Re: [PATCH] tools/memory-model: Weaken ctrl dependency definition in explanation.txt

[Joel Fernandes]

On 8/30/2022 4:44 PM, Paul Heidekrüger wrote:
> The current informal control dependency definition in explanation.txt is
> too broad and, as dicsussed, needs to be updated.
> 
> Consider the following example:
> 
>> if(READ_ONCE(x))
>> 	return 42;
>>
>> 	WRITE_ONCE(y, 42);
>>
>> 	return 21;
> 
> The read event determines whether the write event will be executed "at
> all" - as per the current definition - but the formal LKMM does not
> recognize this as a control dependency.
> 
> Introduce a new defintion which includes the requirement for the second
> memory access event to syntactically lie within the arm of a non-loop
> conditional.
> 
> Link: https://lore.kernel.org/all/20220615114330.2573952-1-paul.heidekrueger@xxxxxxxxx/
> Cc: Marco Elver <elver@xxxxxxxxxx>
> 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 Heidekrüger <paul.heidekrueger@xxxxxxxxx>
> Co-developed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> ---
> 
> @Alan:
> 
> Since I got it wrong the last time, I'm adding you as a co-developer after my
> SOB. I'm sorry if this creates extra work on your side due to you having to
> resubmit the patch now with your SOB if I understand correctly, but since it's
> based on your wording from the other thread, I definitely wanted to give you
> credit.
> 
>  tools/memory-model/Documentation/explanation.txt | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
> index ee819a402b69..0bca50cac5f4 100644
> --- a/tools/memory-model/Documentation/explanation.txt
> +++ b/tools/memory-model/Documentation/explanation.txt
> @@ -464,9 +464,10 @@ to address dependencies, since the address of a location accessed
>  through a pointer will depend on the value read earlier from that
>  pointer.
> 
> -Finally, a read event and another memory access event are linked by a
> -control dependency if the value obtained by the read affects whether
> -the second event is executed at all.  Simple example:
> +Finally, a read event X and another memory access event Y are linked by
> +a control dependency if Y syntactically lies within an arm of an if,
> +else or switch statement and the condition guarding Y is either data or
> +address-dependent on X.  Simple example:

'conditioning guarding Y' sounds confusing to me as it implies to me that the
condition's evaluation depends on Y. I much prefer Alan's wording from the
linked post saying something like 'the branch condition is data or address
dependent on X, and Y lies in one of the arms'.

I have to ask though, why doesn't this imply that the second instruction never
executes at all? I believe that would break the MP-pattern if it were not true.

cheers,

 - Joel