On Thu, Jun 18, 2009 at 12:29:31PM -0400, Gregory Haskins wrote: > Michael S. Tsirkin wrote: > > On Thu, Jun 18, 2009 at 10:03:36AM -0400, Gregory Haskins wrote: > > > >> Michael S. Tsirkin wrote: > >> > >>> On Thu, Jun 18, 2009 at 08:00:39AM -0400, Gregory Haskins wrote: > >>> > >>> > >>>> Michael S. Tsirkin wrote: > >>>> > >>>> > >>>> > >>>>> BTW, Gregory, this can be used to fix the race in the design: create a > >>>>> thread and let it drop the module reference with module_put_and_exit. > >>>>> > >>>>> > >>>>> > >>>> I had thought of doing something like this initially too, but I think > >>>> its racy as well. Ultimately, you need to make sure the eventfd > >>>> callback is completely out before its safe to run, and deferring to a > >>>> thread would not change this race. The only sane way I can see to do > >>>> that is to have the caller infrastructure annotate the event somehow > >>>> (either directly with a module_put(), or indirectly with some kind of > >>>> state transition that can be tracked with something like > >>>> synchronize_sched(). > >>>> > >>>> > >>> Here's what one could do: create a thread for each irqfd, and increment > >>> module ref count, put that thread to sleep. When done with > >>> irqfd, don't delete it and don't decrement module refcount, wake thread > >>> instead. thread kills irqfd and calls module_put_and_exit. > >>> > >>> I don't think it's racy > >>> > >> I believe it is. How would you prevent the thread from doing the > >> module_put_and_exit() before the eventfd callback thread is known to > >> have exited the relevant .text section? > >> > > > > Right. > > > > > >> All this talk does give me an idea, tho. Ill make a patch. > >> > > > > OK, but ask yourself whether this bag of tricks is worth it, and whether > > we'll find another hole later. Let's reserve the trickiness for > > fast-path, where it's needed, and keep at least the assign/deassign simple. > > > > Understood. OTOH, going back to the model where two steps are needed > for close() is ugly too, so I don't want to just give up and revert that > fix too easily. At some point we will call it one way or the other, but > I am not there quite yet. > > > >>> > >>> > >>>>> Which will work, but I guess at this point we should ask ourselves > >>>>> whether all the hearburn with srcu, threads and module references is > >>>>> better than just asking the user to call and ioctl. > >>>>> > >>>>> > >>>>> > >>>> I am starting to agree with you, here. :) > >>>> > >>>> Note one thing: the SRCU stuff is mostly orthogonal from the rest of the > >>>> conversation re: the module_put() races. I only tied it into the > >>>> current thread because the eventfd_notifier_register() thread gave me a > >>>> convenient way to hook some other context to do the module_put(). In > >>>> the long term, the srcu changes are for the can_sleep() stuff. So on > >>>> that note, lets see if I can convince Davide that the srcu stuff is not > >>>> so evil before we revert the POLLHUP patches, since the module_put() fix > >>>> is trivial once that is in place. > >>>> > >>>> > >>> Can this help with DEASSIGN as well? We need it for migration. > >>> > >>> > >> No, but afaict you do not need this for migration anyway. Migrate the > >> GSI and re-call kvm_irqfd() on the other side. Would the fd even be > >> relevant across a migration anyway? I would think not, but admittedly I > >> know little about how qemu/kvm migration actually works. > >> > > > > Yes but that's not live migration. For live migration, the trick is that > > you are running locally but send changes to remote guest. For that, we > > need to put qemu in the middle between the device and the guest, so it > > can detect activity and update the remote side. > > > > And the best way to do that is to take poll eventfd that device assigns > > and push eventfd that kvm polls. To switch between this setup > > and the one where kvm polls the ventfd from device directly, > > you need deassign. > > > > So its still not clear why the distinction between > deassign-the-gsi-but-leave-the-fd-valid is needed over a simple > close(). Can you elaborate? The fd needs to be left assigned to the device, so that we can poll the fd and get events, then forward them to kvm. -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
