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

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Date:   Thu, 3 Aug 2023 11:52:06 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH 2/2] docs: memory-barriers: Add note on plain-accesses to
 address-dependency barriers

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@...lfernandes.org>
> ---
>  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
>