linux-kernel - Re: [RFC][PATCH 0/5] arch: atomic rework

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Message-ID: <20140210150443.GJ4250@linux.vnet.ibm.com>
Date:	Mon, 10 Feb 2014 07:04:43 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Torvald Riegel <triegel@...hat.com>,
	Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
	David Howells <dhowells@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"gcc@....gnu.org" <gcc@....gnu.org>
Subject: Re: [RFC][PATCH 0/5] arch: atomic rework

On Mon, Feb 10, 2014 at 11:49:29AM +0000, Will Deacon wrote:
> On Mon, Feb 10, 2014 at 11:48:13AM +0000, Peter Zijlstra wrote:
> > On Fri, Feb 07, 2014 at 10:02:16AM -0800, Paul E. McKenney wrote:
> > > As near as I can tell, compiler writers hate the idea of prohibiting
> > > speculative-store optimizations because it requires them to introduce
> > > both control and data dependency tracking into their compilers.  Many of
> > > them seem to hate dependency tracking with a purple passion.  At least,
> > > such a hatred would go a long way towards explaining the incomplete
> > > and high-overhead implementations of memory_order_consume, the long
> > > and successful use of idioms based on the memory_order_consume pattern
> > > notwithstanding [*].  ;-)
> > 
> > Just tell them that because the hardware provides control dependencies
> > we actually use and rely on them.
> 
> s/control/address/ ?

Both are important, but as Peter's reply noted, it was control
dependencies under discussion.  Data dependencies (which include the
ARM/PowerPC notion of address dependencies) are called out by the standard
already, but control dependencies are not.  I am not all that satisified
by current implementations of data dependencies, admittedly.  Should
be an interesting discussion.  ;-)

							Thanx, Paul

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