[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>

1 files changed, 16 insertions, 22 deletions

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index e5aad8314585..ee5ed98db4cd 100644
--- 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;
 }