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

[Russell King - ARM Linux]

On Wed, Feb 25, 2015 at 05:21:54PM +0100, Arnd Bergmann wrote:
> On Wednesday 25 February 2015 11:11:28 Pranith Kumar wrote:
> > On Wed, Feb 25, 2015 at 10:58 AM, Arnd Bergmann <arnd at arndb.de> 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 at gmail.com>
> > >
> > > Unfortunately, the BUG message seems incomplete, can you reproduce this
> > > with CONFIG_DEBUG_BUGVERBOSE enabled?
> > 
> > The bug here is in a module caused when xchg() was used on uint16_t
> > variable. It is caused by the __bad_xchg() for 2 byte swap.
> > 
> > More information:
> > [   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!
> > [   45.846450] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
> > [   45.852275] Modules linked in: test(O+) nvhost_vi
> > [   45.857012] CPU: 0 PID: 1848 Comm: insmod Tainted: G           O
> > 3.10.24-g6a2d13a #1
> > [   45.864744] task: ee406580 ti: eb18c000 task.ti: eb18c000
> > [   45.870141] PC is at __bad_xchg+0x24/0x28
> > [   45.874146] LR is at __bad_xchg+0x24/0x28
> 
> I'm more interested in the backtrace here, it's possible we should fix the
> driver instead.

Actually, I think we ought to get rid of __bad_xchg() so that cases
like this cause a link error instead of a runtime error, just like we
do in other cases as well.

That's something that goes back ages (it used to be called something
like invalidptr in 2.0 kernels...)

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