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

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Message-ID: <20140210162255.GC12826@mudshark.cambridge.arm.com>
Date:	Mon, 10 Feb 2014 16:22:55 +0000
From:	Will Deacon <will.deacon@....com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 03:04:43PM +0000, Paul E. McKenney wrote:
> 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.  ;-)

Ok, but since you can't use control dependencies to order LOAD -> LOAD, it's
a pretty big ask of the compiler to make use of them for things like
consume, where a data dependency will suffice for any combination of
accesses.

Will
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