src/patches/suse-2.6.27.39/patches.drivers/block-add-timeout-on-dequeue

   1 From: Tejun Heo <tj@kernel.org>
   2 Subject: block: add timer on blkdev_dequeue_request() not elv_next_request()
   3 References: bnc#440076
   4
   5 Block queue supports two usage models - one where block driver peeks
   6 at the front of queue using elv_next_request(), processes it and
   7 finishes it and the other where block driver peeks at the front of
   8 queue, dequeue the request using blkdev_dequeue_request() and finishes
   9 it.  The latter is more flexible as it allows the driver to process
  10 multiple commands concurrently.
  11
  12 These two inconsistent usage models affect the block layer
  13 implementation confusing.  For some, elv_next_request() is considered
  14 the issue point while others consider blkdev_dequeue_request() the
  15 issue point.
  16
  17 Till now the inconsistency mostly affect only accounting, so it didn't
  18 really break anything seriously; however, with block layer timeout,
  19 this inconsistency hits hard.  Block layer considers
  20 elv_next_request() the issue point and adds timer but SCSI layer
  21 thinks it was just peeking and when the request can't process the
  22 command right away, it's just left there without further processing.
  23 This makes the request dangling on the timer list and, when the timer
  24 goes off, the request which the SCSI layer and below think is still on
  25 the block queue ends up in the EH queue, causing various problems - EH
  26 hang (failed count goes over busy count and EH never wakes up),
  27 WARN_ON() and oopses as low level driver trying to handle the unknown
  28 command, etc. depending on the timing.
  29
  30 As SCSI midlayer is the only user of block layer timer at the moment,
  31 moving blk_add_timer() to elv_dequeue_request() fixes the problem;
  32 however, this two usage models definitely need to be cleaned up in the
  33 future.
  34
  35 Signed-off-by: Tejun Heo <tj@kernel.org>
  36 Signed-off-by: Tejun Heo <teheo@suse.de>
  37 ---
  38  block/elevator.c |   12 ++++++------
  39  1 file changed, 6 insertions(+), 6 deletions(-)
  40
  41 --- a/block/elevator.c
  42 +++ b/block/elevator.c
  43 @@ -781,12 +781,6 @@ struct request *elv_next_request(struct
  44                          */
  45                         rq->cmd_flags |= REQ_STARTED;
  46                         blk_add_trace_rq(q, rq, BLK_TA_ISSUE);
  47 -
  48 -                       /*
  49 -                        * We are now handing the request to the hardware,
  50 -                        * add the timeout handler
  51 -                        */
  52 -                       blk_add_timer(rq);
  53                 }
  54
  55                 if (!q->boundary_rq || q->boundary_rq == rq) {
  56 @@ -858,6 +852,12 @@ void elv_dequeue_request(struct request_
  57          */
  58         if (blk_account_rq(rq))
  59                 q->in_flight++;
  60 +
  61 +       /*
  62 +        * We are now handing the request to the hardware, add the
  63 +        * timeout handler.
  64 +        */
  65 +       blk_add_timer(rq);
  66  }
  67  EXPORT_SYMBOL(elv_dequeue_request);
  68