ARC: use ACCESS_ONCE in cmpxchg loop

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/kernel/smp.c

@@ -221,7 +221,7 @@ static void ipi_send_msg_one(int cpu, enum ipi_msg_type msg)
 	 * and read back old value
 	 */
 	do {
-		new = old = *ipi_data_ptr;
+		new = old = ACCESS_ONCE(*ipi_data_ptr);
 		new |= 1U << msg;
 	} while (cmpxchg(ipi_data_ptr, old, new) != old);