linux-kernel - [PATCH 01/10] blk-cgroup: use cgroup lock and rcu to protect iterating blkcg blkgs

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Message-Id: <20250925081525.700639-2-yukuai1@huaweicloud.com>
Date: Thu, 25 Sep 2025 16:15:16 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: tj@...nel.org,
	ming.lei@...hat.com,
	nilay@...ux.ibm.com,
	hch@....de,
	josef@...icpanda.com,
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Cc: cgroups@...r.kernel.org,
	linux-block@...r.kernel.org,
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	yukuai3@...wei.com,
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Subject: [PATCH 01/10] blk-cgroup: use cgroup lock and rcu to protect iterating blkcg blkgs

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

It's safe to iterate blkgs with cgroup lock or rcu lock held, prevent
nested queue_lock under rcu lock, and prepare to convert protecting
blkcg with blkcg_mutex instead of queuelock.

Signed-off-by: Yu Kuai <yukuai3@...wei.com>
---
 block/blk-cgroup-rwstat.c |  2 --
 block/blk-cgroup.c        | 16 +++++-----------
 2 files changed, 5 insertions(+), 13 deletions(-)

diff --git a/block/blk-cgroup-rwstat.c b/block/blk-cgroup-rwstat.c
index a55fb0c53558..d41ade993312 100644
--- a/block/blk-cgroup-rwstat.c
+++ b/block/blk-cgroup-rwstat.c
@@ -101,8 +101,6 @@ void blkg_rwstat_recursive_sum(struct blkcg_gq *blkg, struct blkcg_policy *pol,
 	struct cgroup_subsys_state *pos_css;
 	unsigned int i;
 
-	lockdep_assert_held(&blkg->q->queue_lock);
-
 	memset(sum, 0, sizeof(*sum));
 	rcu_read_lock();
 	blkg_for_each_descendant_pre(pos_blkg, pos_css, blkg) {
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index f93de34fe87d..2d767ae61d2f 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -712,12 +712,9 @@ void blkcg_print_blkgs(struct seq_file *sf, struct blkcg *blkcg,
 	u64 total = 0;
 
 	rcu_read_lock();
-	hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node) {
-		spin_lock_irq(&blkg->q->queue_lock);
-		if (blkcg_policy_enabled(blkg->q, pol))
+	hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node)
+		if (blkcg_policy_enabled(blkg->q, pol) && blkg->online)
 			total += prfill(sf, blkg->pd[pol->plid], data);
-		spin_unlock_irq(&blkg->q->queue_lock);
-	}
 	rcu_read_unlock();
 
 	if (show_total)
@@ -1242,13 +1239,10 @@ static int blkcg_print_stat(struct seq_file *sf, void *v)
 	else
 		css_rstat_flush(&blkcg->css);
 
-	rcu_read_lock();
-	hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node) {
-		spin_lock_irq(&blkg->q->queue_lock);
+	spin_lock_irq(&blkcg->lock);
+	hlist_for_each_entry(blkg, &blkcg->blkg_list, blkcg_node)
 		blkcg_print_one_stat(blkg, sf);
-		spin_unlock_irq(&blkg->q->queue_lock);
-	}
-	rcu_read_unlock();
+	spin_unlock_irq(&blkcg->lock);
 	return 0;
 }
 
-- 
2.39.2