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/bio.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) (limited to 'block/bio.c') diff --git a/block/bio.c b/block/bio.c index 81c05ee51d6c..083f1c9cde0a 100644 --- a/block/bio.c +++ b/block/bio.c @@ -1988,13 +1988,19 @@ int bio_associate_blkcg_from_page(struct bio *bio, struct page *page) * * This function takes an extra reference of @blkcg_css which will be put * when @bio is released. The caller must own @bio and is responsible for - * synchronizing calls to this function. + * synchronizing calls to this function. If @blkcg_css is NULL, a call to + * blkcg_get_css finds the current css from the kthread or task. */ int bio_associate_blkcg(struct bio *bio, struct cgroup_subsys_state *blkcg_css) { if (unlikely(bio->bi_css)) return -EBUSY; - css_get(blkcg_css); + + if (blkcg_css) + css_get(blkcg_css); + else + blkcg_css = blkcg_get_css(); + bio->bi_css = blkcg_css; return 0; } -- cgit v1.2.3