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[ Upstream commit 85bd6e61 ]

Florian reported a io hung issue when fsync(). It should be
triggered by following race condition.

data + post flush         a flush

blk_flush_complete_seq
  case REQ_FSEQ_DATA
    blk_flush_queue_rq
    issued to driver      blk_mq_dispatch_rq_list
                            try to issue a flush req
                            failed due to NON-NCQ command
                            .queue_rq return BLK_STS_DEV_RESOURCE

request completion
  req->end_io // doesn't check RESTART
  mq_flush_data_end_io
    case REQ_FSEQ_POSTFLUSH
      blk_kick_flush
        do nothing because previous flush
        has not been completed
     blk_mq_run_hw_queue
                              insert rq to hctx->dispatch
                              due to RESTART is still set, do nothing

To fix this, replace the blk_mq_run_hw_queue in mq_flush_data_end_io
with blk_mq_sched_restart to check and clear the RESTART flag.

Fi...