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? -Greg
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