Re: [Qemu-devel] [RFC] vhost: Can we change synchronize_rcu to call_rcu in vhost_set_memory() in vhost kernel module?

[Paolo Bonzini]

Il 12/05/2014 12:18, Michael S. Tsirkin ha scritto:
> On Mon, May 12, 2014 at 12:14:25PM +0200, Paolo Bonzini wrote:
> > Il 12/05/2014 12:08, Michael S. Tsirkin ha scritto:
> > > On Mon, May 12, 2014 at 11:57:32AM +0200, Paolo Bonzini wrote:
> > > > Perhaps we can check for cases where only the address is changing,
> > > > and poke at an existing struct kvm_kernel_irq_routing_entry without
> > > > doing any RCU synchronization?
> > >
> > > I suspect interrupts can get lost then: e.g. if address didn't match any
> > > cpus, now it matches some. No?
> >
> > Can you explain the problem more verbosely? :)
> >
> > Multiple writers would still be protected by the mutex, so you
> > cannot have an "in-place update" writer racing with a "copy the
> > array" writer.
>
> I am not sure really.
> I'm worried about reader vs writer.
> If reader sees a stale msi value msi will be sent to a wrong
> address.

That shouldn't happen on any cache-coherent system, no?

Or at least, it shouldn't become any worse than what can already happen 
with RCU.

Paolo