On Thu, Dec 31, 2015 at 09:09:47PM +0200, Michael S. Tsirkin wrote:
> At the moment, xchg on sh only supports 4 and 1 byte values, so using it
> from smp_store_mb means attempts to store a 2 byte value using this
> macro fail.
>
> And happens to be exactly what virtio drivers want to do.
>
> Check size and fall back to a slower, but safe, WRITE_ONCE+smp_mb.
>
> Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
> ---
> arch/sh/include/asm/barrier.h | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/arch/sh/include/asm/barrier.h b/arch/sh/include/asm/barrier.h
> index f887c64..0cc5735 100644
> --- a/arch/sh/include/asm/barrier.h
> +++ b/arch/sh/include/asm/barrier.h
> @@ -32,7 +32,15 @@
> #define ctrl_barrier() __asm__ __volatile__
> ("nop;nop;nop;nop;nop;nop;nop;nop")
> #endif
>
> -#define __smp_store_mb(var, value) do { (void)xchg(&var, value); } while (0)
> +#define __smp_store_mb(var, value) do { \
> + if (sizeof(var) != 4 && sizeof(var) != 1) { \
> + WRITE_ONCE(var, value); \
> + __smp_mb(); \
> + } else { \
> + (void)xchg(&var, value); \
> + } \
> +} while (0)
So SH is an orphaned arch, which is also why I did not comment on using
xchg() for the UP smp_store_mb() thing.
But I really think we should try fixing the xchg() implementation
instead of this duct-tape.
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