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