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    Subject[PATCH 03/15] blkcg: use tryget logic when associating a blkg with a bio
    Date
    From: "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>

    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: 08e18eab0c579 ("block: add bi_blkg to the bio for cgroups")
    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>
    ---
    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..c626e1f7cdcd 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)))
    + bio_associate_blkg(bio, bio->bi_disk->queue->root_blkg);
    bio_issue_init(&bio->bi_issue, bio_sectors(bio));
    #endif
    }
    --
    2.17.1
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