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Date: Fri, 31 Aug 2018 11:30:08 -0400 From: Josef Bacik <josef@...icpanda.com> To: Dennis Zhou <dennisszhou@...il.com> Cc: Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>, Josef Bacik <josef@...icpanda.com>, kernel-team@...com, linux-block@...r.kernel.org, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org, Jiufei Xue <jiufei.xue@...ux.alibaba.com>, Joseph Qi <joseph.qi@...ux.alibaba.com> Subject: Re: [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@...il.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@...il.com> > Cc: Jiufei Xue <jiufei.xue@...ux.alibaba.com> > Cc: Joseph Qi <joseph.qi@...ux.alibaba.com> > Cc: Tejun Heo <tj@...nel.org> > Cc: Josef Bacik <josef@...icpanda.com> > Cc: Jens Axboe <axboe@...nel.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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