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

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Date:	Wed, 13 Aug 2014 15:49:43 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	David Howells <dhowells@...hat.com>,
	Masanari Iida <standby24x7@...il.com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] doc: memory-barriers.txt: Minor correction in Control
 dependencies

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@...il.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

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