block: add callback function for unplug notification

MD would like to know when a queue is unplugged, so it can flush
it's bitmap writes. Add such a callback.

Signed-off-by: Jens Axboe <[email protected]>

block/blk-core.c

@@ -2672,6 +2672,9 @@ static void queue_unplugged(struct request_queue *q, unsigned int depth)
 {
 	trace_block_unplug_io(q, depth);
 	__blk_run_queue(q, false);
+
+	if (q->unplugged_fn)
+		q->unplugged_fn(q);
 }
 
 static void flush_plug_list(struct blk_plug *plug)

block/blk-settings.c

@@ -790,6 +790,22 @@ void blk_queue_flush(struct request_queue *q, unsigned int flush)
 }
 EXPORT_SYMBOL_GPL(blk_queue_flush);
 
+/**
+ * blk_queue_unplugged - register a callback for an unplug event
+ * @q:		the request queue for the device
+ * @fn:		the function to call
+ *
+ * Some stacked drivers may need to know when IO is dispatched on an
+ * unplug event. By registrering a callback here, they will be notified
+ * when someone flushes their on-stack queue plug. The function will be
+ * called with the queue lock held.
+ */
+void blk_queue_unplugged(struct request_queue *q, unplugged_fn *fn)
+{
+	q->unplugged_fn = fn;
+}
+EXPORT_SYMBOL(blk_queue_unplugged);
+
 static int __init blk_settings_init(void)
 {
 	blk_max_low_pfn = max_low_pfn - 1;