Re: [PATCH 00/18] locking/atomic: atomic64 type cleanup

Andrea Parri
2019-05-24T22:43:54Z

> ---
> Subject: Documentation/atomic_t.txt: Clarify pure non-rmw usage
>
> Clarify that pure non-RMW usage of atomic_t is pointless, there is
> nothing 'magical' about atomic_set() / atomic_read().
>
> This is something that seems to confuse people, because I happen upon it
> semi-regularly.
>
> Acked-by: Will Deacon <will.deacon@arm.com>
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> Documentation/atomic_t.txt | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
> index dca3fb0554db..89eae7f6b360 100644
> --- a/Documentation/atomic_t.txt
> +++ b/Documentation/atomic_t.txt
> @@ -81,9 +81,11 @@ SEMANTICS
>
> The non-RMW ops are (typically) regular LOADs and STOREs and are canonically
> implemented using READ_ONCE(), WRITE_ONCE(), smp_load_acquire() and
> -smp_store_release() respectively.
> +smp_store_release() respectively. Therefore, if you find yourself only using
> +the Non-RMW operations of atomic_t, you do not in fact need atomic_t at all
> +and are doing it wrong.

The counterargument (not so theoretic, just look around in the kernel!) is:
we all 'forget' to use READ_ONCE() and WRITE_ONCE(), it should be difficult
or more difficult to forget to use atomic_read() and atomic_set()... IAC,
I wouldn't call any of them 'wrong'.

Andrea

>
> -The one detail to this is that atomic_set{}() should be observable to the RMW
> +A subtle detail of atomic_set{}() is that it should be observable to the RMW
> ops. That is:
>
> C atomic-set