Re: [PATCH v2 1/2] KVM: MMIO: Lock coalesced device when checking for available entry

Sasha Levin
2011-07-18T10:16:08Z

On Mon, 2011-07-18 at 12:50 +0300, Avi Kivity wrote:
> On 07/18/2011 12:29 PM, Sasha Levin wrote:
> > > Hmm. This means we take the lock for every I/O, whether it hits
> > > coalesced mmio or not.
> > >
> > > We need to do the range check before taking the lock and the space check
> > > after taking the lock.
> > >
> >
> > I'll fix that.
> >
> > Shouldn't the range check be also locked somehow? Currently it is
> > possible that a coalesced region was removed while we are checking the
> > ranges, and we won't issue a mmio exit as the host expects
>
> It's "locked" using rcu.
>

Where is that happening?

All the coalesced zones are stored under the coalesced "device" in a
simple array. When adding and removing zones, kvm->slots_lock is taken -
I don't see anything which prevents a range check during zone removal
unless slots_lock prevents IO.

--

Sasha.