linux-kernel - [PATCH 03/10] blk-cgroup: don't nest queu_lock under rcu in bio_associate

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Message-Id: <20250925081525.700639-4-yukuai1@huaweicloud.com>
Date: Thu, 25 Sep 2025 16:15:18 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: tj@...nel.org,
	ming.lei@...hat.com,
	nilay@...ux.ibm.com,
	hch@....de,
	josef@...icpanda.com,
	axboe@...nel.dk,
	akpm@...ux-foundation.org,
	vgoyal@...hat.com
Cc: cgroups@...r.kernel.org,
	linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	yukuai3@...wei.com,
	yukuai1@...weicloud.com,
	yi.zhang@...wei.com,
	yangerkun@...wei.com,
	johnny.chenyi@...wei.com
Subject: [PATCH 03/10] blk-cgroup: don't nest queu_lock under rcu in bio_associate_blkg()

From: Yu Kuai <yukuai3@...wei.com>

If bio is already associated with blkg, blkcg is already pinned until
bio is done, no need for rcu protection; Otherwise protect blkcg_css()
with rcu independently. Prepare to convert protecting blkcg with
blkcg_mutex instead of queue_lock.

Signed-off-by: Yu Kuai <yukuai3@...wei.com>
---
 block/blk-cgroup.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 9ffc3195f7e4..53a64bfe4a24 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -2165,16 +2165,20 @@ void bio_associate_blkg(struct bio *bio)
 	if (blk_op_is_passthrough(bio->bi_opf))
 		return;
 
-	rcu_read_lock();
-
-	if (bio->bi_blkg)
+	if (bio->bi_blkg) {
 		css = bio_blkcg_css(bio);
-	else
+		bio_associate_blkg_from_css(bio, css);
+	} else {
+		rcu_read_lock();
 		css = blkcg_css();
+		if (!css_tryget_online(css))
+			css = NULL;
+		rcu_read_unlock();
 
-	bio_associate_blkg_from_css(bio, css);
-
-	rcu_read_unlock();
+		bio_associate_blkg_from_css(bio, css);
+		if (css)
+			css_put(css);
+	}
 }
 EXPORT_SYMBOL_GPL(bio_associate_blkg);
 
-- 
2.39.2