[PATCH BUGFIX IMPROVEMENT 10/14] block, bfq: fix queue removal from weights tree

[Paolo Valente]

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@xxxxxxxxxx>
---
 block/bfq-iosched.c | 17 +++++++++++++----
 block/bfq-wf2q.c    |  6 +++---
 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);
 }
 
 /*
-- 
2.20.1