linux-kernel - Re: [PATCH] Documentation/memory-barriers.txt: fix ACCESS

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Date:	Mon, 23 Nov 2015 15:07:26 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Documentation/memory-barriers.txt: fix ACCESS_ONCE thinko

On Mon, Nov 23, 2015 at 05:04:17PM -0500, Chris Metcalf wrote:
> In commit 2ecf810121c7 ("Documentation/memory-barriers.txt: Add
> needed ACCESS_ONCE() calls to memory-barriers.txt") the statement
> "Q = P" was converted to "ACCESS_ONCE(Q) = P".  This should have
> been "Q = ACCESS_ONCE(P)".  It later became "WRITE_ONCE(Q, P)".
> This doesn't match the following text, which is "Q = LOAD P".
> Change the statement to be "Q = READ_ONCE(P)".
> 
> Signed-off-by: Chris Metcalf <cmetcalf@...hip.com>

Good eyes!  Queued for v4.5.

							Thanx, Paul

> ---
>  Documentation/memory-barriers.txt | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index aef9487303d0..85304ebd187c 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -194,7 +194,7 @@ There are some minimal guarantees that may be expected of a CPU:
>   (*) On any given CPU, dependent memory accesses will be issued in order, with
>       respect to itself.  This means that for:
> 
> -	WRITE_ONCE(Q, P); smp_read_barrier_depends(); D = READ_ONCE(*Q);
> +	Q = READ_ONCE(P); smp_read_barrier_depends(); D = READ_ONCE(*Q);
> 
>       the CPU will issue the following memory operations:
> 
> @@ -202,9 +202,9 @@ There are some minimal guarantees that may be expected of a CPU:
> 
>       and always in that order.  On most systems, smp_read_barrier_depends()
>       does nothing, but it is required for DEC Alpha.  The READ_ONCE()
> -     and WRITE_ONCE() are required to prevent compiler mischief.  Please
> -     note that you should normally use something like rcu_dereference()
> -     instead of open-coding smp_read_barrier_depends().
> +     is required to prevent compiler mischief.  Please note that you
> +     should normally use something like rcu_dereference() instead of
> +     open-coding smp_read_barrier_depends().
> 
>   (*) Overlapping loads and stores within a particular CPU will appear to be
>       ordered within that CPU.  This means that for:
> -- 
> 2.1.2
> 

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