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

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Message-ID: <20160616124949.GF30921@twins.programming.kicks-ass.net>
Date:	Thu, 16 Jun 2016 14:49:49 +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 02:43:29PM +0200, Andreas Schwab wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> >> > +/*
> >> > + * 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?
> >> > + */

> No, there is nothing to fix here.

OK, care to elucidate? Clearly I need help reading this.

I'm more than happy to remove the comment, but I would like to better
understand.