Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost)

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Sat, Feb 14, 2009 at 12:50:43AM +1100, Nick Piggin wrote:
> > On Friday 13 February 2009 08:59:59 Paul E. McKenney wrote:
> > > On Thu, Feb 12, 2009 at 01:15:08PM -0800, Linus Torvalds wrote:
> > > > On Thu, 12 Feb 2009, Paul E. McKenney wrote:
> > > > > In other words, you are arguing for using ACCESS_ONCE() in the loops,
> > > > > but keeping the old ACCESS_ONCE() definition, and declaring BF hardware
> > > > > broken?
> > > >
> > > > Well, I _also_ argue that if you have a busy loop, you'd better have a
> > > > cpu_relax() in there somewhere anyway. If you don't, you have a bug.
> > > >
> > > > So I think the BF approach is "borderline broken", but I think it should
> > > > work, if BF just has whatever appropriate cache flush in its cpu_relax.
> > >
> > > OK, got it.  Keep ACCESS_ONCE() as is, make sure any busy-wait
> > > loops contain a cpu_relax().  A given busy loop might or might not
> > > need ACCESS_ONCE(), but that decision is independent of hardware
> > > considerations.
> > >
> > > Ah, and blackfin's cpu_relax() does seem to have migrated from barrier()
> > > to smp_mb() recently, so sounds good to me!!!
> > 
> > 
> > Interesting. I don't know if you would say it is not cache coherent.
> > Does anything in cache coherency definition require timeliness? Only
> > causality I think.
> > 
> > However I think "infinite write buffering delay", or requiring "cache
> > barriers" is insane to teach any generic code about. BF would be free
> > to optimise arch functions, but for correctness surely it must also
> > have a periodic interrupt that will expose stores to other CPUs.
> 
> I have great sympathy for this point of view!!!  So why not have the
> blackfin folks get the appropriate instructions added in the gcc port
> to their architecture?  (Yeah, I know, gcc has no way of knowing which
> variables are shared and not...)
> 
> But perhaps we could decorate the affected variable declarations with
> a macro that expands to some sort of gcc attribute in the blackfin case?
> 

I think that just for the fact that it help identifying such variable
accesses which are :

- performed atomically
- unprotected by any form of locking

This seems like a good things to wrap such accesses into a macro which
permits easy identification of those sites. A bit like rcu_dereference()
does. Gradual use of this new macro could come incrementally too.

Mathieu


> 							Thanx, Paul
> 
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-- 
Mathieu Desnoyers
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