linux-kernel - Re: [RFC PATCH] arm: asm/cmpxchg.h: Add support half-word xchg()

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Message-ID: <20150225170250.GD8656@n2100.arm.linux.org.uk>
Date:	Wed, 25 Feb 2015 17:02:50 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Pranith Kumar <bobby.prani@...il.com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] arm: asm/cmpxchg.h: Add support half-word xchg()

On Wed, Feb 25, 2015 at 04:58:35PM +0100, Arnd Bergmann wrote:
> On Wednesday 25 February 2015 10:36:20 Pranith Kumar wrote:
> > This patch adds support for a half-word xchg() for ARM using ldrexh/strexh
> > instructions. It also fixes an asm comment  for __cmpxchg2.
> > 
> > Currently using a half-word xchg() results in the following splat on an ARMv7
> > machine.
> > 
> > [   45.833303] xchg: bad data size: pc 0xbe806020, ptr 0xeb18deee, size 2
> > [   45.833324] ------------[ cut here ]------------
> > [   45.837939] kernel BUG at /dvs/git/dirty/git-master_linux/kernel/arch/arm/kernel/traps.c:727!
> > 
> > Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
> 
> Unfortunately, the BUG message seems incomplete, can you reproduce this
> with CONFIG_DEBUG_BUGVERBOSE enabled?

That isn't because CONFIG_DEBUG_BUGVERBOSE isn't set.  It's because
the message author decided that the remainder of the kernel bug message
wasn't useful to report.  After all, it's just a load of hex numbers
and symbolic information.  Who would want that junk.

(Some people really don't get this: we print stuff from the kernel
because it gives _us_ useful information to debug problems like this,
but because they don't understand it themselves, they decide they can
omit it from their bug reports...)

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