linux-kernel - Re: [PATCH 7/8] i386: bitops: Kill needless usage of __asm__ __volatile_

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Date:	Mon, 23 Jul 2007 22:02:31 +0530 (IST)
From:	Satyam Sharma <ssatyam@....iitk.ac.in>
To:	Andi Kleen <ak@...e.de>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 7/8] i386: bitops: Kill needless usage of __asm__
 __volatile__

Hi,

On Mon, 23 Jul 2007, Andi Kleen wrote:

> On Monday 23 July 2007 18:06:03 Satyam Sharma wrote:
> > From: Satyam Sharma <ssatyam@....iitk.ac.in>
> > 
> > [7/8] i386: bitops: Kill needless usage of __asm__ __volatile__
> > 
> > Another oddity I noticed in this file. The semantics of __volatile__
> > when used to qualify inline __asm__ are that the compiler will not
> > (1) elid, or, (2) reorder, or, (3) intersperse, our inline asm with
> > the rest of the generated code.
> > 
> > However, we do not want these guarantees in the unlocked variants of the
> > bitops functions. 
> 
> I thought so too and did a similar transformation while moving
> some string functions out of line. After that recent misadventure
> I would be very very careful with this.

Ah, ok, so this must be the case we'd stumbled upon recently on the
other thread. I hadn't got your fix for this, so didn't know.

> Overall I'm sorry to say, but the risk:gain ratio of this
> patch is imho totally out of whack.

The patch does look correct to me, we're only killing the use of
__volatile__ from places that don't require it (the guarantee-less
variants). Without losing it, I really don't see how the compiler
can ever combine multiple bitops, which does sound beneficial when
the caller has already implemented higher-level locking around
the usage of these operations.

Satyam
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