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

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Date:	Thu, 16 Jun 2016 16:35:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andreas Schwab <schwab@...ux-m68k.org>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	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}()

On Thu, Jun 16, 2016 at 02:53:09PM +0200, Andreas Schwab wrote:
> Peter Zijlstra <peterz@...radead.org> writes:

> > OK, care to elucidate? Clearly I need help reading this.
> 
> grep '2.*atomic_read'

Much thanks to your detailed answer I found yet another obscure inline
asm syntax 'feature'.

So the "2" input operand actually sets the value of "=&d" (tmp), how
creative...

I would find:

#define ATOMIC_OP_RETURN(op, c_op, asm_op)                              \
static inline int atomic_##op##_return(int i, atomic_t *v)              \
{                                                                       \
        int t, tmp = atomic_read(v);                                    \
                                                                        \
        __asm__ __volatile__(                                           \
                        "1:     movel %2,%1\n"                          \
                        "       " #asm_op "l %3,%1\n"                   \
                        "       casl %2,%1,%0\n"                        \
                        "       jne 1b"                                 \
                        : "+m" (*v), "=&d" (t), "+d" (tmp)              \
                        : "g" (i));                                     \
        return t;                                                       \
}

Much more obvious.

But you're right, it seems to be sorted. I'll queue a patch removing
that comment.

Thanks!