Re: [PATCH v2 31/32] sh: support a 2-byte smp_store_mb
From: Peter Zijlstra <peterz@infradead.org>
Date: 2016-01-04 14:05:18
Also in:
linux-arch, linux-arm-kernel, linux-mips, linux-s390, linux-sh, linux-um, lkml, sparclinux
On Thu, Dec 31, 2015 at 09:09:47PM +0200, Michael S. Tsirkin wrote:
quoted hunk ↗ jump to hunk
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 <mst@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.