Date: Mon, 27 Feb 2006 16:54:36 -0800
From: Richard Henderson <rth@redhat.com>
To: Andi Kleen <ak@suse.de>
Cc: Linus Torvalds <torvalds@osdl.org>,
       Chuck Ebbert <76306.1226@compuserve.com>,
       linux-kernel <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@osdl.org>, Ingo Molnar <mingo@elte.hu>
Subject: Re: [patch] i386: make bitops safe
Message-ID: <20060228005436.GA24895@redhat.com>
References: <200602271700_MC3-1-B969-F4A5@compuserve.com> <Pine.LNX.4.64.0602271502410.22647@g5.osdl.org> <200602280047.22909.ak@suse.de>
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On Tue, Feb 28, 2006 at 12:47:22AM +0100, Andi Kleen wrote:
> I remember asking rth about this at some point and IIRC
> he expressed doubts if it would actually do what expected. Richard?

It's a bit dicey to be sure.  GCC may or may not be able to look
through the size of the array and not kill things beyond it.  If
one could be *sure* of some actual maximum index, this would be
fine, but I don't think you can.

One could reasonably argue that if you used a structure with a
flexible array member, that GCC could not look through that.  But
again I'm not 100% positive this is handled properly.

I think the best argument for simply leaving things with a memory
clobber is that these are atomic operations, and are on occasion
used as locks, or parts of locks.


r~
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