000 "Tian, Kevin" <[email protected]> wrote:
> > From: Matthew Rosato <[email protected]> > > Sent: Wednesday, January 18, 2023 10:56 PM > > > > On 1/18/23 4:03 AM, Tian, Kevin wrote: > > >> From: Alex Williamson > > >> Sent: Wednesday, January 18, 2023 5:23 AM > > >> > > >> On Fri, 13 Jan 2023 19:03:51 -0500 > > >> Matthew Rosato <[email protected]> wrote: > > >> > > >>> void vfio_device_group_close(struct vfio_device *device) > > >>> { > > >>> + void (*put_kvm)(struct kvm *kvm); > > >>> + struct kvm *kvm; > > >>> + > > >>> mutex_lock(&device->group->group_lock); > > >>> + kvm = device->kvm; > > >>> + put_kvm = device->put_kvm; > > >>> vfio_device_close(device, device->group->iommufd); > > >>> + if (kvm == device->kvm) > > >>> + kvm = NULL; > > >> > > >> Hmm, so we're using whether the device->kvm pointer gets cleared in > > >> last_close to detect whether we should put the kvm reference. That's a > > >> bit obscure. Our get and put is also asymmetric. > > >> > > >> Did we decide that we couldn't do this via a schedule_work() from the > > >> last_close function, ie. implementing our own version of an async put? > > >> It seems like that potentially has a cleaner implementation, symmetric > > >> call points, handling all the storing and clearing of kvm related > > >> pointers within the get/put wrappers, passing only a vfio_device to the > > >> put wrapper, using the "vfio_device_" prefix for both. Potentially > > >> we'd just want an unconditional flush outside of lock here for > > >> deterministic release. > > >> > > >> What's the downside? Thanks, > > >> > > > > > > btw I guess this can be also fixed by Yi's work here: > > > > > > https://lore.kernel.org/kvm/[email protected]/ > > > > > > with set_kvm(NULL) moved to the release callback of kvm_vfio device, > > > such circular lock dependency can be avoided too. > > > > Oh, interesting... It seems to me that this would eliminate the reported > > call > > chain altogether: > > > > kvm_put_kvm > > -> kvm_destroy_vm > > -> kvm_destroy_devices > > -> kvm_vfio_destroy (starting here -- this would no longer be executed) > > -> kvm_vfio_file_set_kvm > > -> vfio_file_set_kvm > > -> group->group_lock/group_rwsem > > > > because kvm_destroy_devices now can't end up calling kvm_vfio_destroy > > and friends, it won't try and acquire the group lock a 2nd time making a > > kvm_put_kvm while the group lock is held OK to do. The vfio_file_set_kvm > > call will now always come from a separate thread of execution, > > kvm_vfio_group_add, kvm_vfio_group_del or the release thread: > > > > kvm_device_release (where the group->group_lock would not be held since > > vfio does not trigger closing of the kvm fd) > > -> kvm_vfio_destroy (or, kvm_vfio_release) > > -> kvm_vfio_file_set_kvm > > -> vfio_file_set_kvm > > -> group->group_lock/group_rwsem > > Yes, that's my point. If Alex/Jason are also OK with it probably Yi can > send that patch separately as a fix to this issue. It's much simpler. 😊 If we can extract that flow separate from the cdev refactoring, ideally something that matches the stable kernel backport rules, then that sounds like the preferred solution. Thanks, Alex
