Re: [PATCH v2 6/8] kvm tools: Add rwlock wrapper

[Sasha Levin <levinsasha928@gmail.com>]

On Mon, 2011-05-30 at 12:40 +0300, Pekka Enberg wrote:
> Hi Sasha,
> 
> On Mon, May 30, 2011 at 12:34 PM, Sasha Levin <levinsasha928@gmail.com> wrote:
> > It would mean we need that many VCPUs: current br_read_lock() doesn't
> > really do anything, which means that running these tests with dummy
> > threads won't work.
> 
> Heh, sure they're doing something - they're burning CPU. You assume
> that br_write_unlock() will actually resume all of them but I'd really
> like to see a reproducible test case for that. ;-)

I agree with what you're saying: testing whether br_write_lock() makes
all VCPU threads stop and br_write_unlock() makes all VCPU threads
resume should be tested.

I'm just saying that we're limited to as many VCPU threads as we can
create. br_read_lock() won't do anything on a non-VCPU thread, which
makes it impossible to test it on non-VCPUs.

> Do you think it's a bad idea? I don't quite see why - as I've said
> before, this is something we learned the hard way when we implemented
> our own locking primitives and stop-the-world for Jato. Those are just
> so damn easy to break accidentally so having some safety net is
> definitely worth it.
> 
> Also, didn't Paul suggest some debugging magic to detect errors?
> 
>                        Pekka

-- 

Sasha.