Commit c119a8a3 authored May 08, 2020 by Julian Wiedmann Committed by Vasily Gorbik Jun 16, 2020

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>

parent b3a9e3b9

drivers/s390/cio/qdio_main.c

+8 −1

Original line number	Diff line number	Diff line
		@@ -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;
		}

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