linux-kernel - Re: kernel + gcc 4.1 = several problems

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Date:	Thu, 4 Jan 2007 19:10:46 +0000
From:	Al Viro <viro@....linux.org.uk>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	Segher Boessenkool <segher@...nel.crashing.org>,
	Albert Cahalan <acahalan@...il.com>, akpm@...l.org,
	linux-kernel@...r.kernel.org, s0348365@....ed.ac.uk,
	bunk@...sta.de, mikpe@...uu.se
Subject: Re: kernel + gcc 4.1 = several problems

On Thu, Jan 04, 2007 at 09:47:01AM -0800, Linus Torvalds wrote:
> NOBODY will guarantee you that they follow all standards to the letter. 
> Some use compiler extensions knowingly, but pretty much _everybody_ ends 
> up depending on subtle issues without even realizing it. It's almost 
> impossible to write a real program that has no bugs, and if they don't 
> show up in testing (because the compiler didn't generate buggy assembly 
> code from source code that had the _potential_ for bugs), they often won't 
> get fixed.
> 
> The kernel does things like compare pointers across objects, and the 
> kernel EXPECTS it to work. I seriously doubt that the kernel is even 
> unusual in this. The common way to avoid AB-BA deadlocks in any threaded 
> code (whether kernel or user space) is to just take two locks in a 
> specific order, and the common way to do that for locks of the same type 
> is simply to compare the addresses).
> 
> The fact that this is "undefined" behaviour matters not a _whit_. Not for 
> the kernel, and I bet not for a lot of other applications either.

True, but we'd better understand what assumptions we are making.  I have
seen patches seriously attempting to _subtract_ unrelated pointers.  And
that simply doesn't work for obvious reasons...
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