linux-kernel - Re: [PATCH 09/10] Change table chaining layout

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Message-ID: <20071022211617.31f5c63d@the-village.bc.nu>
Date:	Mon, 22 Oct 2007 21:16:17 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linux Kernel Development <linux-kernel@...r.kernel.org>,
	mingo@...e.hu, Linux/m68k <linux-m68k@...r.kernel.org>
Subject: Re: [PATCH 09/10] Change table chaining layout

On Mon, 22 Oct 2007 12:49:40 -0700 (PDT)
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 
> 
> On Mon, 22 Oct 2007, Geert Uytterhoeven wrote:
> >
> > Better safe than sorry...
> > 
> > Is it possible that a chain entry pointer has bit 1 set on architectures
> > (e.g. m68k) where the natural alignment of 32-bit quantities is _2_ bytes,
> > not 4?
> 
> Better make sure that such alignment never happens... But no, I don't 
> think it will, since these things would generally always have to be 
> allocated with an allocator, and the *allocator* won't return 2-byte 
> aligned data structures.

No - but a structure which has other objects in it before the object
being written out may well be 2 byte aligned on M68K and some of the
other externally 16bit platforms - ditto local dynamic objects.

Why can't we just make the list one item longer than the entry count and
stick a NULL on the end of it like normal people ? Then you need one bit
which ought to be safe for everyone (and if the bit is a macro any CPU
warped enough to have byte alignment is surely going to have top bits
spare...)

Alan
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