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

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Message-ID: <20090213145653.GA6854@linux.vnet.ibm.com>
Date:	Fri, 13 Feb 2009 06:56:54 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	ltt-dev@...ts.casi.polymtl.ca, linux-kernel@...r.kernel.org,
	Bryan Wu <cooloney@...nel.org>,
	uclinux-dist-devel@...ckfin.uclinux.org
Subject: Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux
	(repost)

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?

							Thanx, Paul
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