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 <jaxboe@fusionio.com>

3 files changed, 22 insertions, 0 deletions

diff --git a/block/blk-core.c b/block/blk-core.c
index 0c0ea10e61ea..76850fc9cf23 100644
--- a/block/blk-core.c
+++ b/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)
diff --git a/block/blk-settings.c b/block/blk-settings.c
index 1fa769293597..eb949045bb12 100644
--- a/block/blk-settings.c
+++ b/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;
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 32176cc8e715..c07ffafac5d4 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -196,6 +196,7 @@ typedef void (request_fn_proc) (struct request_queue *q);
 typedef int (make_request_fn) (struct request_queue *q, struct bio *bio);
 typedef int (prep_rq_fn) (struct request_queue *, struct request *);
 typedef void (unprep_rq_fn) (struct request_queue *, struct request *);
+typedef void (unplugged_fn) (struct request_queue *);
 
 struct bio_vec;
 struct bvec_merge_data {
@@ -283,6 +284,7 @@ struct request_queue
         rq_timed_out_fn         *rq_timed_out_fn;
         dma_drain_needed_fn     *dma_drain_needed;
         lld_busy_fn             *lld_busy_fn;
+        unplugged_fn            *unplugged_fn;
 
         /*
          * Dispatch queue sorting
@@ -841,6 +843,7 @@ extern void blk_queue_dma_alignment(struct request_queue *, int);
 extern void blk_queue_update_dma_alignment(struct request_queue *, int);
 extern void blk_queue_softirq_done(struct request_queue *, softirq_done_fn *);
 extern void blk_queue_rq_timed_out(struct request_queue *, rq_timed_out_fn *);
+extern void blk_queue_unplugged(struct request_queue *, unplugged_fn *);
 extern void blk_queue_rq_timeout(struct request_queue *, unsigned int);
 extern void blk_queue_flush(struct request_queue *q, unsigned int flush);
 extern struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev);