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

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, Jun 14, 2022 at 03:48:11PM +0000, Paul Heidekrüger 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@in.tum.de>
> Co-developed-by: Alan Stern <stern@rowland.harvard.edu>
> Cc: Marco Elver <elver@google.com>
> Cc: Charalampos Mainas <charalampos.mainas@gmail.com>
> Cc: Pramod Bhatotia <pramod.bhatotia@in.tum.de>
> Cc: Soham Chakraborty <s.s.chakraborty@tudelft.nl>
> Cc: Martin Fink <martin.fink@in.tum.de>
> ---

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

thanks,

 - Joel


> v2:
> - Incorporate Alan Stern's feedback.
> - Add suggested text by Alan Stern to clearly state how the branch and the
>   smp_mb() affect ordering.
> - Add "Co-developed-by: Alan Stern <stern@rowland.harvard.edu>" based on the
>   above.
> 
>  .../Documentation/litmus-tests.txt            | 37 ++++++++++++++-----
>  1 file changed, 27 insertions(+), 10 deletions(-)
> 
> diff --git a/tools/memory-model/Documentation/litmus-tests.txt b/tools/memory-model/Documentation/litmus-tests.txt
> index 8a9d5d2787f9..cc355999815c 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.
> -- 
> 2.35.1
>