Re: [PATCH 2/2] docs: memory-barriers: Add note on plain-accesses to address-dependency barriers

[Paul E. McKenney]

On Thu, Aug 03, 2023 at 03:24:07AM +0000, Joel Fernandes (Google) 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 and point to rcu-dereference.rst
> for more information.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---
>  Documentation/memory-barriers.txt | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index 06e14efd8662..acc8ec5ce563 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -435,6 +435,11 @@ Memory barriers come in four basic varieties:
>       variables such as READ_ONCE() and rcu_dereference() provide implicit
>       address-dependency barriers.
>  
> +     [!] Note that address dependency barriers can be destroyed by comparison
> +     of a pointer obtained by a marked accessor such as READ_ONCE() or
> +     rcu_dereference() with some value.  For an example of this, see
> +     rcu_dereference.rst (part where the comparison of pointers is discussed).

Hmmm...

Given that this is in a section marked "historical" (for the old
smp_read_barrier_depends() API), why not instead add a pointer to
Documentation/RCU/rcu_dereference.rst to the beginning of the section,
noted as the updated material?

							Thanx, Paul

> +
>   (3) Read (or load) memory barriers.
>  
>       A read barrier is an address-dependency barrier plus a guarantee that all
> -- 
> 2.41.0.585.gd2178a4bd4-goog
>