path: root/block
diff options
authorTejun Heo <>2008-10-30 08:32:29 +0100
committerJens Axboe <>2008-11-06 08:41:55 +0100
commit2920ebbd65f3e80c318adf5191ac0987142bda80 (patch)
tree1f45e880b1cfe86040b4acf834a1d04faf9804f3 /block
parentf92131c3dd567fc6df18ce3f46fcf57ecbdefbe0 (diff)
block: add timer on blkdev_dequeue_request() not elv_next_request()
Block queue supports two usage models - one where block driver peeks at the front of queue using elv_next_request(), processes it and finishes it and the other where block driver peeks at the front of queue, dequeue the request using blkdev_dequeue_request() and finishes it. The latter is more flexible as it allows the driver to process multiple commands concurrently. These two inconsistent usage models affect the block layer implementation confusing. For some, elv_next_request() is considered the issue point while others consider blkdev_dequeue_request() the issue point. Till now the inconsistency mostly affect only accounting, so it didn't really break anything seriously; however, with block layer timeout, this inconsistency hits hard. Block layer considers elv_next_request() the issue point and adds timer but SCSI layer thinks it was just peeking and when the request can't process the command right away, it's just left there without further processing. This makes the request dangling on the timer list and, when the timer goes off, the request which the SCSI layer and below think is still on the block queue ends up in the EH queue, causing various problems - EH hang (failed count goes over busy count and EH never wakes up), WARN_ON() and oopses as low level driver trying to handle the unknown command, etc. depending on the timing. As SCSI midlayer is the only user of block layer timer at the moment, moving blk_add_timer() to elv_dequeue_request() fixes the problem; however, this two usage models definitely need to be cleaned up in the future. Signed-off-by: Tejun Heo <> Signed-off-by: Jens Axboe <>
Diffstat (limited to 'block')
1 files changed, 6 insertions, 6 deletions
diff --git a/block/elevator.c b/block/elevator.c
index 59173a69ebdf..9ac82dde99dd 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -773,12 +773,6 @@ struct request *elv_next_request(struct request_queue *q)
rq->cmd_flags |= REQ_STARTED;
blk_add_trace_rq(q, rq, BLK_TA_ISSUE);
- /*
- * We are now handing the request to the hardware,
- * add the timeout handler
- */
- blk_add_timer(rq);
if (!q->boundary_rq || q->boundary_rq == rq) {
@@ -850,6 +844,12 @@ void elv_dequeue_request(struct request_queue *q, struct request *rq)
if (blk_account_rq(rq))
+ /*
+ * We are now handing the request to the hardware, add the
+ * timeout handler.
+ */
+ blk_add_timer(rq);