dm thin: speed up cell_defer_no_holder()

Reduce the time that a spinlock is held in cell_defer_no_holder().
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-thin.c

@@ -883,15 +883,17 @@ static void cell_defer_no_holder(struct thin_c *tc, struct dm_bio_prison_cell *c
 {
 	struct pool *pool = tc->pool;
 	unsigned long flags;
-	int has_work;
+	struct bio_list bios;
 
-	spin_lock_irqsave(&tc->lock, flags);
-	cell_release_no_holder(pool, cell, &tc->deferred_bio_list);
-	has_work = !bio_list_empty(&tc->deferred_bio_list);
-	spin_unlock_irqrestore(&tc->lock, flags);
+	bio_list_init(&bios);
+	cell_release_no_holder(pool, cell, &bios);
 
-	if (has_work)
+	if (!bio_list_empty(&bios)) {
+		spin_lock_irqsave(&tc->lock, flags);
+		bio_list_merge(&tc->deferred_bio_list, &bios);
+		spin_unlock_irqrestore(&tc->lock, flags);
 		wake_worker(pool);
+	}
 }
 
 static void thin_defer_bio(struct thin_c *tc, struct bio *bio);