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