linux-kernel - Re: Proposal for new memory_order

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Message-ID: <20160119222512.GA30041@linux.vnet.ibm.com>
Date:	Tue, 19 Jan 2016 14:25:12 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	c++std-parallel@...u.org
Cc:	linux-kernel@...r.kernel.org, triegel@...hat.com,
	jeff@...shing.com, boehm@....org, clark.nelson@...el.com,
	OGiroux@...dia.com, Lawrence@...wl.org, dhowells@...hat.com,
	joseph@...esourcery.com, torvalds@...ux-foundation.org,
	Mark.Batty@...cam.ac.uk, Peter.Sewell@...cam.ac.uk,
	peterz@...radead.org, will.deacon@....com,
	behanw@...verseincode.com, jfb@...gle.com, Jens.Maurer@....net,
	michaelw@...ibm.com
Subject: Re: Proposal for new memory_order_consume definition

On Mon, Jan 11, 2016 at 03:11:42PM -0800, Paul E. McKenney wrote:
> Hello!
> 
> As requested at the October 2015 C++ Standards Committee Meeting, I have
> created a single proposal for memory_order_consume in C++:
> 
> http://www2.rdrop.com/users/paulmck/submission/consume.2016.01.11b.pdf
> 
> This contains an informal description of the proposal, rough-draft
> wording changes, and a number of litmus tests demonstrating how the
> proposal works.
> 
> The required changes to compilers appears to be extremely small,
> however, I would like to get more compiler writers' thoughts on the
> pointer_cmp_eq_dep(), pointer_cmp_ne_dep(), pointer_cmp_gt_dep(),
> pointer_cmp_ge_dep(), pointer_cmp_lt_dep(), and pointer_cmp_le_dep()
> intrinsics that do pointer comparisons without breaking dependencies on
> their first argument.  Figures 25 and 26 on page 16 demonstrate their use.
> These intrinsics were suggested at the October meeting, but it would be
> good to get wider feedback on them.
> 
> Note that last I checked, the Linux kernel actually does not depend
> on pointer comparisons not breaking dependency chains, because all
> comparisons are against NULL or a list-head structure, in which case
> the pointer is not going to be dereferenced after an equals comparison.
> But I do believe that some past versions of the Linux kernel have depended
> on this.

And an update based on considerable off-list feedback.

More thoughts?

 							Thanx, Paul

> PS.  For more background information, please see:
> 
> 	http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0098r0.pdf

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