[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>

1 files changed, 6 insertions, 1 deletions

diff --git a/drivers/block/elevator.c b/drivers/block/elevator.c
index 36f1057084b0..d4a49a3df829 100644
--- a/drivers/block/elevator.c
+++ b/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: