[PATCH 3/3] blkcg: use tryget logic when associating a blkg with a bio

[Dennis Zhou]

From: "Dennis Zhou (Facebook)" <dennisszhou@xxxxxxxxx>

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.

v2:
 - In blk-throttle, be explicit that we only associate with the
   root_blkg if tryget failed.

Fixes: 08e18eab0c579 ("block: add bi_blkg to the bio for cgroups")
Signed-off-by: Dennis Zhou <dennisszhou@xxxxxxxxx>
Cc: Jiufei Xue <jiufei.xue@xxxxxxxxxxxxxxxxx>
Cc: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Josef Bacik <josef@xxxxxxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
---
 block/bio.c          | 3 ++-
 block/blk-throttle.c | 5 +++--
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 04969b392c72..4473ccd22987 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1987,7 +1987,8 @@ 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;
 }
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index a3eede00d302..01d0620a4e4a 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -2129,8 +2129,9 @@ static inline void throtl_update_latency_buckets(struct throtl_data *td)
 static void blk_throtl_assoc_bio(struct throtl_grp *tg, struct bio *bio)
 {
 #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
-	if (bio->bi_css)
-		bio_associate_blkg(bio, tg_to_blkg(tg));
+	/* fallback to root_blkg if we fail to get a blkg ref */
+	if (bio->bi_css && (bio_associate_blkg(bio, tg_to_blkg(tg)) == -ENODEV))
+		bio_associate_blkg(bio, bio->bi_disk->queue->root_blkg);
 	bio_issue_init(&bio->bi_issue, bio_sectors(bio));
 #endif
 }
-- 
2.17.1