[PATCH] blk: fix dangling pointer access in __elv_add_request

cfq's add_req_fn callback may invoke q->request_fn directly and
depending on low-level driver used and timing, a queued request may be
finished & deallocated before add_req_fn callback returns.  So,
__elv_add_request must not access rq after it's passed to add_req_fn
callback.

This patch moves rq_mergeable test above add_req_fn().  This may
result in q->last_merge pointing to REQ_NOMERGE request if add_req_fn
callback sets it but as RQ_NOMERGE is checked again when blk layer
actually tries to merge requests, this does not cause any problem.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/block/elevator.c

@@ -369,9 +369,14 @@ void __elv_add_request(request_queue_t *q, struct request *rq, int where,
 	case ELEVATOR_INSERT_SORT:
 		BUG_ON(!blk_fs_request(rq));
 		rq->flags |= REQ_SORTED;
-		q->elevator->ops->elevator_add_req_fn(q, rq);
 		if (q->last_merge == NULL && rq_mergeable(rq))
 			q->last_merge = rq;
+		/*
+		 * Some ioscheds (cfq) run q->request_fn directly, so
+		 * rq cannot be accessed after calling
+		 * elevator_add_req_fn.
+		 */
+		q->elevator->ops->elevator_add_req_fn(q, rq);
 		break;
 
 	default: