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

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Message-Id: <200707231833.20678.ak@suse.de>
Date:	Mon, 23 Jul 2007 18:33:20 +0200
From:	Andi Kleen <ak@...e.de>
To:	Satyam Sharma <ssatyam@....iitk.ac.in>
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 6/8] i386: bitops: Don't mark memory as clobbered unnecessarily


> Yes, but _that_ address (of the bit-string) is protected already -- by the
> implicit memory barrier due to the LOCK prefix.

Compiler barrier != CPU barrier.

The memory clobber is a compiler barrier that prevents its global optimizer
from moving memory references. The CPU memory ordering guarantees are completely 
independent from this.


> We shouldn't really be 
> caring about any other memory addresses, so it doesn't affect the
> correctness of the bitops API at all.

The problem is the relationship to other operations.

This is not theoretic and we have had bugs because of this.

-Andi

 
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