blk-mq: insert rq with DONTPREP to hctx dispatch list when requeue

When requeue, if RQF_DONTPREP, rq has contained some driver
specific data, so insert it to hctx dispatch list to avoid any
merge. Take scsi as example, here is the trace event log (no
io scheduler, because RQF_STARTED would prevent merging),

   kworker/0:1H-339   [000] ...1  2037.209289: block_rq_insert: 8,0 R 4096 () 32768 + 8 [kworker/0:1H]
scsi_inert_test-1987  [000] ....  2037.220465: block_bio_queue: 8,0 R 32776 + 8 [scsi_inert_test]
scsi_inert_test-1987  [000] ...2  2037.220466: block_bio_backmerge: 8,0 R 32776 + 8 [scsi_inert_test]
   kworker/0:1H-339   [000] ....  2047.220913: block_rq_issue: 8,0 R 8192 () 32768 + 16 [kworker/0:1H]
scsi_inert_test-1996  [000] ..s1  2047.221007: block_rq_complete: 8,0 R () 32768 + 8 [0]
scsi_inert_test-1996  [000] .Ns1  2047.221045: block_rq_requeue: 8,0 R () 32776 + 8 [0]
   kworker/0:1H-339   [000] ...1  2047.221054: block_rq_insert: 8,0 R 4096 () 32776 + 8 [kworker/0:1H]
   kworker/0:1H-339   [000] ...1  2047.221056: block_rq_issue: 8,0 R 4096 () 32776 + 8 [kworker/0:1H]
scsi_inert_test-1986  [000] ..s1  2047.221119: block_rq_complete: 8,0 R () 32776 + 8 [0]

(32768 + 8) was requeued by scsi_queue_insert and had RQF_DONTPREP.
Then it was merged with (32776 + 8) and issued. Due to RQF_DONTPREP,
the sdb only contained the part of (32768 + 8), then only that part
was completed. The lucky thing was that scsi_io_completion detected
it and requeued the remaining part. So we didn't get corrupted data.
However, the requeue of (32776 + 8) is not expected.

Suggested-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 8f5b533764ca0c286ec0d34b3462d7e64e3713d9..9437a5eb07cff63062ed459afc0b6b90e685e6f7 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -737,12 +737,20 @@ static void blk_mq_requeue_work(struct work_struct *work)
        spin_unlock_irq(&q->requeue_lock);
 
        list_for_each_entry_safe(rq, next, &rq_list, queuelist) {
-               if (!(rq->rq_flags & RQF_SOFTBARRIER))
+               if (!(rq->rq_flags & (RQF_SOFTBARRIER | RQF_DONTPREP)))
                        continue;
 
                rq->rq_flags &= ~RQF_SOFTBARRIER;
                list_del_init(&rq->queuelist);
-               blk_mq_sched_insert_request(rq, true, false, false);
+               /*
+                * If RQF_DONTPREP, rq has contained some driver specific
+                * data, so insert it to hctx dispatch list to avoid any
+                * merge.
+                */
+               if (rq->rq_flags & RQF_DONTPREP)
+                       blk_mq_request_bypass_insert(rq, false);
+               else
+                       blk_mq_sched_insert_request(rq, true, false, false);
        }
 
        while (!list_empty(&rq_list)) {