block: blk-flush shouldn't call directly into q->request_fn() __blk_run_queue()

blk-flush decomposes a flush into sequence of multiple requests.  On
completion of a request, the next one is queued; however, block layer
must not implicitly call into q->request_fn() directly from completion
path.  This makes the queue behave unexpectedly when seen from the
drivers and violates the assumption that q->request_fn() is called
with process context + queue_lock.

This patch makes blk-flush the following two changes to make sure
q->request_fn() is not called directly from request completion path.

- blk_flush_complete_seq_end_io() now asks __blk_run_queue() to always
  use kblockd instead of calling directly into q->request_fn().

- queue_next_fseq() uses ELEVATOR_INSERT_REQUEUE instead of
  ELEVATOR_INSERT_FRONT so that elv_insert() doesn't try to unplug the
  request queue directly.

Reported by Jan in the following threads.

 http://thread.gmane.org/gmane.linux.ide/48778
 http://thread.gmane.org/gmane.linux.ide/48786

stable: applicable to v2.6.37.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Jan Beulich <JBeulich@novell.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/block/blk-flush.c b/block/blk-flush.c
index 56adaa8d55c..b27d0208611 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -66,10 +66,12 @@ static void blk_flush_complete_seq_end_io(struct request_queue *q,
 
         /*
          * Moving a request silently to empty queue_head may stall the
-         * queue.  Kick the queue in those cases.
+         * queue.  Kick the queue in those cases.  This function is called
+         * from request completion path and calling directly into
+         * request_fn may confuse the driver.  Always use kblockd.
          */
         if (was_empty && next_rq)
-                __blk_run_queue(q, false);
+                __blk_run_queue(q, true);
 }
 
 static void pre_flush_end_io(struct request *rq, int error)
@@ -130,7 +132,7 @@ static struct request *queue_next_fseq(struct request_queue *q)
                 BUG();
         }
 
-        elv_insert(q, rq, ELEVATOR_INSERT_FRONT);
+        elv_insert(q, rq, ELEVATOR_INSERT_REQUEUE);
         return rq;
 }