block: make queue flags non-atomic

We can save some atomic ops in the IO path, if we clearly define
the rules of how to modify the queue flags.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 18 insertions, 13 deletions

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 67f412bb4974..d545ad1cf47a 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -536,6 +536,9 @@ static void scsi_run_queue(struct request_queue *q)
 	       !shost->host_blocked && !shost->host_self_blocked &&
 		!((shost->can_queue > 0) &&
 		  (shost->host_busy >= shost->can_queue))) {
+
+		int flagset;
+
 		/*
 		 * As long as shost is accepting commands and we have
 		 * starved queues, call blk_run_queue. scsi_request_fn
@@ -549,19 +552,20 @@ static void scsi_run_queue(struct request_queue *q)
 		sdev = list_entry(shost->starved_list.next,
 					  struct scsi_device, starved_entry);
 		list_del_init(&sdev->starved_entry);
-		spin_unlock_irqrestore(shost->host_lock, flags);
-
+		spin_unlock(shost->host_lock);
+
+		spin_lock(sdev->request_queue->queue_lock);
+		flagset = test_bit(QUEUE_FLAG_REENTER, &q->queue_flags) &&
+				!test_bit(QUEUE_FLAG_REENTER,
+					&sdev->request_queue->queue_flags);
+		if (flagset)
+			queue_flag_set(QUEUE_FLAG_REENTER, sdev->request_queue);
+		__blk_run_queue(sdev->request_queue);
+		if (flagset)
+			queue_flag_clear(QUEUE_FLAG_REENTER, sdev->request_queue);
+		spin_unlock(sdev->request_queue->queue_lock);
 
-		if (test_bit(QUEUE_FLAG_REENTER, &q->queue_flags) &&
-		    !test_and_set_bit(QUEUE_FLAG_REENTER,
-				      &sdev->request_queue->queue_flags)) {
-			blk_run_queue(sdev->request_queue);
-			clear_bit(QUEUE_FLAG_REENTER,
-				  &sdev->request_queue->queue_flags);
-		} else
-			blk_run_queue(sdev->request_queue);
-
-		spin_lock_irqsave(shost->host_lock, flags);
+		spin_lock(shost->host_lock);
 		if (unlikely(!list_empty(&sdev->starved_entry)))
 			/*
 			 * sdev lost a race, and was put back on the
@@ -1585,8 +1589,9 @@ struct request_queue *__scsi_alloc_queue(struct Scsi_Host *shost,
 
 	blk_queue_max_segment_size(q, dma_get_max_seg_size(dev));
 
+	/* New queue, no concurrency on queue_flags */
 	if (!shost->use_clustering)
-		clear_bit(QUEUE_FLAG_CLUSTER, &q->queue_flags);
+		queue_flag_clear_unlocked(QUEUE_FLAG_CLUSTER, q);
 
 	/*
 	 * set a reasonable default alignment on word boundaries: the