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

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Message-ID: <20090212215959.GN6759@linux.vnet.ibm.com>
Date:	Thu, 12 Feb 2009 13:59:59 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	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 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!!!

							Thanx, Paul
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