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    SubjectRe: [PATCH 03/15] blkcg: use tryget logic when associating a blkg with a bio
    On Thu, Aug 30, 2018 at 09:53:44PM -0400, Dennis Zhou wrote:
    > 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);

    Except if we've already assocated a blkg this is just extra, can we do

    if (bio->bi_css && (bio_associate_blkg(bio, tg_to_blkg(tg)) == -ENODEV))

    to make it clear that we're only attaching it to the root if we failed to attach
    a blkg at all? Thanks,

    Josef

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