[PATCH] request references and list deletion/insertion checking

o Always use list_del_init() on request queuelist, this allows us to
  sanity check the integrity of the request on insertion and removal.
  So we can complain loudly instead of silently corrupting memory.

o Add references to requests. This is cheap, since we dont have to use
  an atomic variable for it (all puts are inside queue lock). We've had
  a bug in IDE for years where we want to inspect request state after
  io completion, but this is not possible to do race free right now.
  REQ_BLOCK_PC and sgio will need this too, for checking io residual
  etc. This is not just a theoretical race, I've seen it happen.

2 files changed, 5 insertions, 1 deletions

diff --git a/include/linux/blk.h b/include/linux/blk.h
index 775d021167c1..b225609f2ee7 100644
--- a/include/linux/blk.h
+++ b/include/linux/blk.h
@@ -44,7 +44,9 @@ struct request *elv_next_request(request_queue_t *q);
 
 static inline void blkdev_dequeue_request(struct request *req)
 {
-	list_del(&req->queuelist);
+	BUG_ON(list_empty(&req->queuelist));
+
+	list_del_init(&req->queuelist);
 
 	if (req->q)
 		elv_remove_request(req->q, req);
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index cd9b016a8993..de87dee16f83 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -26,6 +26,8 @@ struct request {
 	struct list_head queuelist; /* looking for ->queue? you must _not_
 				     * access it directly, use
 				     * blkdev_dequeue_request! */
+	int ref_count;
+
 	void *elevator_private;
 
 	unsigned char cmd[16];