block, bfq: fix queue removal from weights tree

bfq maintains an ordered list, through a red-black tree, of unique
weights of active bfq_queues. This list is used to detect whether there
are active queues with differentiated weights. The weight of a queue is
removed from the list when both the following two conditions become
true:

(1) the bfq_queue is flagged as inactive
(2) the has no in-flight request any longer;

Unfortunately, in the rare cases where condition (2) becomes true before
condition (1), the removal fails, because the function to remove the
weight of the queue (bfq_weights_tree_remove) is rightly invoked in the
path that deactivates the bfq_queue, but mistakenly invoked *before* the
function that actually performs the deactivation (bfq_deactivate_bfqq).

This commits moves the invocation of bfq_weights_tree_remove for
condition (1) to after bfq_deactivate_bfqq. As a consequence of this
move, it is necessary to add a further reference to the queue when the
weight of a queue is added, because the queue might otherwise be freed
before bfq_weights_tree_remove is invoked. This commit adds this
reference and makes all related modifications.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

2 files changed, 16 insertions, 7 deletions

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 12228af16198..bf585ad29bb5 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -754,6 +754,7 @@ void bfq_weights_tree_add(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 
 inc_counter:
         bfqq->weight_counter->num_active++;
+        bfqq->ref++;
 }
 
 /*
@@ -778,6 +779,7 @@ void __bfq_weights_tree_remove(struct bfq_data *bfqd,
 
 reset_entity_pointer:
         bfqq->weight_counter = NULL;
+        bfq_put_queue(bfqq);
 }
 
 /*
@@ -789,9 +791,6 @@ void bfq_weights_tree_remove(struct bfq_data *bfqd,
 {
         struct bfq_entity *entity = bfqq->entity.parent;
 
-        __bfq_weights_tree_remove(bfqd, bfqq,
-                                  &bfqd->queue_weights_tree);
-
         for_each_entity(entity) {
                 struct bfq_sched_data *sd = entity->my_sched_data;
 
@@ -825,6 +824,15 @@ void bfq_weights_tree_remove(struct bfq_data *bfqd,
                         bfqd->num_groups_with_pending_reqs--;
                 }
         }
+
+        /*
+         * Next function is invoked last, because it causes bfqq to be
+         * freed if the following holds: bfqq is not in service and
+         * has no dispatched request. DO NOT use bfqq after the next
+         * function invocation.
+         */
+        __bfq_weights_tree_remove(bfqd, bfqq,
+                                  &bfqd->queue_weights_tree);
 }
 
 /*
@@ -1020,7 +1028,8 @@ bfq_bfqq_resume_state(struct bfq_queue *bfqq, struct bfq_data *bfqd,
 
 static int bfqq_process_refs(struct bfq_queue *bfqq)
 {
-        return bfqq->ref - bfqq->allocated - bfqq->entity.on_st;
+        return bfqq->ref - bfqq->allocated - bfqq->entity.on_st -
+                (bfqq->weight_counter != NULL);
 }
 
 /* Empty burst list and add just bfqq (see comments on bfq_handle_burst) */
diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
index ce37d709a34f..63311d1ff1ed 100644
--- a/block/bfq-wf2q.c
+++ b/block/bfq-wf2q.c
@@ -1673,15 +1673,15 @@ void bfq_del_bfqq_busy(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 
         bfqd->busy_queues[bfqq->ioprio_class - 1]--;
 
-        if (!bfqq->dispatched)
-                bfq_weights_tree_remove(bfqd, bfqq);
-
         if (bfqq->wr_coeff > 1)
                 bfqd->wr_busy_queues--;
 
         bfqg_stats_update_dequeue(bfqq_group(bfqq));
 
         bfq_deactivate_bfqq(bfqd, bfqq, true, expiration);
+
+        if (!bfqq->dispatched)
+                bfq_weights_tree_remove(bfqd, bfqq);
 }
 
 /*