Commit aef1897c authored Feb 12, 2019 by Jianchao Wang Committed by Jens Axboe Feb 11, 2019

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>

parent 26984841

block/blk-mq.c

+10 −2

Original line number	Diff line number	Diff line
		@@ -737,11 +737,19 @@ 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);
		/*
		* 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);
		}