Re: [PATCH 1/2] Documentation/memory-barriers.txt: cross-reference "tools/memory-model/"

[Paul E. McKenney]

On Sun, Feb 04, 2018 at 07:37:08PM +0100, Andrea Parri wrote:
> Hi Akira,
> 
> On Mon, Feb 05, 2018 at 01:14:10AM +0900, Akira Yokosawa wrote:
> > Hi Paul,
> > CC: Andrea
> > 
> > This is intentionally off the list, as I was not cc'd in the thread.
> > If you think it is worthwhile, could you help me join the thread by
> > forwarding the following part as a reply to your message, plus CC: to me.
> 
> [CCing lists and other people]
> 
> 
> > 
> > On Fri, Feb 02, 2018 at 17:21:03AM -0800, Paul E. McKenney wrote:
> > > On Fri, Feb 02, 2018 at 10:12:48AM +0100, Andrea Parri wrote:
> > >> Recent efforts led to the specification of a memory consistency model
> > >> for the Linux kernel [1], which "can (roughly speaking) be thought of
> > >> as an automated version of memory-barriers.txt" and which is (in turn)
> > >> "accompanied by extensive documentation on its use and its design".
> > >> 
> > >> Make sure that the (occasional) reader of memory-barriers.txt will be
> > >> aware of these developments.
> > >> 
> > >> [1] https://marc.info/?l=linux-kernel&m=151687290114799&w=2
> > >> 
> > >> Signed-off-by: Andrea Parri <parri.andrea@xxxxxxxxx>
> > > 
> > > I am inclined to pull in something along these lines, but would like
> > > some feedback on the wording, especially how "official" we want to
> > > make the memory model to be.
> > > 
> > > Thoughts?
> > 
> > The change log of commit e7720af5f9ac ("locking/Documentation: Add disclaimer") says:
> >     
> >     It appears people are reading this document as a requirements list for
> >     building hardware. This is not the intent of this document. Nor is it
> >     particularly suited for this purpose.
> >     
> >     The primary purpose of this document is our collective attempt to define
> >     a set of primitives that (hopefully) allow us to write correct code on
> >     the myriad of SMP platforms Linux supports.
> >     
> >     Its a definite work in progress as our understanding of these platforms,
> >     and memory ordering in general, progresses.
> >     
> >     Nor does being mentioned in this document mean we think its a
> >     particularly good idea; the data dependency barrier required by Alpha
> >     being a prime example. Yes we have it, no you're insane to require it
> >     when building new hardware.
> > 
> > My take on the Linux Kernel memory-consistency model is a supplement of
> > memory-barriers.txt and the disclaimer also applies to the memory model.
> > 
> > > 
> > > If I don't hear otherwise in a couple of days, I will pull this as is.
> > > 
> > > 							Thanx, Paul
> > > 
> > >> ---
> > >>  Documentation/memory-barriers.txt | 4 +++-
> > >>  1 file changed, 3 insertions(+), 1 deletion(-)
> > >> 
> > >> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> > >> index a863009849a3b..8cc3f098f4a7d 100644
> > >> --- a/Documentation/memory-barriers.txt
> > >> +++ b/Documentation/memory-barriers.txt
> > >> @@ -17,7 +17,9 @@ meant as a guide to using the various memory barriers provided by Linux, but
> > >>  in case of any doubt (and there are many) please ask.
> > >> 
> > >>  To repeat, this document is not a specification of what Linux expects from
> > >> -hardware.
> > >> +hardware.  For such a specification, in the form of a memory consistency
> > >> +model, and for documentation about its usage and its design, the reader is
> > >> +referred to "tools/memory-model/".
> > >> 
> > 
> > Adding cross-reference in this way can _weaken_ the message of the disclaimer.
> 
> Thank you for your remarks; I do share the same concern.
> 
> > What about adding it in the previous sentence as the patch appended bellow?
> 
> I do like this idea: I believe that my phrasing (and that "what Linux
> expects from hardware") may be easily subject to misinterpretation...
> which your solution can avoid.

Any objections to Akira's patch below?  (Give or take the usual
wordsmithing.)

Andrea, should I interpret your paragraph above ask an Acked-by?

							Thanx, Paul

>   Andrea
> 
> 
> > 
> > The tag use in the change log may need adjustments. I'm not familiar with the
> > manner in modifying other persons' patches. Of course, the wording itself can
> > be improved further.  Any feedback is welcome.
> > 
> >      Thanks, Akira
> > 
> > >>  The purpose of this document is twofold:
> > >> 
> > >> -- 
> > >> 2.7.4
> > >> 
> > 
> > ----8<-------
> > From 714e8c4b09acd6e965de116532dce05070b9e636 Mon Sep 17 00:00:00 2001
> > From: Akira Yokosawa <akiyks@xxxxxxxxx>
> > Date: Mon, 5 Feb 2018 00:28:36 +0900
> > Subject: [PATCH] Documentation/memory-barriers.txt: cross-reference "tools/memory-model/"
> > 
> > Recent efforts led to the specification of a memory consistency model
> > for the Linux kernel [1], which "can (roughly speaking) be thought of
> > as an automated version of memory-barriers.txt" and which is (in turn)
> > "accompanied by extensive documentation on its use and its design".
> > 
> > Make sure that the (occasional) reader of memory-barriers.txt will be
> > aware of these developments.
> > 
> > [1] https://marc.info/?l=linux-kernel&m=151687290114799&w=2
> > 
> > Signed-off-by: Andrea Parri <parri.andrea@xxxxxxxxx>
> > Signed-off-by: Akira Yokosawa <akiyks@xxxxxxxxx>
> > ---
> >  Documentation/memory-barriers.txt | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> > index 479ecec..975488d 100644
> > --- a/Documentation/memory-barriers.txt
> > +++ b/Documentation/memory-barriers.txt
> > @@ -14,7 +14,9 @@ DISCLAIMER
> >  This document is not a specification; it is intentionally (for the sake of
> >  brevity) and unintentionally (due to being human) incomplete. This document is
> >  meant as a guide to using the various memory barriers provided by Linux, but
> > -in case of any doubt (and there are many) please ask.
> > +in case of any doubt (and there are many) please ask. For clarification of such
> > +doubt, in the form of a memory consistency model, and for documentation about
> > +its usage and its design, the reader is referred to "tools/memory-model/".
> >  
> >  To repeat, this document is not a specification of what Linux expects from
> >  hardware.
> > -- 
> > 2.7.4
> > 
> > 
> > 
>