blkcg: fix ref count issue with bio_blkcg using task_css

The accessor function bio_blkcg either returns the blkcg associated with
the bio or finds one in the current context. This can cause an issue
when trying to associate a bio with a blkcg. Particularly, it's the
third case that is problematic:

	return css_to_blkcg(task_css(current, io_cgrp_id));

As the above may race against task migration and the cgroup exiting, it
is not always ok to take a reference on the blkcg returned from
bio_blkcg.

This patch adds association ahead of calling bio_blkcg rather than
after. This makes association a required and explicit step along the
code paths for calling bio_blkcg. blk_get_rl is modified as well to get
a reference to the blkcg it may use and blk_put_rl will always put the
reference back. Association is also moved above the bio_blkcg call to
ensure it will not return NULL in blk-iolatency.

BFQ and CFQ utilize this flaw, but due to the complexity, I do not want
to address this in this series. I've created a private version of the
function with notes not to use it describing the flaw. Hopefully soon,
that code can be cleaned up.

Signed-off-by: Dennis Zhou <dennisszhou@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 2 insertions, 2 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 2eb87444b157..d219e9a1af65 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3753,7 +3753,7 @@ static void check_blkcg_changed(struct cfq_io_cq *cic, struct bio *bio)
 	uint64_t serial_nr;
 
 	rcu_read_lock();
-	serial_nr = bio_blkcg(bio)->css.serial_nr;
+	serial_nr = __bio_blkcg(bio)->css.serial_nr;
 	rcu_read_unlock();
 
 	/*
@@ -3818,7 +3818,7 @@ cfq_get_queue(struct cfq_data *cfqd, bool is_sync, struct cfq_io_cq *cic,
 	struct cfq_group *cfqg;
 
 	rcu_read_lock();
-	cfqg = cfq_lookup_cfqg(cfqd, bio_blkcg(bio));
+	cfqg = cfq_lookup_cfqg(cfqd, __bio_blkcg(bio));
 	if (!cfqg) {
 		cfqq = &cfqd->oom_cfqq;
 		goto out;