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

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Message-ID: <20090212203924.GK6759@linux.vnet.ibm.com>
Date:	Thu, 12 Feb 2009 12:39:24 -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 12:13:29PM -0800, Linus Torvalds wrote:
> 
> 
> On Thu, 12 Feb 2009, Paul E. McKenney wrote:
> > 
> > Actually the best way to do this would be:
> > 
> > 	while (ACCESS_ONCE(sig_done) < 1)
> > 		continue;
> > 
> > If ACCESS_ONCE() needs to be made architecture-specific to make this
> > really work on Blackfin, we should make that change.
> 
> I really wouldn't want to mix up compiler barriers and cache barriers this 
> way. 
> 
> I think "cpu_relax()" is likely the right thing to piggy-back on for 
> broken cache-coherency.
> 
> > And, now that you mention it, I have heard rumors that other CPU 
> > families can violate cache coherence in some circumstances.
> 
> I personally suspect that the BF pseudo-SMP code is just broken, and that 
> it likely has tons of subtle bugs and races - because we _do_ depend on 
> cache coherency at least for accessing objects next to each other. I just 
> never personally felt like I had the energy to care deeply enough.
> 
> But I draw the line at making ACCESS_ONCE() imply anything but a compiler 
> optimization issue.

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?

I am OK with that, just wanting to make sure I understand what you are
asking us to do.

							Thanx, Paul
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