Re: [PATCH memory-model] docs: memory-barriers: Add note on compiler transformation and address deps

[Andrea Parri]

On Thu, Oct 05, 2023 at 09:53:12AM -0700, Paul E. McKenney wrote:
> The compiler has the ability to cause misordering by destroying
> address-dependency barriers if comparison operations are used. Add a
> note about this to memory-barriers.txt in the beginning of both the
> historical address-dependency sections and point to rcu-dereference.rst
> for more information.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

Reviewed-by: Andrea Parri <parri.andrea@xxxxxxxxx>

Thanks,
  Andrea


> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index 06e14efd8662..d414e145f912 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -396,6 +396,10 @@ Memory barriers come in four basic varieties:
>  
>  
>   (2) Address-dependency barriers (historical).
> +     [!] This section is marked as HISTORICAL: For more up-to-date
> +     information, including how compiler transformations related to pointer
> +     comparisons can sometimes cause problems, see
> +     Documentation/RCU/rcu_dereference.rst.
>  
>       An address-dependency barrier is a weaker form of read barrier.  In the
>       case where two loads are performed such that the second depends on the
> @@ -556,6 +560,9 @@ There are certain things that the Linux kernel memory barriers do not guarantee:
>  
>  ADDRESS-DEPENDENCY BARRIERS (HISTORICAL)
>  ----------------------------------------
> +[!] This section is marked as HISTORICAL: For more up-to-date information,
> +including how compiler transformations related to pointer comparisons can
> +sometimes cause problems, see Documentation/RCU/rcu_dereference.rst.
>  
>  As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for
>  DEC Alpha, which means that about the only people who need to pay attention