Message-ID: <20180209150053.GA24203@andrea>
Date:   Fri, 9 Feb 2018 16:00:53 +0100
From:   Andrea Parri <parri.andrea@...il.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Akira Yokosawa <akiyks@...il.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, stern@...land.harvard.edu,
        will.deacon@....com, peterz@...radead.org, boqun.feng@...il.com,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, corbet@....net
Subject: Re: [PATCH 1/2] Documentation/memory-barriers.txt: cross-reference
 "tools/memory-model/"

On Fri, Feb 09, 2018 at 06:29:23AM -0800, Paul E. McKenney wrote:
> On Fri, Feb 09, 2018 at 01:50:51PM +0100, Andrea Parri wrote:
> > On Fri, Feb 09, 2018 at 04:31:00AM -0800, Paul E. McKenney wrote:
> > > 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@...xxxxxx>
> > > > > > 
> > > > > > 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?
> > 
> > Well, I am among the Signed-off-by: of the patch; it didn't seem too fair
> > to me to Ack my own patch... ;-) Is the wording sound? other suggestions?
> 
> Good point, too many all-day meetings last week.  ;-)
> 
> How about the following?

Even better IMO,

Thanks!

  Andrea


> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 9370f98c312d658afe88e548d469549d8f31e402
> Author: Andrea Parri <parri.andrea@...il.com>
> Date:   Fri Feb 9 06:26:08 2018 -0800
> 
>     Documentation/memory-barriers.txt: Cross-reference "tools/memory-model/"
>     
>     A memory consistency model is now available 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".
>     
>     Inform the (occasional) reader of memory-barriers.txt of these
>     developments.
>     
>     [1] https://marc.info/?l=linux-kernel&m=151687290114799&w=2
>     
>     Co-developed-by: Andrea Parri <parri.andrea@...il.com>
>     Co-developed-by: Akira Yokosawa <akiyks@...il.com>
>     Signed-off-by: Andrea Parri <parri.andrea@...il.com>
>     Signed-off-by: Akira Yokosawa <akiyks@...il.com>
>     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index 479ecec80593..74ad222d11ed 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -14,7 +14,11 @@ 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.  Some doubts may be
> +resolved by referring to the formal memory consistency model and related
> +documentation at tools/memory-model/.  Nevertheless, even this memory
> +model should be viewed as the collective opinion of its maintainers rather
> +than as an infallible oracle.
>  
>  To repeat, this document is not a specification of what Linux expects from
>  hardware.
> 

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