Re: [PATCH 3/4] kvm tools: Add a brlock

[Sasha Levin <levinsasha928@gmail.com>]

On Sun, 2011-05-29 at 20:47 +0200, Ingo Molnar wrote:
> * Sasha Levin <levinsasha928@gmail.com> wrote:
> 
> > +++ b/tools/kvm/include/kvm/brlock.h
> > @@ -0,0 +1,12 @@
> > +#ifndef KVM__BRLOCK_H
> > +#define KVM__BRLOCK_H
> > +
> > +#include "kvm/kvm.h"
> > +#include "kvm/barrier.h"
> > +
> > +#define br_read_lock()		mb()
> > +#define br_read_unlock()	mb()
> 
> These only need to be compiler barrier()s AFAICS, because the 'pause' 
> op will signal back to the requestor thread - which whole operation 
> is a barrier to begin with.

I'm wondering why we need a barrier here at all. In this brlock
implementation the readers are waiting on a mutex in their main loop -
right before a call to KVM_RUN. They can't get anywhere near a
br_read_lock() once br_write_lock() has completed.

> > +#define br_write_lock()		kvm__pause()
> > +#define br_write_unlock()	kvm__continue()
> > +#endif
> 
> Btw., it might make sense to add a comment to this header file, 
> explaining what a 'big reader lock' is :-)

I'll put the commit message into the header, should be enough?

-- 

Sasha.