Re: [PATCH] doc: memory-barriers.txt: Minor correction in Control dependencies

Paul E. McKenney
2014-08-13T22:58:03Z

On Mon, Aug 11, 2014 at 06:51:39PM -0400, Pranith Kumar wrote:
> Minor corrections in memory-barriers.txt document.
>
> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
> ---
> Documentation/memory-barriers.txt | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index abec3f9..41a6c9c 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -627,7 +627,7 @@ proving the value of 'a', and the pair of barrier() invocations are
> required to prevent the compiler from pulling the two identical stores
> to 'b' out from the legs of the "if" statement.
>
> -It is important to note that control dependencies absolutely require a
> +It is important to note that control dependencies absolutely require
> a conditional. For example, the following "optimized" version of
> the above example breaks ordering, which is why the barrier() invocations
> are absolutely required if you have identical stores in both legs of
> @@ -652,7 +652,7 @@ for example, as follows:
> do_something();
> } else {
> barrier();
> - ACCESS_ONCE(b) = q / 3;
> + ACCESS_ONCE(b) = q / 2;
> do_something_else();
> }

This one is gone due to the two-legged "if" statements being reworked.

> @@ -710,7 +710,7 @@ x and y both being zero:
>
> The above two-CPU example will never trigger the assert(). However,
> if control dependencies guaranteed transitivity (which they do not),
> -then adding the following two CPUs would guarantee a related assertion:
> +then adding the following CPU would guarantee a related assertion:
>
> CPU 2
> =====================
> @@ -719,8 +719,8 @@ then adding the following two CPUs would guarantee a related assertion:
> assert(!(r1 == 2 && r2 == 1 && x == 2)); /* FAILS!!! */
>
> But because control dependencies do -not- provide transitivity, the
> -above assertion can fail after the combined four-CPU example completes.
> -If you need the four-CPU example to provide ordering, you will need
> +above assertion can fail after the combined three-CPU example completes.
> +If you need the three-CPU example to provide ordering, you will need
> smp_mb() between the loads and stores in the CPU 0 and CPU 1 code fragments,
> that is, just before or just after the "if" statements.

Nikolay Samofatov beat you to this one by a couple of weeks.

Nevertheless, thank you for your careful review!

Thanx, Paul