[PATCH] fix ordering on requeued request drainage

Previously, if a fs request which was being drained failed and got
requeued, blk_do_ordered() didn't allow it to be reissued, which causes
queue stall.  This patch makes blk_do_ordered() use the sequence of each
request to determine whether a request can be issued or not.  This fixes
the bug and simplifies code.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Acked-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index e5aad8314585ef6c434917ffb4a42b068d61ad94..ee5ed98db4cd15ff2ab7f5ae5b3a893466691abe 100644 (file)
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -508,7 +508,7 @@ static inline struct request *start_ordered(request_queue_t *q,
 
 int blk_do_ordered(request_queue_t *q, struct request **rqp)
 {
-       struct request *rq = *rqp, *allowed_rq;
+       struct request *rq = *rqp;
        int is_barrier = blk_fs_request(rq) && blk_barrier_rq(rq);
 
        if (!q->ordseq) {
@@ -532,32 +532,26 @@ int blk_do_ordered(request_queue_t *q, struct request **rqp)
                }
        }
 
+       /*
+        * Ordered sequence in progress
+        */
+
+       /* Special requests are not subject to ordering rules. */
+       if (!blk_fs_request(rq) &&
+           rq != &q->pre_flush_rq && rq != &q->post_flush_rq)
+               return 1;
+
        if (q->ordered & QUEUE_ORDERED_TAG) {
+               /* Ordered by tag.  Blocking the next barrier is enough. */
                if (is_barrier && rq != &q->bar_rq)
                        *rqp = NULL;
-               return 1;
+       } else {
+               /* Ordered by draining.  Wait for turn. */
+               WARN_ON(blk_ordered_req_seq(rq) < blk_ordered_cur_seq(q));
+               if (blk_ordered_req_seq(rq) > blk_ordered_cur_seq(q))
+                       *rqp = NULL;
        }
 
-       switch (blk_ordered_cur_seq(q)) {
-       case QUEUE_ORDSEQ_PREFLUSH:
-               allowed_rq = &q->pre_flush_rq;
-               break;
-       case QUEUE_ORDSEQ_BAR:
-               allowed_rq = &q->bar_rq;
-               break;
-       case QUEUE_ORDSEQ_POSTFLUSH:
-               allowed_rq = &q->post_flush_rq;
-               break;
-       default:
-               allowed_rq = NULL;
-               break;
-       }
-
-       if (rq != allowed_rq &&
-           (blk_fs_request(rq) || rq == &q->pre_flush_rq ||
-            rq == &q->post_flush_rq))
-               *rqp = NULL;
-
        return 1;
 }