Message-Id: <EE1854E3-C33D-4A4E-AC31-4194A701052B@in.tum.de>
Date:   Mon, 11 Jul 2022 17:14:55 +0200
From:   Paul Heidekrüger <Paul.Heidekrueger@...tum.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Marco Elver <elver@...gle.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Charalampos Mainas <charalampos.mainas@...il.com>,
        Pramod Bhatotia <pramod.bhatotia@...tum.de>,
        Soham Chakraborty <s.s.chakraborty@...elft.nl>,
        Martin Fink <martin.fink@...tum.de>
Subject: Re: [PATCH v2] tools/memory-model: Clarify LKMM's limitations in
 litmus-tests.txt

> On 8. Jul 2022, at 20:47, Paul E. McKenney <paulmck@...nel.org> wrote:
> 
> On Fri, Jul 08, 2022 at 10:45:06AM -0400, Alan Stern wrote:
>> On Fri, Jul 08, 2022 at 01:44:06PM +0200, Marco Elver wrote:
>>> On Tue, 14 Jun 2022 at 17:49, Paul Heidekrüger
>>> <paul.heidekrueger@...tum.de> wrote:
>>>> As discussed, clarify LKMM not recognizing certain kinds of orderings.
>>>> In particular, highlight the fact that LKMM might deliberately make
>>>> weaker guarantees than compilers and architectures.
>>>> 
>>>> Link: https://lore.kernel.org/all/YpoW1deb%2FQeeszO1@ethstick13.dse.in.tum.de/T/#u
>>>> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@...tum.de>
>>>> Co-developed-by: Alan Stern <stern@...land.harvard.edu>
>>> 
>>> Reviewed-by: Marco Elver <elver@...gle.com>
>>> 
>>> However with the Co-developed-by, this is missing Alan's SOB.
>> 
>> For the record:
>> 
>> Signed-off-by: Alan Stern <stern@...land.harvard.edu>
>> 
>> (Note that according to Documentation/process/submitting-patches.rst, 
>> the submitting author's SOB is supposed to come last.)
> 
> And this is what I ended up with. Please provide additional feedback
> as needed, and in the meantime, thank you all!
> 
> 							Thanx, Paul

Looks great - my first commit in the Linux kernel!

Thanks everyone!

Paul

> ------------------------------------------------------------------------
> 
> commit 3c7753e959706f39e1ee183ef8dcde3b4cfbb4c7
> Author: Paul Heidekrüger <paul.heidekrueger@...tum.de>
> Date: Tue Jun 14 15:48:11 2022 +0000
> 
> tools/memory-model: Clarify LKMM's limitations in litmus-tests.txt
> 
> As discussed, clarify LKMM not recognizing certain kinds of orderings.
> In particular, highlight the fact that LKMM might deliberately make
> weaker guarantees than compilers and architectures.
> 
> Link: https://lore.kernel.org/all/YpoW1deb%2FQeeszO1@ethstick13.dse.in.tum.de/T/#u
> Co-developed-by: Alan Stern <stern@...land.harvard.edu>
> Signed-off-by: Alan Stern <stern@...land.harvard.edu>
> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@...tum.de>
> Reviewed-by: Marco Elver <elver@...gle.com>
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> Cc: Charalampos Mainas <charalampos.mainas@...il.com>
> Cc: Pramod Bhatotia <pramod.bhatotia@...tum.de>
> Cc: Soham Chakraborty <s.s.chakraborty@...elft.nl>
> Cc: Martin Fink <martin.fink@...tum.de>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> 
> diff --git a/tools/memory-model/Documentation/litmus-tests.txt b/tools/memory-model/Documentation/litmus-tests.txt
> index 8a9d5d2787f9e..cc355999815cb 100644
> --- a/tools/memory-model/Documentation/litmus-tests.txt
> +++ b/tools/memory-model/Documentation/litmus-tests.txt
> @@ -946,22 +946,39 @@ Limitations of the Linux-kernel memory model (LKMM) include:
> 	carrying a dependency, then the compiler can break that dependency
> 	by substituting a constant of that value.
> 
> -	Conversely, LKMM sometimes doesn't recognize that a particular
> -	optimization is not allowed, and as a result, thinks that a
> -	dependency is not present (because the optimization would break it).
> -	The memory model misses some pretty obvious control dependencies
> -	because of this limitation. A simple example is:
> +	Conversely, LKMM will sometimes overestimate the amount of
> +	reordering compilers and CPUs can carry out, leading it to miss
> +	some pretty obvious cases of ordering. A simple example is:
> 
> 		r1 = READ_ONCE(x);
> 		if (r1 == 0)
> 			smp_mb();
> 		WRITE_ONCE(y, 1);
> 
> -	There is a control dependency from the READ_ONCE to the WRITE_ONCE,
> -	even when r1 is nonzero, but LKMM doesn't realize this and thinks
> -	that the write may execute before the read if r1 != 0. (Yes, that
> -	doesn't make sense if you think about it, but the memory model's
> -	intelligence is limited.)
> +	The WRITE_ONCE() does not depend on the READ_ONCE(), and as a
> +	result, LKMM does not claim ordering. However, even though no
> +	dependency is present, the WRITE_ONCE() will not be executed before
> +	the READ_ONCE(). There are two reasons for this:
> +
> + The presence of the smp_mb() in one of the branches
> + prevents the compiler from moving the WRITE_ONCE()
> + up before the "if" statement, since the compiler has
> + to assume that r1 will sometimes be 0 (but see the
> + comment below);
> +
> + CPUs do not execute stores before po-earlier conditional
> + branches, even in cases where the store occurs after the
> + two arms of the branch have recombined.
> +
> +	It is clear that it is not dangerous in the slightest for LKMM to
> +	make weaker guarantees than architectures. In fact, it is
> +	desirable, as it gives compilers room for making optimizations. 
> +	For instance, suppose that a 0 value in r1 would trigger undefined
> +	behavior elsewhere. Then a clever compiler might deduce that r1
> +	can never be 0 in the if condition. As a result, said clever
> +	compiler might deem it safe to optimize away the smp_mb(),
> +	eliminating the branch and any ordering an architecture would
> +	guarantee otherwise.
> 
> 2.	Multiple access sizes for a single variable are not supported,
> 	and neither are misaligned or partially overlapping accesses.

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