[BLOCK] Implement elv_drain_elevator for improved switch error detection

This patch adds request_queue->nr_sorted which keeps the number of
requests in the iosched and implement elv_drain_elevator which
performs forced dispatching.  elv_drain_elevator checks whether
iosched actually dispatches all requests it has and prints error
message if it doesn't.  As buggy forced dispatching can result in
wrong barrier operations, I think this extra check is worthwhile.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <axboe@suse.de>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 025a7f084dbd..a33a31e71bbc 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -406,6 +406,7 @@ struct request_queue
 
         atomic_t                refcnt;
 
+        unsigned int            nr_sorted;
         unsigned int            in_flight;
 
         /*
@@ -631,6 +632,7 @@ static inline void elv_dispatch_add_tail(struct request_queue *q,
 {
         if (q->last_merge == rq)
                 q->last_merge = NULL;
+        q->nr_sorted--;
 
         q->end_sector = rq_end_sector(rq);
         q->boundary_rq = rq;