block, bfq: move forward the getting of an extra ref in bfq_bfqq_move

Commit ecedd3d7e199 ("block, bfq: get extra ref to prevent a queue
from being freed during a group move") gets an extra reference to a
bfq_queue before possibly deactivating it (temporarily), in
bfq_bfqq_move(). This prevents the bfq_queue from disappearing before
being reactivated in its new group.

Yet, the bfq_queue may also be expired (i.e., its service may be
stopped) before the bfq_queue is deactivated. And also an expiration
may lead to a premature freeing. This commit fixes this issue by
simply moving forward the getting of the extra reference already
introduced by commit ecedd3d7e199 ("block, bfq: get extra ref to
prevent a queue from being freed during a group move").

Reported-by: cki-project@redhat.com
Tested-by: cki-project@redhat.com
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 f0ff6654af2894359e54395fe72d0ce03f207406..9d963ed518d1f218e1dcab66ab53f36c2e39df5c 100644 (file)
--- a/block/bfq-cgroup.c
+++ b/block/bfq-cgroup.c
@@ -642,6 +642,12 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 {
        struct bfq_entity *entity = &bfqq->entity;
 
+       /*
+        * Get extra reference to prevent bfqq from being freed in
+        * next possible expire or deactivate.
+        */
+       bfqq->ref++;
+
        /* If bfqq is empty, then bfq_bfqq_expire also invokes
         * bfq_del_bfqq_busy, thereby removing bfqq and its entity
         * from data structures related to current group. Otherwise we
@@ -652,12 +658,6 @@ 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_or_in_serv)
@@ -677,7 +677,7 @@ 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 */
+       /* release extra ref taken above, bfqq may happen to be freed now */
        bfq_put_queue(bfqq);
 }