1 files changed, 25 insertions, 2 deletions

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index db08cb868652..ef37f043fd17 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -1613,14 +1613,34 @@ static int __spi_pump_transfer_message(struct spi_controller *ctlr,
 		}
 	}
 
+	/*
+	 * Drivers implementation of transfer_one_message() must arrange for
+	 * spi_finalize_current_message() to get called. Most drivers will do
+	 * this in the calling context, but some don't. For those cases, a
+	 * completion is used to guarantee that this function does not return
+	 * until spi_finalize_current_message() is done accessing
+	 * ctlr->cur_msg.
+	 * Use of the following two flags enable to opportunistically skip the
+	 * use of the completion since its use involves expensive spin locks.
+	 * In case of a race with the context that calls
+	 * spi_finalize_current_message() the completion will always be used,
+	 * due to strict ordering of these flags using barriers.
+	 */
+	WRITE_ONCE(ctlr->cur_msg_incomplete, true);
+	WRITE_ONCE(ctlr->cur_msg_need_completion, false);
 	reinit_completion(&ctlr->cur_msg_completion);
+	smp_wmb(); /* make these available to spi_finalize_current_message */
+
 	ret = ctlr->transfer_one_message(ctlr, msg);
 	if (ret) {
 		dev_err(&ctlr->dev,
 			"failed to transfer one message from queue\n");
 		return ret;
 	} else {
-		wait_for_completion(&ctlr->cur_msg_completion);
+		WRITE_ONCE(ctlr->cur_msg_need_completion, true);
+		smp_mb(); /* see spi_finalize_current_message()... */
+		if (READ_ONCE(ctlr->cur_msg_incomplete))
+			wait_for_completion(&ctlr->cur_msg_completion);
 	}
 
 	return 0;
@@ -1942,7 +1962,10 @@ void spi_finalize_current_message(struct spi_controller *ctlr)
 
 	mesg->prepared = false;
 
-	complete(&ctlr->cur_msg_completion);
+	WRITE_ONCE(ctlr->cur_msg_incomplete, false);
+	smp_mb(); /* See __spi_pump_transfer_message()... */
+	if (READ_ONCE(ctlr->cur_msg_need_completion))
+		complete(&ctlr->cur_msg_completion);
 
 	trace_spi_message_done(mesg);