s390/qdio: fine-tune SLSB update

xchg() for a single-byte location assembles to a 4-byte Compare&Swap,
wrapped into a non-trivial amount of retry code that deals with
concurrent modifications to the unaffected bytes.

Change it to a simple byte-store, but preserve the memory ordering
semantics that the CS provided.
This simplifies the generated code for a hot path, and in theory also
allows us to amortize the memory barriers over multiple SLSB updates.

CC: Andreas Krebbel <krebbel@linux.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

drivers/s390/cio/qdio_main.c

@@ -254,10 +254,17 @@ static inline int set_buf_states(struct qdio_q *q, int bufnr,
 	if (is_qebsm(q))
 		return qdio_do_sqbs(q, state, bufnr, count);
 
+	/* Ensure that all preceding changes to the SBALs are visible: */
+	mb();
+
 	for (i = 0; i < count; i++) {
-		xchg(&q->slsb.val[bufnr], state);
+		WRITE_ONCE(q->slsb.val[bufnr], state);
 		bufnr = next_buf(bufnr);
 	}
+
+	/* Make our SLSB changes visible: */
+	mb();
+
 	return count;
 }