blkcg: use tryget logic when associating a blkg with a bio

dennisszhou · axboe · commit 3111885015b4 · 2018-08-31T14:48:58.000-06:00
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: 08e18ea ("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>
diff --git 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;
 }
diff --git 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
 }

Original file line number	Diff line number	Diff line change
`@@ -2015,7 +2015,8 @@ int bio_associate_blkg(struct bio bio, struct blkcg_gq blkg)`
`2015`	`2015`	`{`
`2016`	`2016`	`if (unlikely(bio->bi_blkg))`
`2017`	`2017`	`return -EBUSY;`
`2018`		`- blkg_get(blkg);`
	`2018`	`+ if (!blkg_try_get(blkg))`
	`2019`	`+ return -ENODEV;`
`2019`	`2020`	`bio->bi_blkg = blkg;`
`2020`	`2021`	`return 0;`
`2021`	`2022`	`}`
Original file line number	Diff line number	Diff line change
`@@ -2129,8 +2129,9 @@ static inline void throtl_update_latency_buckets(struct throtl_data *td)`
`2129`	`2129`	`static void blk_throtl_assoc_bio(struct throtl_grp tg, struct bio bio)`
`2130`	`2130`	`{`
`2131`	`2131`	`#ifdef CONFIG_BLK_DEV_THROTTLING_LOW`
`2132`		`- if (bio->bi_css)`
`2133`		`- bio_associate_blkg(bio, tg_to_blkg(tg));`
	`2132`	`+ /* fallback to root_blkg if we fail to get a blkg ref */`
	`2133`	`+ if (bio->bi_css && (bio_associate_blkg(bio, tg_to_blkg(tg)) == -ENODEV))`
	`2134`	`+ bio_associate_blkg(bio, bio->bi_disk->queue->root_blkg);`
`2134`	`2135`	`bio_issue_init(&bio->bi_issue, bio_sectors(bio));`
`2135`	`2136`	`#endif`
`2136`	`2137`	`}`