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Date:	Fri, 10 Jul 2015 09:05:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"rth@...ddle.net" <rth@...ddle.net>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"will.deacon@....com" <will.deacon@....com>,
	"hskinnemoen@...il.com" <hskinnemoen@...il.com>,
	"realmz6@...il.com" <realmz6@...il.com>,
	"dhowells@...hat.com" <dhowells@...hat.com>,
	"rkuo@...eaurora.org" <rkuo@...eaurora.org>,
	"tony.luck@...el.com" <tony.luck@...el.com>,
	"geert@...ux-m68k.org" <geert@...ux-m68k.org>,
	"james.hogan@...tec.com" <james.hogan@...tec.com>,
	"ralf@...ux-mips.org" <ralf@...ux-mips.org>,
	"jejb@...isc-linux.org" <jejb@...isc-linux.org>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	"heiko.carstens@...ibm.com" <heiko.carstens@...ibm.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"cmetcalf@...hip.com" <cmetcalf@...hip.com>,
	"mingo@...nel.org" <mingo@...nel.org>
Subject: Re: [RFC][PATCH 02/24] arc: Provide atomic_{or,xor,and}

On Fri, Jul 10, 2015 at 04:30:46AM +0000, Vineet Gupta wrote:
> 
> Since we are on the topic, the cmpxchg() loop in arch/arc/kernel/smp.c still
> irritates me.
> Do we need a new set of primitives to operate atomically on non atomic_t data or
> does that mean that the data *not* being atomic_t but requiring such semantics is
> the fundamental problem and thus needs to be converted first.

So if you look at the last patch, there's already a few sites that do
things like:

+       atomic_or(*mask, (atomic_t *)&flushcache_cpumask);

Which is of course ugly as hell, but does work.

Esp. inside arch code.

Now the 'problem' with cmpxchg/xchg, the instructions working on !atomic
data is:

  http://lkml.kernel.org/r/alpine.LRH.2.02.1406011342470.20831@file01.intranet.prod.int.rdu2.redhat.com
  http://lkml.kernel.org/r/20140606175316.GV13930@laptop.programming.kicks-ass.net

And note that includes some arc.

Adding more such primitives will only make it harder on those already
'broken' archs.
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