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

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Date:	Wed, 1 Feb 2012 19:58:19 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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

> So here's basically what the kernel needs:
> 
>  - if we don't touch a field, the compiler doesn't touch it.
> 
>    This is the rule that gcc now violates with bitfields.
> 
>    This is a gcc bug. End of story. The "volatile" example proves it -
> anybody who argues otherwise is simply wrong, and is just trying to
> make excuses.

C historically didn't make this guarantee because a lot of processors
couldn't make it because they didn't have things like byte accessors (In
fact I suspect early ARM cannot make it for example).

Not meeting it for types where you can do is a bit rude however and
really ought to be an option (speed v sanity).

> See above: it's not the "state" that is accessed concurrently. It's
> the code. If you ever try to mark state, you've already lost. The same
> "state" can be atomic or not depending on context. It's not about the
> state or the data structures, and it never will be.

There are optimisation cases - where you can prove access properties are
safe (eg local variables some times) but they should be exactly that -
optimisations.

Alan
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