linux-kernel - Re: [RFC PATCH 1/4] asm-generic: cmpxchg: implement dummy cmpxchg64

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Date:	Thu, 26 Sep 2013 08:34:04 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Will Deacon <will.deacon@....com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [RFC PATCH 1/4] asm-generic: cmpxchg: implement dummy
 cmpxchg64_relaxed operation

On Thu, Sep 26, 2013 at 8:13 AM, Will Deacon <will.deacon@....com> wrote:
>
> This patch implements a dummy implementation for asm-generic, falling
> back to the usual cmpxchg64 code.

I don't like the "let's add dummy operations for everybody who doesn't
care" when it is this specialized.

I'd much rather just add a single

   #ifndef cmpxchg64_relaxed
   # define cmpxchg64_relaxed cmpxchg64
   #endif

to the LOCKREF code, and then ARM (and others) can define it as they wish.

And *if* anybody else ever realizes that they want this outside of the
lockref code, let's look at doing that then. Right now I don't know of
any users, and I'd be leery of people using this willy-nilly, because
very few people really understand memory ordering.

               Linus
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