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Date:   Sat, 25 May 2019 00:43:40 +0200
From:   Andrea Parri <andrea.parri@amarulasolutions.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Will Deacon <will.deacon@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        linux-kernel@vger.kernel.org, aou@eecs.berkeley.edu, arnd@arndb.de,
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        fenghua.yu@intel.com, heiko.carstens@de.ibm.com,
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        aditr@vmware.com, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH 00/18] locking/atomic: atomic64 type cleanup
Message-ID: <20190524224340.GA3792@andrea>
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> ---
> 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