block, bfq: get extra ref to prevent a queue from being freed during a group move

In bfq_bfqq_move(), the bfq_queue, say Q, to be moved to a new group
may happen to be deactivated in the scheduling data structures of the
source group (and then activated in the destination group). If Q is
referred only by the data structures in the source group when the
deactivation happens, then Q is freed upon the deactivation.

This commit addresses this issue by getting an extra reference before
the possible deactivation, and releasing this extra reference after Q
has been moved.

Tested-by: Chris Evich <cevich@redhat.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
index e1419ed..8ab7f18 100644 (file)
--- a/block/bfq-cgroup.c
+++ b/block/bfq-cgroup.c
@@ -651,6 +651,12 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
                bfq_bfqq_expire(bfqd, bfqd->in_service_queue,
                                false, BFQQE_PREEMPTED);
 
+       /*
+        * get extra reference to prevent bfqq from being freed in
+        * next possible deactivate
+        */
+       bfqq->ref++;
+
        if (bfq_bfqq_busy(bfqq))
                bfq_deactivate_bfqq(bfqd, bfqq, false, false);
        else if (entity->on_st)
@@ -670,6 +676,8 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 
        if (!bfqd->in_service_queue && !bfqd->rq_in_driver)
                bfq_schedule_dispatch(bfqd);
+       /* release extra ref taken above */
+       bfq_put_queue(bfqq);
 }
 
 /**