scsi: scsi_debug: Refine sdebug_blk_mq_poll()

Refine the sdebug_blk_mq_poll() function so it only takes the spinlock on
the queue when it can see one or more requests with the in_use bitmap flag
set.

Link: https://lore.kernel.org/r/20220109012853.301953-5-dgilbert@interlog.comSigned-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_debug.c

@@ -7396,6 +7396,7 @@ static int sdebug_blk_mq_poll(struct Scsi_Host *shost, unsigned int queue_num)
 {
 	bool first;
 	bool retiring = false;
+	bool locked = false;
 	int num_entries = 0;
 	unsigned int qc_idx = 0;
 	unsigned long iflags;
@@ -7407,16 +7408,23 @@ static int sdebug_blk_mq_poll(struct Scsi_Host *shost, unsigned int queue_num)
 	struct sdebug_defer *sd_dp;
 
 	sqp = sdebug_q_arr + queue_num;
-	spin_lock_irqsave(&sqp->qc_lock, iflags);
+	qc_idx = find_first_bit(sqp->in_use_bm, sdebug_max_queue);
+	if (qc_idx >= sdebug_max_queue)
+		return 0;
 
 	for (first = true; first || qc_idx + 1 < sdebug_max_queue; )   {
+		if (!locked) {
+			spin_lock_irqsave(&sqp->qc_lock, iflags);
+			locked = true;
+		}
 		if (first) {
-			qc_idx = find_first_bit(sqp->in_use_bm, sdebug_max_queue);
 			first = false;
+			if (!test_bit(qc_idx, sqp->in_use_bm))
+				continue;
 		} else {
 			qc_idx = find_next_bit(sqp->in_use_bm, sdebug_max_queue, qc_idx + 1);
 		}
-		if (unlikely(qc_idx >= sdebug_max_queue))
+		if (qc_idx >= sdebug_max_queue)
 			break;
 
 		sqcp = &sqp->qc_arr[qc_idx];
@@ -7465,11 +7473,14 @@ static int sdebug_blk_mq_poll(struct Scsi_Host *shost, unsigned int queue_num)
 		}
 		WRITE_ONCE(sd_dp->defer_t, SDEB_DEFER_NONE);
 		spin_unlock_irqrestore(&sqp->qc_lock, iflags);
+		locked = false;
 		scsi_done(scp); /* callback to mid level */
-		spin_lock_irqsave(&sqp->qc_lock, iflags);
 		num_entries++;
+		if (find_first_bit(sqp->in_use_bm, sdebug_max_queue) >= sdebug_max_queue)
+			break;	/* if no more then exit without retaking spinlock */
 	}
-	spin_unlock_irqrestore(&sqp->qc_lock, iflags);
+	if (locked)
+		spin_unlock_irqrestore(&sqp->qc_lock, iflags);
 	if (num_entries > 0)
 		atomic_add(num_entries, &sdeb_mq_poll_count);
 	return num_entries;