Commit 31118850 authored Aug 31, 2018 by Dennis Zhou (Facebook) Committed by Jens Axboe Aug 31, 2018

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: 08e18eab ("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>

parent 59b57717

block/bio.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -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;
		}

block/blk-throttle.c

+3 −2

Original line number	Diff line number	Diff line
		@@ -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
		}

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