[PATCH 4/5] scsi_io_completion: conditional hints on fastpath

[Douglas Gilbert <dgilbert@xxxxxxxxxxxx>]

ChangeLog:
  - add likely() and unlikely() hints to conditionals on or near the
    fastpath
  - fix a comment in scsi_io_completion_action()

Signed-off-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>
---
 drivers/scsi/scsi_lib.c | 26 ++++++++++++--------------
 1 file changed, 12 insertions(+), 14 deletions(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 8d2d12a69508..23e59281bd02 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -818,7 +818,7 @@ static void scsi_io_completion_action(struct scsi_cmnd *cmd, int result)
 	unsigned long wait_for = (cmd->allowed + 1) * req->timeout;
 	struct scsi_sense_hdr sshdr;
 	bool sense_valid_and_current = false;
-	blk_status_t blk_stat;		/* enum, BLK_STS_OK is 0 */
+	blk_status_t blk_stat;		/* u8, BLK_STS_OK is only 0 */
 
 	/* sense not about current command is termed: deferred */
 	if (scsi_command_normalize_sense(cmd, &sshdr) &&
@@ -1009,7 +1009,7 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
 	struct request *req = cmd->request;
 	blk_status_t blk_stat = BLK_STS_OK;	/* u8: BLK_STS_OK is only 0 */
 
-	if (result) {
+	if (unlikely(result)) {
 		blk_stat = scsi_io_completion_nz_result(cmd, result);
 		if (blk_stat == BLK_STS_OK)
 			result = 0;
@@ -1017,14 +1017,14 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
 			blk_stat = BLK_STS_OK;
 	}
 
-	if (blk_rq_is_passthrough(req)) {
+	if (unlikely(blk_rq_is_passthrough(req))) {
 		/*
 		 * __scsi_error_from_host_byte may have reset the host_byte
 		 */
 		scsi_req(req)->result = cmd->result;
 		scsi_req(req)->resid_len = scsi_get_resid(cmd);
 
-		if (scsi_bidi_cmnd(cmd)) {
+		if (unlikely(scsi_bidi_cmnd(cmd))) {
 			/*
 			 * Bidi commands Must be complete as a whole,
 			 * both sides at once.
@@ -1037,7 +1037,7 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
 		}
 	}
 
-	/* no bidi support for !blk_rq_is_passthrough yet */
+	/* no bidi support yet, other than in pass-through */
 	BUG_ON(blk_bidi_rq(req));
 
 	SCSI_LOG_HLCOMPLETE(1, scmd_printk(KERN_INFO, cmd,
@@ -1049,15 +1049,13 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
 	 * handle. Failed, zero length commands always need to drop down
 	 * to retry code. Fast path should return in this block.
 	 */
-	if (blk_rq_bytes(req) > 0 || blk_stat == BLK_STS_OK) {
-		if (!scsi_end_request(req, blk_stat, good_bytes, 0))
-			return; /* no bytes remaining */
+	if (likely(blk_rq_bytes(req) > 0 || blk_stat == BLK_STS_OK)) {
+		if (likely(!scsi_end_request(req, blk_stat, good_bytes, 0)))
+			return;	/* no bytes remaining */
 	}
 
-	/*
-	 * Kill remainder if no retrys.
-	 */
-	if (blk_stat && scsi_noretry_cmd(cmd)) {
+	/* Kill remainder if no retrys  */
+	if (unlikely(blk_stat && scsi_noretry_cmd(cmd))) {
 		if (scsi_end_request(req, blk_stat, blk_rq_bytes(req), 0))
 			BUG();
 		return;
@@ -1067,14 +1065,14 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
 	 * If there had been no error, but we have leftover bytes in the
 	 * requeues just queue the command up again.
 	 */
-	if (result == 0) {
+	if (likely(result == 0)) {
 		/*
 		 * Unprep the request and put it back at the head of the
 		 * queue. A new command will be prepared and issued.
 		 * This block is the same as case ACTION_REPREP in
 		 * scsi_io_completion_action() above.
 		 */
-		if (q->mq_ops)
+		if (likely(q->mq_ops))
 			scsi_mq_requeue_cmd(cmd);
 		else {
 			scsi_release_buffers(cmd);
-- 
2.14.1