linux-kernel - Re: Memory corruption due to word sharing

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Message-ID: <CA+55aFyzYO0CMZOgdrVV0PdG2DkbwTQG7eE7N3bp=0tQmruFEQ@mail.gmail.com>
Date:	Wed, 1 Feb 2012 15:11:00 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Torvald Riegel <triegel@...hat.com>, Jan Kara <jack@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>, linux-ia64@...r.kernel.org,
	dsterba@...e.cz, ptesarik@...e.cz, rguenther@...e.de,
	gcc@....gnu.org
Subject: Re: Memory corruption due to word sharing

On Wed, Feb 1, 2012 at 2:45 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
>
> My (perhaps forlorn and naive) hope is that C++11 memory_order_relaxed
> will eventually allow ACCESS_ONCE() to be upgraded so that (for example)
> access-once increments can generate a single increment-memory instruction
> on x86.

I don't think that is a semantic issue.

gcc could do it *today* with volatile accesses. It doesn't, because
volatiles are scary and basically disables a lot of optimizations. Why
would memory ordering be substantially different just because it has a
different name?

> New architectures might eventually might define things like atomic_inc()
> in terms of C++11 atomics, but let's start with the straightforward stuff
> as and if it makes sense.

SMP-atomic or percpu atomic? Or both?

We need both variants in the kernel. If the compiler generates one of
them for us, that doesn't really much help.

                          Linus
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