umem: fix up unplugging

Fix a regression introduced by 7eaceacc ("block: remove per-queue
plugging").  In that patch, Jens removed the whole mm_unplug_device()
function, which used to be the trigger to make umem start to work.

We need to implement unplugging to make umem start to work, or I/O will
never be triggered.
Signed-off-by: Tao Guo <Tao.Guo@emc.com>
Cc: Neil Brown <neilb@suse.de>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Shaohua Li <shli@kernel.org>
Cc: <stable@vger.kernel.org>
Acked-by: NeilBrown <neilb@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/umem.c

@@ -513,6 +513,44 @@ static void process_page(unsigned long data)
 	}
 }
 
+struct mm_plug_cb {
+	struct blk_plug_cb cb;
+	struct cardinfo *card;
+};
+
+static void mm_unplug(struct blk_plug_cb *cb)
+{
+	struct mm_plug_cb *mmcb = container_of(cb, struct mm_plug_cb, cb);
+
+	spin_lock_irq(&mmcb->card->lock);
+	activate(mmcb->card);
+	spin_unlock_irq(&mmcb->card->lock);
+	kfree(mmcb);
+}
+
+static int mm_check_plugged(struct cardinfo *card)
+{
+	struct blk_plug *plug = current->plug;
+	struct mm_plug_cb *mmcb;
+
+	if (!plug)
+		return 0;
+
+	list_for_each_entry(mmcb, &plug->cb_list, cb.list) {
+		if (mmcb->cb.callback == mm_unplug && mmcb->card == card)
+			return 1;
+	}
+	/* Not currently on the callback list */
+	mmcb = kmalloc(sizeof(*mmcb), GFP_ATOMIC);
+	if (!mmcb)
+		return 0;
+
+	mmcb->card = card;
+	mmcb->cb.callback = mm_unplug;
+	list_add(&mmcb->cb.list, &plug->cb_list);
+	return 1;
+}
+
 static void mm_make_request(struct request_queue *q, struct bio *bio)
 {
 	struct cardinfo *card = q->queuedata;
@@ -523,6 +561,8 @@ static void mm_make_request(struct request_queue *q, struct bio *bio)
 	*card->biotail = bio;
 	bio->bi_next = NULL;
 	card->biotail = &bio->bi_next;
+	if (bio->bi_rw & REQ_SYNC || !mm_check_plugged(card))
+		activate(card);
 	spin_unlock_irq(&card->lock);
 
 	return;