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    Subject[PATCH 6.0 173/862] blk-wbt: call rq_qos_add() after wb_normal is initialized
    Date
    From: Yu Kuai <yukuai3@huawei.com>

    commit 8c5035dfbb9475b67c82b3fdb7351236525bf52b upstream.

    Our test found a problem that wbt inflight counter is negative, which
    will cause io hang(noted that this problem doesn't exist in mainline):

    t1: device create t2: issue io
    add_disk
    blk_register_queue
    wbt_enable_default
    wbt_init
    rq_qos_add
    // wb_normal is still 0
    /*
    * in mainline, disk can't be opened before
    * bdev_add(), however, in old kernels, disk
    * can be opened before blk_register_queue().
    */
    blkdev_issue_flush
    // disk size is 0, however, it's not checked
    submit_bio_wait
    submit_bio
    blk_mq_submit_bio
    rq_qos_throttle
    wbt_wait
    bio_to_wbt_flags
    rwb_enabled
    // wb_normal is 0, inflight is not increased

    wbt_queue_depth_changed(&rwb->rqos);
    wbt_update_limits
    // wb_normal is initialized
    rq_qos_track
    wbt_track
    rq->wbt_flags |= bio_to_wbt_flags(rwb, bio);
    // wb_normal is not 0,wbt_flags will be set
    t3: io completion
    blk_mq_free_request
    rq_qos_done
    wbt_done
    wbt_is_tracked
    // return true
    __wbt_done
    wbt_rqw_done
    atomic_dec_return(&rqw->inflight);
    // inflight is decreased

    commit 8235b5c1e8c1 ("block: call bdev_add later in device_add_disk") can
    avoid this problem, however it's better to fix this problem in wbt:

    1) Lower kernel can't backport this patch due to lots of refactor.
    2) Root cause is that wbt call rq_qos_add() before wb_normal is
    initialized.

    Fixes: e34cbd307477 ("blk-wbt: add general throttling mechanism")
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Yu Kuai <yukuai3@huawei.com>
    Link: https://lore.kernel.org/r/20220913105749.3086243-1-yukuai1@huaweicloud.com
    Signed-off-by: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    block/blk-wbt.c | 9 ++++-----
    1 file changed, 4 insertions(+), 5 deletions(-)

    --- a/block/blk-wbt.c
    +++ b/block/blk-wbt.c
    @@ -843,6 +843,10 @@ int wbt_init(struct request_queue *q)
    rwb->enable_state = WBT_STATE_ON_DEFAULT;
    rwb->wc = 1;
    rwb->rq_depth.default_depth = RWB_DEF_DEPTH;
    + rwb->min_lat_nsec = wbt_default_latency_nsec(q);
    +
    + wbt_queue_depth_changed(&rwb->rqos);
    + wbt_set_write_cache(q, test_bit(QUEUE_FLAG_WC, &q->queue_flags));

    /*
    * Assign rwb and add the stats callback.
    @@ -853,11 +857,6 @@ int wbt_init(struct request_queue *q)

    blk_stat_add_callback(q, rwb->cb);

    - rwb->min_lat_nsec = wbt_default_latency_nsec(q);
    -
    - wbt_queue_depth_changed(&rwb->rqos);
    - wbt_set_write_cache(q, test_bit(QUEUE_FLAG_WC, &q->queue_flags));
    -
    return 0;

    err_free:

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