Re: [tip:locking/core] Documentation/atomic_t.txt: Clarify pure non-rmw usage

Andrea Parri
2019-06-06T08:44:30Z

On Mon, Jun 03, 2019 at 06:46:54AM -0700, tip-bot for Peter Zijlstra wrote:
> Commit-ID: fff9b6c7d26943a8eb32b58364b7ec6b9369746a
> Gitweb: https://git.kernel.org/tip/fff9b6c7d26943a8eb32b58364b7ec6b9369746a
> Author: Peter Zijlstra <peterz@infradead.org>
> AuthorDate: Fri, 24 May 2019 13:52:31 +0200
> Committer: Ingo Molnar <mingo@kernel.org>
> CommitDate: Mon, 3 Jun 2019 12:32:57 +0200
>
> 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.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Acked-by: Will Deacon <will.deacon@arm.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Link: https://lkml.kernel.org/r/20190524115231.GN2623@hirez.programming.kicks-ass.net
> Signed-off-by: Ingo Molnar <mingo@kernel.org>

I'd appreciate if you could Cc: me in future changes to this doc.
(as currently suggested by get_maintainer.pl).

This is particularly annoying when you spend time to review such
changes:

https://lkml.kernel.org/r/20190528111558.GA9106@andrea

Thanks,
Andrea

> ---
> 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 @@ Non-RMW ops:
>
> 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 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