• Ming Lei's avatar
    blk-mq: introduce BLK_STS_DEV_RESOURCE · 86ff7c2a
    Ming Lei authored
    
    
    This status is returned from driver to block layer if device related
    resource is unavailable, but driver can guarantee that IO dispatch
    will be triggered in future when the resource is available.
    
    Convert some drivers to return BLK_STS_DEV_RESOURCE.  Also, if driver
    returns BLK_STS_RESOURCE and SCHED_RESTART is set, rerun queue after
    a delay (BLK_MQ_DELAY_QUEUE) to avoid IO stalls.  BLK_MQ_DELAY_QUEUE is
    3 ms because both scsi-mq and nvmefc are using that magic value.
    
    If a driver can make sure there is in-flight IO, it is safe to return
    BLK_STS_DEV_RESOURCE because:
    
    1) If all in-flight IOs complete before examining SCHED_RESTART in
    blk_mq_dispatch_rq_list(), SCHED_RESTART must be cleared, so queue
    is run immediately in this case by blk_mq_dispatch_rq_list();
    
    2) if there is any in-flight IO after/when examining SCHED_RESTART
    in blk_mq_dispatch_rq_list():
    - if SCHED_RESTART isn't set, queue is run immediately as handled in 1)
    - otherwise, this request will be dispatched after any in-flight IO is
      completed via blk_mq_sched_restart()
    
    3) if SCHED_RESTART is set concurently in context because of
    BLK_STS_RESOURCE, blk_mq_delay_run_hw_queue() will cover the above two
    cases and make sure IO hang can be avoided.
    
    One invariant is that queue will be rerun if SCHED_RESTART is set.
    Suggested-by: default avatarJens Axboe <axboe@kernel.dk>
    Tested-by: default avatarLaurence Oberman <loberman@redhat.com>
    Signed-off-by: default avatarMing Lei <ming.lei@redhat.com>
    Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    86ff7c2a