From: Julian Wiedmann <jwi@linux.ibm.com>
Date: Fri, 8 May 2020 17:00:21 +0200
Subject: s390/qdio: fine-tune SLSB update
Git-commit: c119a8a3c395b0850f131728bdddd166d172842f
Patch-mainline: v5.8-rc2
References: jsc#SLE-13690

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>
Acked-by: Petr Tesarik <ptesarik@suse.com>
---
 drivers/s390/cio/qdio_main.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/s390/cio/qdio_main.c b/drivers/s390/cio/qdio_main.c
index 610c05f59589..bb137f962c3f 100644
--- a/drivers/s390/cio/qdio_main.c
+++ b/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;
 }