Use "mb" not the nonexistent "rmb" for pg_read_barrier() on Alpha.

tglsfdc · tglsfdc · commit 747ca66977d5 · 2015-08-29T16:34:30.000-04:00
It's only necessary to fix this in 9.4; later versions don't have this
code (because we ripped out Alpha support entirely), while earlier ones
aren't actually using pg_read_barrier() anywhere.

Per rather belated report from Christoph Berg.
diff --git a/src/include/storage/barrier.h b/src/include/storage/barrier.h
@@ -117,7 +117,7 @@ extern slock_t dummy_spinlock;
  * read barrier to cover that case.  We might need to add that later.
  */
 #define pg_memory_barrier()		__asm__ __volatile__ ("mb" : : : "memory")
-#define pg_read_barrier()		__asm__ __volatile__ ("rmb" : : : "memory")
+#define pg_read_barrier()		__asm__ __volatile__ ("mb" : : : "memory")
 #define pg_write_barrier()		__asm__ __volatile__ ("wmb" : : : "memory")
 #elif defined(__hppa) || defined(__hppa__)		/* HP PA-RISC */
 
