On Sun, Jun 14, 2009 at 11:39:21PM -0400, Gregory Haskins wrote: > Michael S. Tsirkin wrote: > > On Sun, Jun 14, 2009 at 08:53:11AM -0400, Gregory Haskins wrote: > > > >> Michael S. Tsirkin wrote: > >> > >>> On Thu, Jun 04, 2009 at 08:48:12AM -0400, Gregory Haskins wrote: > >>> > >>> > >>>> +static void > >>>> +irqfd_disconnect(struct _irqfd *irqfd) > >>>> +{ > >>>> + struct kvm *kvm; > >>>> + > >>>> + mutex_lock(&irqfd->lock); > >>>> + > >>>> + kvm = rcu_dereference(irqfd->kvm); > >>>> + rcu_assign_pointer(irqfd->kvm, NULL); > >>>> + > >>>> + mutex_unlock(&irqfd->lock); > >>>> + > >>>> + if (!kvm) > >>>> + return; > >>>> > >>>> mutex_lock(&kvm->lock); > >>>> - kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 1); > >>>> - kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0); > >>>> + list_del(&irqfd->list); > >>>> mutex_unlock(&kvm->lock); > >>>> + > >>>> + /* > >>>> + * It is important to not drop the kvm reference until the next grace > >>>> + * period because there might be lockless references in flight up > >>>> + * until then > >>>> + */ > >>>> + synchronize_srcu(&irqfd->srcu); > >>>> + kvm_put_kvm(kvm); > >>>> } > >>>> > >>>> > >>> So irqfd object will persist after kvm goes away, until eventfd is closed? > >>> > >>> > >> Yep, by design. It becomes part of the eventfd and is thus associated > >> with its lifetime. Consider it as if we made our own anon-fd > >> implementation for irqfd and the lifetime looks similar. The difference > >> is that we are reusing eventfd and its interface semantics. > >> > >>> > >>> > >>>> > >>>> static int > >>>> irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key) > >>>> { > >>>> struct _irqfd *irqfd = container_of(wait, struct _irqfd, wait); > >>>> + unsigned long flags = (unsigned long)key; > >>>> > >>>> - /* > >>>> - * The wake_up is called with interrupts disabled. Therefore we need > >>>> - * to defer the IRQ injection until later since we need to acquire the > >>>> - * kvm->lock to do so. > >>>> - */ > >>>> - schedule_work(&irqfd->work); > >>>> + if (flags & POLLIN) > >>>> + /* > >>>> + * The POLLIN wake_up is called with interrupts disabled. > >>>> + * Therefore we need to defer the IRQ injection until later > >>>> + * since we need to acquire the kvm->lock to do so. > >>>> + */ > >>>> + schedule_work(&irqfd->inject); > >>>> + > >>>> + if (flags & POLLHUP) { > >>>> + /* > >>>> + * The POLLHUP is called unlocked, so it theoretically should > >>>> + * be safe to remove ourselves from the wqh using the locked > >>>> + * variant of remove_wait_queue() > >>>> + */ > >>>> + remove_wait_queue(irqfd->wqh, &irqfd->wait); > >>>> + flush_work(&irqfd->inject); > >>>> + irqfd_disconnect(irqfd); > >>>> + > >>>> + cleanup_srcu_struct(&irqfd->srcu); > >>>> + kfree(irqfd); > >>>> + } > >>>> > >>>> return 0; > >>>> } > >>>> > >>>> > >>> And it is removed by this function when eventfd is closed. > >>> But what prevents the kvm module from going away, meanwhile? > >>> > >>> > >> Well, we hold a reference to struct kvm until we call > >> irqfd_disconnect(). If kvm closes first, we disconnect and disassociate > >> all references to kvm leaving irqfd->kvm = NULL. Likewise, if irqfd > >> closes first, we disassociate with kvm with the above quoted logic. In > >> either case, we are holding a kvm reference up until that "disconnect" > >> point. Therefore kvm should not be able to disappear before that > >> disconnect, and after that point we do not care. > >> > > > > Yes, we do care. > > > > Here's the scenario in more detail: > > > > - kvm is closed > > - irq disconnect is called > > - kvm is put > > - kvm module is removed: all irqs are disconnected > > - eventfd closes and triggers callback into removed kvm module > > - crash > > > > [ lightbulb turns on] > > Ah, now I see the point you were making. I thought you were talking > about the .text in kvm_set_irq() (which would be protected by my > kvm_get_kvm() reference afaict). But you are actually talking about the > irqfd .text itself. Indeed, you are correct that is this currently a > race. Good catch! > > > > >> If that is not sufficient to prevent kvm.ko from going away in the > >> middle, then IMO kvm_get_kvm() has a bug, not irqfd. ;) However, I > >> believe everything is actually ok here. > >> > >> -Greg > >> > >> > > > > > > BTW, why can't we remove irqfds in kvm_release? > > > > Well, this would be ideal but we run into that bi-directional reference > thing that we talked about earlier and we both agree is non-trivial to > solve. Solving this locking problem would incidentally also pave the > way for restoring the DEASSIGN feature, so patches welcome! So far the only workable approach that I see is reverting the POLLHUP patch. I agree it looks pretty, but DEASSIGN and closing the races is more important IMO. And locking will definitely become much simpler. > In the meantime, I think we can close the hole you found with the > following patch (build-tested only): > > commit f3a8dccc9e815599438e9feb0ea53e8eb10ad2b3 > Author: Gregory Haskins <ghaskins@xxxxxxxxxx> > Date: Sun Jun 14 23:37:49 2009 -0400 > > KVM: make irqfd take kvm.ko module reference > > Michael Tsirkin pointed out that we currently have a race between someone > holding an irqfd reference and an rmmod against kvm.ko. This patch closes > that hole by making sure that irqfd holds a kvm.ko reference for its lifetime. > > Found-by: Michael S. Tsirkin <mst@xxxxxxxxxx> > Signed-off-by: Gregory Haskins <ghaskins@xxxxxxxxxx> > > diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c > index 2c8028c..67e4eca 100644 > --- a/virt/kvm/eventfd.c > +++ b/virt/kvm/eventfd.c > @@ -29,6 +29,7 @@ > #include <linux/list.h> > #include <linux/eventfd.h> > #include <linux/srcu.h> > +#include <linux/module.h> > > /* > * -------------------------------------------------------------------- > @@ -123,6 +124,7 @@ irqfd_wakeup(wait_queue_t *wait, unsigned mode, int > sync, void > *key) > > cleanup_srcu_struct(&irqfd->srcu); > kfree(irqfd); > + module_put(THIS_MODULE); > } > > return 0; module_put(THIS_MODULE) is always a bug unless you know that someone has a reference to the current module: the module could go away between this call and returning from function. > @@ -176,6 +178,7 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags) > if (ret < 0) > goto fail; > > + __module_get(THIS_MODULE); > kvm_get_kvm(kvm); > > mutex_lock(&kvm->lock); > > > -- 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