linux-kernel - Re: [patch][rfc] rwsem: generic rwsem

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Message-ID: <20061211040334.GA11246@wotan.suse.de>
Date:	Mon, 11 Dec 2006 05:03:34 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	David Howells <dhowells@...hat.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org, Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [patch][rfc] rwsem: generic rwsem

On Fri, Dec 08, 2006 at 01:58:42PM +0000, David Howells wrote:
> Nick Piggin <npiggin@...e.de> wrote:
> 
> > > Look at how the counter works in the XADD-based version.  That's the way
> > > it is *because* I'm using XADD.  That's quite limiting.
> > 
> > Not really. ll/sc architectures "emulate" xadd the same as they would
> > emulate a spinlock. There is nothing suboptimal about it.
> 
> Yes, really.  You've missed the point entirely.  Look at *how* the counter
> *works*.

OK, I've looked but I can't see how you would make it more optimal on
an ll/sc architecture. The atomic_add_return variant has no more atomic
or barrier operations than the spinlock one, and it has less loads and
branches.
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