• Jianchao Wang's avatar
    blk-mq: insert rq with DONTPREP to hctx dispatch list when requeue · aef1897c
    Jianchao Wang authored
    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: default avatarJens Axboe <axboe@kernel.dk>
    Signed-off-by: default avatarJianchao Wang <jianchao.w.wang@oracle.com>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    aef1897c
blk-mq.c 84.1 KB