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

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Date:	Thu, 12 Feb 2009 12:13:29 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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, 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.

		Linus
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