On Wed, Sep 19, 2012 at 01:23:35PM -0600, Alex Williamson wrote:
> On Wed, 2012-09-19 at 21:48 +0300, Michael S. Tsirkin wrote:
> > On Wed, Sep 19, 2012 at 12:23:13PM -0600, Alex Williamson wrote:
> > > On Wed, 2012-09-19 at 11:59 +0300, Avi Kivity wrote:
> > > > On 09/18/2012 06:16 AM, Alex Williamson wrote:
> > > > > @@ -92,6 +156,43 @@ irqfd_shutdown(struct work_struct *work)
> > > > > */
> > > > > flush_work_sync(&irqfd->inject);
> > > > >
> > > > > + if (irqfd->resampler) {
> > > > > + struct _irqfd_resampler *resampler = irqfd->resampler;
> > > > > + struct kvm *kvm = resampler->kvm;
> > > > > +
> > > > > + mutex_lock(&kvm->irq_lock);
> > > > > + spin_lock_irq(&irqfd->kvm->irqfds.lock);
> > > > > +
> > > > > + list_del_rcu(&irqfd->resampler_list);
> > > > > +
> > > > > + /*
> > > > > + * On removal of the last irqfd in the resampler list,
> > > > > + * remove the resampler and unregister the irq ack
> > > > > + * notifier. It's possible to race the ack of the final
> > > > > + * injection here, so manually de-assert the gsi to avoid
> > > > > + * leaving an unmanaged, asserted interrupt line.
> > > > > + */
> > > > > + if (list_empty(&resampler->irqfds)) {
> > > > > + list_del(&resampler->list);
> > > > > + __kvm_unregister_irq_ack_notifier(kvm,
> > > > > + &resampler->notifier);
> > > > > + kvm_set_irq(kvm, KVM_IRQFD_RESAMPLE_IRQ_SOURCE_ID,
> > > > > + resampler->notifier.gsi, 0);
> > > > > + kfree(resampler);
> > > >
> > > > Is this rcu safe?
> > >
> > > No it's not and unfortunately this also points out another race in
> > > trying to use a single source ID...
> > >
> > > > > + }
> > > > > +
> > > > > + spin_unlock_irq(&irqfd->kvm->irqfds.lock);
> > > > > + mutex_unlock(&kvm->irq_lock);
> > > > > +
> > > > > + /*
> > > > > + * Both list_del_rcu & __kvm_unregister_irq_ack_notifier
> > > > > + * require an rcu grace period/
> > > > > + */
> > > > > + synchronize_rcu();
> > >
> > > The kfree can't be done until here and we also have to assume that ack
> > > notifies are firing until here. That means that between the
> > > mutex_unlock and the end of synchronize_rcu another resampling irqfd can
> > > be registered, post an interrupt, and have it de-asserted by the wrong
> > > resampler. Maybe the conversion wasn't as clean as I first thought :(
> > > > Quite ugly to expose the internals this way.
> > >
> > > Yep. I don't know how to clean it up though; between all the different
> > > rcu operations and locks, it's a mess. Thanks,
> > >
> > > Alex
> >
> > Add another mutex for the resamplers, keep it during the whole
> > operation? This also removes the need for exposing the internals.
> > If you do pls document lock nesting rules.
>
> How does that hide the internals?
You can call kvm_unregister_irq_ack_notifier now.
> Seems like we'd just wrap this in yet
> another mutex, but be largely the same.
The key is synchronize_rcu is under mutex.
> Thanks,
>
> Alex
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