From c119a8a3c395b0850f131728bdddd166d172842f Mon Sep 17 00:00:00 2001 From: Julian Wiedmann Date: Fri, 8 May 2020 17:00:21 +0200 Subject: [PATCH] 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 Signed-off-by: Julian Wiedmann Signed-off-by: Vasily Gorbik --- 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 610c05f..bb137f9 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; } -- 2.7.4