Because:

arch/frv/include/asm/smp.h:#error SMP not supported

smp_mb() is barrier() and we can use the default implementation that
uses smp_mb().
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/n/tip-n296g51yzdu5ru1vp7mccxmf@git.kernel.org
Cc: David Howells <dhowells@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>