From 3111885015b458c97b4cf272e2a87f1d6f0ed06a Mon Sep 17 00:00:00 2001
From: "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>
Date: Fri, 31 Aug 2018 16:22:44 -0400
Subject: blkcg: use tryget logic when associating a blkg with a bio

There is a very small change a bio gets caught up in a really
unfortunate race between a task migration, cgroup exiting, and itself
trying to associate with a blkg. This is due to css offlining being
performed after the css->refcnt is killed which triggers removal of
blkgs that reach their blkg->refcnt of 0.

To avoid this, association with a blkg should use tryget and fallback to
using the root_blkg.

Fixes: 08e18eab0c579 ("block: add bi_blkg to the bio for cgroups")
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Dennis Zhou <dennisszhou@gmail.com>
Cc: Jiufei Xue <jiufei.xue@linux.alibaba.com>
Cc: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 block/bio.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

(limited to 'block/bio.c')

diff --git a/block/bio.c b/block/bio.c
index b12966e415d3..8c680a776171 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -2015,7 +2015,8 @@ int bio_associate_blkg(struct bio *bio, struct blkcg_gq *blkg)
 {
 	if (unlikely(bio->bi_blkg))
 		return -EBUSY;
-	blkg_get(blkg);
+	if (!blkg_try_get(blkg))
+		return -ENODEV;
 	bio->bi_blkg = blkg;
 	return 0;
 }
-- 
cgit