linux-kernel - Re: [PATCH -v2 14/33] locking,m68k: Implement atomic_fetch

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Date:	Thu, 16 Jun 2016 12:08:27 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <will.deacon@....com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	boqun.feng@...il.com, waiman.long@....com,
	Frédéric Weisbecker <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	Richard Henderson <rth@...ddle.net>,
	Vineet Gupta <vgupta@...opsys.com>,
	Russell King <linux@....linux.org.uk>,
	Hans-Christian Noren Egtvedt <egtvedt@...fundet.no>,
	Miao Steven <realmz6@...il.com>,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>,
	Richard Kuo <rkuo@...eaurora.org>,
	Tony Luck <tony.luck@...el.com>,
	James Hogan <james.hogan@...tec.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	David Howells <dhowells@...hat.com>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Rich Felker <dalias@...c.org>,
	"David S. Miller" <davem@...emloft.net>, cmetcalf@...lanox.com,
	Max Filippov <jcmvbkbc@...il.com>,
	Arnd Bergmann <arnd@...db.de>, dbueso@...e.de,
	Wu Fengguang <fengguang.wu@...el.com>,
	linux-m68k <linux-m68k@...ts.linux-m68k.org>
Subject: Re: [PATCH -v2 14/33] locking,m68k: Implement atomic_fetch_{add,sub,and,or,xor}()

Hi Peter,

On Tue, May 31, 2016 at 12:19 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> Implement FETCH-OP atomic primitives, these are very similar to the
> existing OP-RETURN primitives we already have, except they return the
> value of the atomic variable _before_ modification.
>
> This is especially useful for irreversible operations -- such as
> bitops (because it becomes impossible to reconstruct the state prior
> to modification).
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/m68k/include/asm/atomic.h |   53 +++++++++++++++++++++++++++++++++++++----
>  1 file changed, 49 insertions(+), 4 deletions(-)
>
> --- a/arch/m68k/include/asm/atomic.h
> +++ b/arch/m68k/include/asm/atomic.h
> @@ -38,6 +38,13 @@ static inline void atomic_##op(int i, at
>
>  #ifdef CONFIG_RMW_INSNS
>
> +/*
> + * Am I reading these CAS loops right in that %2 is the old value and the first
> + * iteration uses an uninitialized value?
> + *
> + * Would it not make sense to add: tmp = atomic_read(v); to avoid this?
> + */
> +
>  #define ATOMIC_OP_RETURN(op, c_op, asm_op)                             \
>  static inline int atomic_##op##_return(int i, atomic_t *v)             \
>  {                                                                      \

Do we want the above comment in the code?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds