linux-kernel - Re: [PATCH 3/8] i386: bitops: Rectify bogus "+m" constraints

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Message-ID: <18925.1185268964@redhat.com>
Date:	Tue, 24 Jul 2007 10:22:44 +0100
From:	David Howells <dhowells@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Satyam Sharma <ssatyam@....iitk.ac.in>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <nickpiggin@...oo.com.au>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/8] i386: bitops: Rectify bogus "+m" constraints

Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> > So, using the "+" constraint modifier for memory, like "+m" is bogus.
> > We must simply specify "=m" which handles the case correctly.
> 
> No. This is wrong.

Agreed.

> "=m" means that the new value of the memory location is *set*.
> 
> Which means that gcc will potentially optimize away any previous stores to 
> that memory location.
> 
> And yes, it happens, and yes, we've seen it.

I had a lot of "fun" with this when mucking around with the asm-optimised R/W
semaphores.

David
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