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

Arnd Bergmann
2015-02-25T15:58:35Z

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@gmail.com>

Unfortunately, the BUG message seems incomplete, can you reproduce this
with CONFIG_DEBUG_BUGVERBOSE enabled?

> arch/arm/include/asm/cmpxchg.h | 18 +++++++++++++++++-
> 1 file changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm/include/asm/cmpxchg.h b/arch/arm/include/asm/cmpxchg.h
> index abb2c37..9505cca 100644
> --- a/arch/arm/include/asm/cmpxchg.h
> +++ b/arch/arm/include/asm/cmpxchg.h
> @@ -50,6 +50,16 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr, int size
> : "r" (x), "r" (ptr)
> : "memory", "cc");
> break;
> + case 2:
> + asm volatile("@ __xchg2\n"
> + "1: ldrexh %0, [%3]\n"
> + " strexh %1, %2, [%3]\n"
> + " teq %1, #0\n"
> + " bne 1b"
> + : "=&r" (ret), "=&r" (tmp)
> + : "r" (x), "r" (ptr)
> + : "memory", "cc");
> + break;
> case 4:
> asm volatile("@ __xchg4\n"
> "1: ldrex %0, [%3]\n"

Does this work on all ARMv6 or just ARMv6k?

Arnd