linux-kernel - Re: [PATCH 3/7] barrier: a scalable synchonisation barrier

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Message-ID: <20070204054603.GI5647@linux.vnet.ibm.com>
Date:	Sat, 3 Feb 2007 21:46:03 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Oleg Nesterov <oleg@...sign.ru>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/7] barrier: a scalable synchonisation barrier

On Sat, Feb 03, 2007 at 10:24:04PM -0500, Alan Stern wrote:
> On Sat, 3 Feb 2007, Paul E. McKenney wrote:
> 
> > > And another note: this all assumes that STORE-MB-LOAD works "correctly", yes?
> > > We have other code which relies on that, should not be a problem.
> > 
> > We have been working with Doug Lea of SUNY Oswego, Sebatian Burckhardt of
> > University of Pennsylvania, and Vijay Saraswat of IBM Research towards
> > a "universal memory model" that accommodates all machines.  Currently,
> > it does in fact handle store-mb-load the way we all want, thankfully!
> 
> We should add that many places in the kernel do depend on proper behavior 
> for this data access pattern.  So whatever "universal memory model" we end 
> up with, it had better handle the pattern correctly if Linux is to support 
> it.

Agreed!

> It's interesting to note, however, that this does exclude simple MESI.

Yep!  And also a number of compiler optimizations, as it turns out.  ;-)

There is a tension between nice-to-software memory-barrier properties
on the one hand and easily understood code on the other.  But I guess
that this is true of pretty much any software tool.

							Thanx, Paul
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