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SubjectRe: [PATCH] Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}_atomic()
On Fri, Apr 19, 2019 at 01:21:45PM -0400, Alan Stern wrote:
> The description of smp_mb__before_atomic() and smp_mb__after_atomic()
> in Documentation/atomic_t.txt is slightly terse and misleading. It
> does not clearly state that these barriers only affect the ordering of
> other instructions with respect to the atomic operation.
>
> This improves the text to make the actual ordering implications clear,
> and also to explain how these barriers differ from a RELEASE or
> ACQUIRE ordering.
>
> Signed-off-by: Alan Stern <stern@rowland.harvard.edu>

Queued for further review, thank you, Alan!

Thanx, Paul

> ---
>
>
> Documentation/atomic_t.txt | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
>
> Index: usb-devel/Documentation/atomic_t.txt
> ===================================================================
> --- usb-devel.orig/Documentation/atomic_t.txt
> +++ usb-devel/Documentation/atomic_t.txt
> @@ -171,7 +171,10 @@ The barriers:
> smp_mb__{before,after}_atomic()
>
> only apply to the RMW ops and can be used to augment/upgrade the ordering
> -inherent to the used atomic op. These barriers provide a full smp_mb().
> +inherent to the used atomic op. Unlike normal smp_mb() barriers, they order
> +only the RMW op itself against the instructions preceding the
> +smp_mb__before_atomic() or following the smp_mb__after_atomic(); they do
> +not order instructions on the other side of the RMW op at all.
>
> These helper barriers exist because architectures have varying implicit
> ordering on their SMP atomic primitives. For example our TSO architectures
> @@ -195,7 +198,8 @@ Further, while something like:
> atomic_dec(&X);
>
> is a 'typical' RELEASE pattern, the barrier is strictly stronger than
> -a RELEASE. Similarly for something like:
> +a RELEASE because it orders preceding instructions against both the read
> +and write parts of the atomic_dec(). Similarly, something like:
>
> atomic_inc(&X);
> smp_mb__after_atomic();
> @@ -227,7 +231,8 @@ strictly stronger than ACQUIRE. As illus
>
> This should not happen; but a hypothetical atomic_inc_acquire() --
> (void)atomic_fetch_inc_acquire() for instance -- would allow the outcome,
> -since then:
> +because it would not order the W part of the RMW against the following
> +WRITE_ONCE. Thus:
>
> P1 P2
>
>

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