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 <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

block/blk-iolatency.c

@@ -401,8 +401,8 @@ static void blkcg_iolatency_throttle(struct rq_qos *rqos, struct bio *bio,
 		return;
 
 	rcu_read_lock();
+	bio_associate_blkcg(bio, NULL);
 	blkcg = bio_blkcg(bio);
-	bio_associate_blkcg(bio, &blkcg->css);
 	blkg = blkg_lookup(blkcg, q);
 	if (unlikely(!blkg)) {
 		if (!lock)