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Date: Fri, 31 Aug 2018 16:22:44 -0400 From: Dennis Zhou <dennisszhou@...il.com> To: Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>, Josef Bacik <josef@...icpanda.com> Cc: kernel-team@...com, linux-block@...r.kernel.org, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org, "Dennis Zhou (Facebook)" <dennisszhou@...il.com>, Jiufei Xue <jiufei.xue@...ux.alibaba.com>, Joseph Qi <joseph.qi@...ux.alibaba.com> Subject: [PATCH 3/3] blkcg: use tryget logic when associating a blkg with a bio 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. v2: - In blk-throttle, be explicit that we only associate with the root_blkg if tryget failed. 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..01d0620a4e4a 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)) == -ENODEV)) + 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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