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    Subject[PATCH 5.6 038/166] block, bfq: invoke flush_idle_tree after reparent_active_queues in pd_offline
    Date
    From: Paolo Valente <paolo.valente@linaro.org>

    commit 4d38a87fbb77fb9ff2ff4e914162a8ae6453eff5 upstream.

    In bfq_pd_offline(), the function bfq_flush_idle_tree() is invoked to
    flush the rb tree that contains all idle entities belonging to the pd
    (cgroup) being destroyed. In particular, bfq_flush_idle_tree() is
    invoked before bfq_reparent_active_queues(). Yet the latter may happen
    to add some entities to the idle tree. It happens if, in some of the
    calls to bfq_bfqq_move() performed by bfq_reparent_active_queues(),
    the queue to move is empty and gets expired.

    This commit simply reverses the invocation order between
    bfq_flush_idle_tree() and bfq_reparent_active_queues().

    Tested-by: cki-project@redhat.com
    Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
    Signed-off-by: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    block/bfq-cgroup.c | 20 +++++++++++++-------
    1 file changed, 13 insertions(+), 7 deletions(-)

    --- a/block/bfq-cgroup.c
    +++ b/block/bfq-cgroup.c
    @@ -894,13 +894,6 @@ static void bfq_pd_offline(struct blkg_p
    st = bfqg->sched_data.service_tree + i;

    /*
    - * The idle tree may still contain bfq_queues belonging
    - * to exited task because they never migrated to a different
    - * cgroup from the one being destroyed now.
    - */
    - bfq_flush_idle_tree(st);
    -
    - /*
    * It may happen that some queues are still active
    * (busy) upon group destruction (if the corresponding
    * processes have been forced to terminate). We move
    @@ -913,6 +906,19 @@ static void bfq_pd_offline(struct blkg_p
    * scheduler has taken no reference.
    */
    bfq_reparent_active_queues(bfqd, bfqg, st, i);
    +
    + /*
    + * The idle tree may still contain bfq_queues
    + * belonging to exited task because they never
    + * migrated to a different cgroup from the one being
    + * destroyed now. In addition, even
    + * bfq_reparent_active_queues() may happen to add some
    + * entities to the idle tree. It happens if, in some
    + * of the calls to bfq_bfqq_move() performed by
    + * bfq_reparent_active_queues(), the queue to move is
    + * empty and gets expired.
    + */
    + bfq_flush_idle_tree(st);
    }

    __bfq_deactivate_entity(entity, false);

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