| Message ID | 1451572003-2440-32-git-send-email-mst@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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 <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.
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) + #define smp_store_mb(var, value) __smp_store_mb(var, value) #include <asm-generic/barrier.h>
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(-)