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

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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: 08e18eab0c ("block: add bi_blkg to the bio for cgroups")
Reviewed-by: Josef Bacik <[email protected]>
Signed-off-by: Dennis Zhou <[email protected]>
Cc: Jiufei Xue <[email protected]>
Cc: Joseph Qi <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Josef Bacik <[email protected]>
Cc: Jens Axboe <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
This commit is contained in:
Dennis Zhou (Facebook) authored and Jens Axboe committed 2018-08-31 14:48:58 -06:00
1 parent 59b57717ff
commit 3111885015
2 files changed
+5 -3
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block/bio.c
+2 -1
View File
@@ -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
View File
@@ -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
}