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Commit 31118850 authored by Dennis Zhou (Facebook)'s avatar Dennis Zhou (Facebook) Committed by Jens Axboe
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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: default avatarJosef Bacik <josef@toxicpanda.com>
Signed-off-by: default avatarDennis 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: default avatarJens 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

}