linux-kernel - Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered unnecessarily

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Message-Id: <1185270756.5439.256.camel@localhost.localdomain>
Date:	Tue, 24 Jul 2007 19:52:36 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Satyam Sharma <ssatyam@....iitk.ac.in>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered
	unnecessarily

On Mon, 2007-07-23 at 10:55 -0700, Linus Torvalds wrote:
> > The goal is to let gcc generate good, beautiful, optimized code.
> 
> No. The point is to let gcc generate *correct* code.
> 
> It's either "=m" together with "memory", or it's "+m".

In fact, it's more than that... the bitops that return a value are often
used to have hand-made spinlock semantics. I'm sure we would get funky
bugs if loads or stores leaked out of the locked region. I think a full
"memory" clobber should be kept around for those cases.

(That's also why on ppc, we give them a few more barriers)

Ben.

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