On Tue, 2012-07-17 at 19:21 +0300, Michael S. Tsirkin wrote:
> On Tue, Jul 17, 2012 at 10:17:03AM -0600, Alex Williamson wrote:
> > > > > > > > > And current code looks buggy if yes we need to fix it
> > > > > > > > > somehow.
> > > > > > > >
> > > > > > > >
> > > > > > > > Which to me seems to indicate this should be handled as a
> > > > > > > > separate
> > > > > > > > effort.
> > > > > > >
> > > > > > > A separate patchset, sure. But likely a prerequisite: we still
> > > > > > > need to
> > > > > > > look at all the code. Let's not copy bugs, need to fix them.
> > > > > >
> > > > > > This looks tangential to me unless you can come up with an actual
> > > > > > reason
> > > > > > the above spinlock usage is incorrect or insufficient.
> > > > >
> > > > > You copy the same pattern that seems racy. So you double the
> > > > > amount of code that woul need to be fixed.
> > > >
> > > >
> > > > _Seems_ racy, or _is_ racy? Please identify the race.
> > >
> > > Look at this:
> > >
> > > static inline int kvm_irq_line_state(unsigned long *irq_state,
> > > int irq_source_id, int level)
> > > {
> > > /* Logical OR for level trig interrupt */
> > > if (level)
> > > set_bit(irq_source_id, irq_state);
> > > else
> > > clear_bit(irq_source_id, irq_state);
> > >
> > > return !!(*irq_state);
> > > }
> > >
> > >
> > > Now:
> > > If other CPU changes some other bit after the atomic change,
> > > it looks like !!(*irq_state) might return a stale value.
> > >
> > > CPU 0 clears bit 0. CPU 1 sets bit 1. CPU 1 sets level to 1.
> > > If CPU 0 sees a stale value now it will return 0 here
> > > and interrupt will get cleared.
> > >
> > >
> > > Maybe this is not a problem. But in that case IMO it needs
> > > a comment explaining why and why it's not a problem in
> > > your code.
> >
> > So you want to close the door on anything that uses kvm_set_irq until
> > this gets fixed... that's insane.
>
> What does kvm_set_irq use have to do with it? You posted this patch:
>
> +static int kvm_clear_pic_irq(struct kvm_kernel_irq_routing_entry *e,
> + struct kvm *kvm, int irq_source_id)
> +{
> +#ifdef CONFIG_X86
> + struct kvm_pic *pic = pic_irqchip(kvm);
> + int level =
> kvm_clear_irq_line_state(&pic->irq_states[e->irqchip.pin],
> + irq_source_id);
> + if (level)
> + kvm_pic_set_irq(pic, e->irqchip.pin,
> + !!pic->irq_states[e->irqchip.pin]);
> + return level;
> +#else
> + return -1;
> +#endif
> +}
> +
>
> it seems racy in the same way.
Now you're just misrepresenting how we got here, which was:
> > > > > > IMHO, we're going off into the weeds again with these last
> > > > > > two patches. It may be a valid optimization, but it really has no
> > > > > > bearing on the meat of the series (and afaict, no significant
> > > > > > performance difference either).
> > > > >
> > > > > For me it's not a performance thing. IMO code is cleaner without this
> > > > > locking:
> > > > > we add a lock but only use it in some cases, so the rules become
> > > > > really
> > > > > complex.
So I'm happy to drop the last 2 patches, which were done at your request
anyway, but you've failed to show how the locking in patches 1&2 is
messy, inconsistent, or complex and now you're asking to block all
progress. Those patches are just users of kvm_set_irq.
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