From 27e6fa996c534c32702aa4d32db0ffa383acd050 Mon Sep 17 00:00:00 2001 From: "Dennis Zhou (Facebook)" Date: Tue, 11 Sep 2018 14:41:26 -0400 Subject: 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 Acked-by: Tejun Heo Signed-off-by: Jens Axboe --- block/cfq-iosched.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'block/cfq-iosched.c') 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; -- cgit v1.2.3