Re: [PATCH v2] tools/memory-model: Clarify LKMM's limitations in litmus-tests.txt

[Paul Heidekrüger]

> On 8. Jul 2022, at 20:47, Paul E. McKenney <paulmck@xxxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxx/T/#u
>>>> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@xxxxxxxxx>
>>>> Co-developed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
>>> 
>>> Reviewed-by: Marco Elver <elver@xxxxxxxxxx>
>>> 
>>> However with the Co-developed-by, this is missing Alan's SOB.
>> 
>> For the record:
>> 
>> Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
>> 
>> (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@xxxxxxxxx>
> 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@xxxxxxxxxxxxxxxxxxxxxxxx/T/#u
> Co-developed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@xxxxxxxxx>
> Reviewed-by: Marco Elver <elver@xxxxxxxxxx>
> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Cc: Charalampos Mainas <charalampos.mainas@xxxxxxxxx>
> Cc: Pramod Bhatotia <pramod.bhatotia@xxxxxxxxx>
> Cc: Soham Chakraborty <s.s.chakraborty@xxxxxxxxxx>
> Cc: Martin Fink <martin.fink@xxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> 
> 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.