4 files changed, 21 insertions, 3 deletions

diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index ed5217867555..371d8800b48a 100644
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -402,6 +402,12 @@ static void blk_mq_sysfs_init(struct request_queue *q)
         }
 }
 
+/* see blk_register_queue() */
+void blk_mq_finish_init(struct request_queue *q)
+{
+        percpu_ref_switch_to_percpu(&q->mq_usage_counter);
+}
+
 int blk_mq_register_disk(struct gendisk *disk)
 {
         struct device *dev = disk_to_dev(disk);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index d85fe01c44ef..38f4a165640d 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1795,8 +1795,12 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
         if (!q)
                 goto err_hctxs;
 
+        /*
+         * Init percpu_ref in atomic mode so that it's faster to shutdown.
+         * See blk_register_queue() for details.
+         */
         if (percpu_ref_init(&q->mq_usage_counter, blk_mq_usage_counter_release,
-                            0, GFP_KERNEL))
+                            PERCPU_REF_INIT_ATOMIC, GFP_KERNEL))
                 goto err_map;
 
         setup_timer(&q->timeout, blk_mq_rq_timer, (unsigned long) q);
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 17f5c84ce7bf..521ae9089c50 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -551,12 +551,19 @@ int blk_register_queue(struct gendisk *disk)
                 return -ENXIO;
 
         /*
-         * Initialization must be complete by now.  Finish the initial
-         * bypass from queue allocation.
+         * SCSI probing may synchronously create and destroy a lot of
+         * request_queues for non-existent devices.  Shutting down a fully
+         * functional queue takes measureable wallclock time as RCU grace
+         * periods are involved.  To avoid excessive latency in these
+         * cases, a request_queue starts out in a degraded mode which is
+         * faster to shut down and is made fully functional here as
+         * request_queues for non-existent devices never get registered.
          */
         if (!blk_queue_init_done(q)) {
                 queue_flag_set_unlocked(QUEUE_FLAG_INIT_DONE, q);
                 blk_queue_bypass_end(q);
+                if (q->mq_ops)
+                        blk_mq_finish_init(q);
         }
 
         ret = blk_trace_init_sysfs(dev);
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index a1e31f274fcd..c13a0c09faea 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -140,6 +140,7 @@ enum {
 };
 
 struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *);
+void blk_mq_finish_init(struct request_queue *q);
 int blk_mq_register_disk(struct gendisk *);
 void blk_mq_unregister_disk(struct gendisk *);