[lttng-dev] Quick questions about liburcu and RCU in general

[Paul E. McKenney]

On Tue, May 07, 2013 at 07:59:14AM -0400, Mathieu Desnoyers wrote:
> Hi Richard,
> 
> * Richard Braun (rbraun at sceen.net) wrote:
> > Hello,
> > 
> > I'm currently studying RCU/URCU, and I have a few questions that I wasn't
> > sure where to ask.
> > 
> > 1/ Why use poll instead of sched_yield in e.g. force_mb_all_readers ?
> > (I guess it's about portability and the effect is expected to be the same,
> > but is there another reason ?)
> 
> poll() allow us to do a millisecond-level wait (timer-based).
> sched_yield() is pretty much a scheduler hack that just says "be nice to
> other scheduled processes here". Quoting sched_yield(2):
> 
>        If the calling thread is the only thread in the highest  priority list
>        at that time, it will continue to run after a call to sched_yield().
> 
> This is a kind of behavior we don't want.
> 
> > 2/ What was the conclusion of the discussion regarding sys_membarrier ?
> > (I couldn't find it in the main mail thread, and it looks quite interesting,
> > even though I expect most carefully written applications not to exceed
> > one thread per processor too much)
> 
> The conclusion so far was that:
> 
> - the implementation was fine,
> - we had to show there were enough users of this new ABI to justify its
>   inclusion and maintenance cost. Currently, it's pretty much just
>   liburcu that uses it. As the number of liburcu users grows, this
>   helps, but what would help even more would be to have other libraries
>   and applications using sys_membarrier() (other people interested and
>   weighting in).
> 
> > 
> > 3/ Do you know if IBM allows the use of patented RCU techniques in GPLv3+
> > code as well ? ("GPL" is mentioned in the Linux documentation, and
> > apparently liburcu is covered by LGPLv2+ so I expect that to be the case,
> > just looking for a confirmation)
> 
> AFAIK, LGPLv2+ code can migrate into GPLv3+, it's the other way around
> that is not permitted. Therefore, I would expect it is allowed, but I
> will let Paul answer to this one on behalf of IBM.

Yes, LGPLv2+ allows you to migrate to LGPLv3+, which is compatible with
GPLv3+.  So as long as you derive the code from lttng's userspace-rcu
library, you are set.

That said, you could also just link userspace-rcu into your GPLv3+
application.

							Thanx, Paul

> Thanks,
> 
> Mathieu
> 
> 
> > 
> > Thanks for your answers.
> > 
> > -- 
> > Richard Braun
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>