linux-kernel - Re: [PATCH tip/core/rcu 2/6] Documentation/memory-barriers.txt: ACCESS

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Message-ID: <20140217214056.GB7941@thin>
Date:	Mon, 17 Feb 2014 13:40:56 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, oleg@...hat.com, sbw@....edu
Subject: Re: [PATCH tip/core/rcu 2/6] Documentation/memory-barriers.txt:
 ACCESS_ONCE() provides cache coherence

On Mon, Feb 17, 2014 at 01:26:49PM -0800, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> The ACCESS_ONCE() primitive provides cache coherence, but the
> documentation does not clearly state this.  This commit therefore upgrades
> the documentation.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Punctuation nit below; otherwise:
Reviewed-by: Josh Triplett <josh@...htriplett.org>

>  Documentation/memory-barriers.txt | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index 102dc19c4119..ad6db1d48f1f 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1249,6 +1249,23 @@ The ACCESS_ONCE() function can prevent any number of optimizations that,
>  while perfectly safe in single-threaded code, can be fatal in concurrent
>  code.  Here are some examples of these sorts of optimizations:
>  
> + (*) The compiler is within its rights to reorder loads and stores
> +     to the same variable, and in some cases, the CPU is within its
> +     rights to reorder loads to the same variable.  This means that
> +     the following code:
> +
> +	a[0] = x;
> +	a[1] = x;
> +
> +     Might result in an older value of x stored in a[1] than in a[0].
> +     Prevent both the compiler and the CPU from doing this as follows:
> +
> +	a[0] = ACCESS_ONCE(x);
> +	a[1] = ACCESS_ONCE(x);
> +
> +     In short, ACCESS_ONCE() provides "cache coherence" for accesses from
> +     multiple CPUs to a single variable.

You don't need to "quote" the well-established term "cache coherence".

>   (*) The compiler is within its rights to merge successive loads from
>       the same variable.  Such merging can cause the compiler to "optimize"
>       the following code:
> -- 
> 1.8.1.5
> 
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