From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dennis Zhou Subject: [PATCH 3/3] blkcg: use tryget logic when associating a blkg with a bio Date: Fri, 31 Aug 2018 16:22:44 -0400 Message-ID: <20180831202244.21678-4-dennisszhou@gmail.com> References: <20180831202244.21678-1-dennisszhou@gmail.com> Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=from:to:cc:subject:date:message-id:in-reply-to:references; bh=4ycn/g7fw8SqfXjt7f6eHXQQdDKhRrTOaowx1xDnWeg=; b=bZuVkyCU1I70NNU/Nt6qA+o76T3OmFM2XGSQlTC/lPJT7C5jA/ts+UF53fAXi4g3tG 0Q2vQwCwAOE53QDwEOSFpaV0duuAdctBWbjH9Z+cZK5/PkbO2nqkAnO2Zz5Hj6REO2B2 Q771+5202p1QLs/shU8sZyELGxXPSy33lvsMWYvYk9LLDCwyJdcJDjgR432TelMGTm4n 2wcf8pmAJUalwtdh61M77DtcLaXCGL0/saDQ++CaWXD67LXH+uu58ycs24LQgz+/5fEy 1D3UR5eGeoKR8zwFnozt+gSTc0I6HMXMVho2482Fx0l6F8WBILRUmnmUpUJUODnwdPfn pG0g== In-Reply-To: <20180831202244.21678-1-dennisszhou@gmail.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Jens Axboe , Tejun Heo , Johannes Weiner , Josef Bacik Cc: kernel-team@fb.com, linux-block@vger.kernel.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, "Dennis Zhou (Facebook)" , Jiufei Xue , Joseph Qi From: "Dennis Zhou (Facebook)" 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 Cc: Jiufei Xue Cc: Joseph Qi Cc: Tejun Heo Cc: Josef Bacik Cc: Jens Axboe --- 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