block: reorganize queue draining

Reorganize queue draining related code in preparation of queue exit
changes.

* Factor out actual draining from elv_quiesce_start() to
  blk_drain_queue().

* Make elv_quiesce_start/end() responsible for their own locking.

* Replace open-coded ELVSWITCH clearing in elevator_switch() with
  elv_quiesce_end().

This patch doesn't cause any visible functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 28 insertions, 0 deletions

diff --git a/block/blk-core.c b/block/blk-core.c
index a3d2fdc8ed1c..149149dd7f7b 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -28,6 +28,7 @@
 #include <linux/task_io_accounting_ops.h>
 #include <linux/fault-inject.h>
 #include <linux/list_sort.h>
+#include <linux/delay.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/block.h>
@@ -345,6 +346,33 @@ void blk_put_queue(struct request_queue *q)
 }
 EXPORT_SYMBOL(blk_put_queue);
 
+/**
+ * blk_drain_queue - drain requests from request_queue
+ * @q: queue to drain
+ *
+ * Drain ELV_PRIV requests from @q.  The caller is responsible for ensuring
+ * that no new requests which need to be drained are queued.
+ */
+void blk_drain_queue(struct request_queue *q)
+{
+	while (true) {
+		int nr_rqs;
+
+		spin_lock_irq(q->queue_lock);
+
+		elv_drain_elevator(q);
+
+		__blk_run_queue(q);
+		nr_rqs = q->rq.elvpriv;
+
+		spin_unlock_irq(q->queue_lock);
+
+		if (!nr_rqs)
+			break;
+		msleep(10);
+	}
+}
+
 /*
  * Note: If a driver supplied the queue lock, it is disconnected
  * by this function. The actual state of the lock doesn't matter