[PATCH 5.4 103/214] bfq: fix blkio cgroup leakage v4

Greg Kroah-Hartman
2020-09-01T16:23:09Z

From: Dmitry Monakhov <dmtrmonakhov@yandex-team.ru>

[ Upstream commit 2de791ab4918969d8108f15238a701968375f235 ]

Changes from v1:
- update commit description with proper ref-accounting justification

commit db37a34c563b ("block, bfq: get a ref to a group when adding it to a service tree")
introduce leak forbfq_group and blkcg_gq objects because of get/put
imbalance.
In fact whole idea of original commit is wrong because bfq_group entity
can not dissapear under us because it is referenced by child bfq_queue's
entities from here:
-> bfq_init_entity()
->bfqg_and_blkg_get(bfqg);
->entity->parent = bfqg->my_entity

-> bfq_put_queue(bfqq)
FINAL_PUT
->bfqg_and_blkg_put(bfqq_group(bfqq))
->kmem_cache_free(bfq_pool, bfqq);

So parent entity can not disappear while child entity is in tree,
and child entities already has proper protection.
This patch revert commit db37a34c563b ("block, bfq: get a ref to a group when adding it to a service tree")

bfq_group leak trace caused by bad commit:
-> blkg_alloc
-> bfq_pq_alloc
-> bfqg_get (+1)
->bfq_activate_bfqq
->bfq_activate_requeue_entity
-> __bfq_activate_entity
->bfq_get_entity
->bfqg_and_blkg_get (+1) <==== : Note1
->bfq_del_bfqq_busy
->bfq_deactivate_entity+0x53/0xc0 [bfq]
->__bfq_deactivate_entity+0x1b8/0x210 [bfq]
-> bfq_forget_entity(is_in_service = true)
entity->on_st_or_in_serv = false <=== :Note2
if (is_in_service)
return; ==> do not touch reference
-> blkcg_css_offline
-> blkcg_destroy_blkgs
-> blkg_destroy
-> bfq_pd_offline
-> __bfq_deactivate_entity
if (!entity->on_st_or_in_serv) /* true, because (Note2)
return false;
-> bfq_pd_free
-> bfqg_put() (-1, byt bfqg->ref == 2) because of (Note2)
So bfq_group and blkcg_gq will leak forever, see test-case below.

##TESTCASE_BEGIN:
#!/bin/bash

max_iters=${1:-100}
#prep cgroup mounts
mount -t tmpfs cgroup_root /sys/fs/cgroup
mkdir /sys/fs/cgroup/blkio
mount -t cgroup -o blkio none /sys/fs/cgroup/blkio

# Prepare blkdev
grep blkio /proc/cgroups
truncate -s 1M img
losetup /dev/loop0 img
echo bfq > /sys/block/loop0/queue/scheduler

grep blkio /proc/cgroups
for ((i=0;i<max_iters;i++))
do
mkdir -p /sys/fs/cgroup/blkio/a
echo 0 > /sys/fs/cgroup/blkio/a/cgroup.procs
dd if=/dev/loop0 bs=4k count=1 of=/dev/null iflag=direct 2> /dev/null
echo 0 > /sys/fs/cgroup/blkio/cgroup.procs
rmdir /sys/fs/cgroup/blkio/a
grep blkio /proc/cgroups
done
##TESTCASE_END:

Fixes: db37a34c563b ("block, bfq: get a ref to a group when adding it to a service tree")
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Dmitry Monakhov <dmtrmonakhov@yandex-team.ru>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
block/bfq-cgroup.c | 2 +-
block/bfq-iosched.h | 1 -
block/bfq-wf2q.c | 12 ++----------
3 files changed, 3 insertions(+), 12 deletions(-)

diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
index 12b707a4e52fd..342a1cfa48c57 100644
--- a/block/bfq-cgroup.c
+++ b/block/bfq-cgroup.c
@@ -332,7 +332,7 @@ static void bfqg_put(struct bfq_group *bfqg)
kfree(bfqg);
}

-void bfqg_and_blkg_get(struct bfq_group *bfqg)
+static void bfqg_and_blkg_get(struct bfq_group *bfqg)
{
/* see comments in bfq_bic_update_cgroup for why refcounting bfqg */
bfqg_get(bfqg);
diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
index c0232975075d0..de98fdfe9ea17 100644
--- a/block/bfq-iosched.h
+++ b/block/bfq-iosched.h
@@ -980,7 +980,6 @@ struct bfq_group *bfq_find_set_group(struct bfq_data *bfqd,
struct blkcg_gq *bfqg_to_blkg(struct bfq_group *bfqg);
struct bfq_group *bfqq_group(struct bfq_queue *bfqq);
struct bfq_group *bfq_create_group_hierarchy(struct bfq_data *bfqd, int node);
-void bfqg_and_blkg_get(struct bfq_group *bfqg);
void bfqg_and_blkg_put(struct bfq_group *bfqg);

#ifdef CONFIG_BFQ_GROUP_IOSCHED
diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
index 44079147e396e..05f0bf4a1144d 100644
--- a/block/bfq-wf2q.c
+++ b/block/bfq-wf2q.c
@@ -536,9 +536,7 @@ static void bfq_get_entity(struct bfq_entity *entity)
bfqq->ref++;
bfq_log_bfqq(bfqq->bfqd, bfqq, "get_entity: %p %d",
bfqq, bfqq->ref);
- } else
- bfqg_and_blkg_get(container_of(entity, struct bfq_group,
- entity));
+ }
}

/**
@@ -652,14 +650,8 @@ static void bfq_forget_entity(struct bfq_service_tree *st,

entity->on_st = false;
st->wsum -= entity->weight;
- if (is_in_service)
- return;
-
- if (bfqq)
+ if (bfqq && !is_in_service)
bfq_put_queue(bfqq);
- else
- bfqg_and_blkg_put(container_of(entity, struct bfq_group,
- entity));
}

/**
--
2.25.1