Re: [PATCH 4/6] kvm tools: Add rwlock wrapper

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sun, May 29, 2011 at 08:31:30AM -0700, Paul E. McKenney wrote:
> On Sun, May 29, 2011 at 09:19:48AM +0200, Ingo Molnar wrote:
> > 
> > * Avi Kivity <avi@redhat.com> wrote:
> > 
> > > Yes, this is equivalent to the kernel's stop_machine_run().  It's a 
> > > heavyweight method but it should work just fine.
> > 
> > Yeah. It is fine for reconfiguration/configuration-only kind of write 
> > paths - i.e. the mmio lookup path should be ok.
> > 
> > There's only one thing i'd like to warn about: please still shape it 
> > as a br_read_lock()/unlock(), br_write_lock()/unlock() operation.
> > 
> > This way all the SMP read paths remain identified properly, even if 
> > br_write_lock() does a stop_machine_run() equivalent. This still 
> > leaves options open for an easy conversion to rwlock or urcu (should 
> > any particular write path become more than just occasional).
> 
> This is very important even if no write path ever becomes more than
> just occasional.  If you don't mark the read paths like Ingo suggests,
> your one-year-from-now self will be very annoyed at you, as the code
> will be quite difficult to understand.  And anyone else trying to
> read your code will be even more annoyed.  ;-)

And the other reason that you want to mark the readers is for debug
purposes.  Murphy being who he is, you will some day need to check
for someone calling the "OK to update" function while they are acting
as a reader.

							Thanx, Paul