linux-kernel - [PATCH 1/4] blk-mq-debugfs: warn about possible deadlock

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Message-ID: <20251014022149.947800-2-yukuai3@huawei.com>
Date: Tue, 14 Oct 2025 10:21:46 +0800
From: Yu Kuai <yukuai3@...wei.com>
To: <nilay@...ux.ibm.com>, <ming.lei@...hat.com>, <tj@...nel.org>,
	<josef@...icpanda.com>, <axboe@...nel.dk>
CC: <cgroups@...r.kernel.org>, <linux-block@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <yukuai3@...wei.com>,
	<yukuai1@...weicloud.com>, <yi.zhang@...wei.com>, <yangerkun@...wei.com>,
	<johnny.chenyi@...wei.com>
Subject: [PATCH 1/4] blk-mq-debugfs: warn about possible deadlock

Creating new debugfs entries can trigger fs reclaim, hence we can't do
this with queue freezed, meanwhile, other locks that can be held while
queue is freezed should not be held as well.

Signed-off-by: Yu Kuai <yukuai3@...wei.com>
---
 block/blk-mq-debugfs.c | 31 ++++++++++++++++++++++++-------
 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index 4896525b1c05..66864ed0b77f 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -608,9 +608,23 @@ static const struct blk_mq_debugfs_attr blk_mq_debugfs_ctx_attrs[] = {
 	{},
 };
 
-static void debugfs_create_files(struct dentry *parent, void *data,
+static void debugfs_create_files(struct request_queue *q, struct dentry *parent,
+				 void *data,
 				 const struct blk_mq_debugfs_attr *attr)
 {
+	/*
+	 * Creating new debugfs entries with queue freezed has the rist of
+	 * deadlock.
+	 */
+	WARN_ON_ONCE(q->mq_freeze_depth != 0);
+	/*
+	 * debugfs_mutex should not be nested under other locks that can be
+	 * grabbed while queue is freezed.
+	 */
+	lockdep_assert_not_held(&q->elevator_lock);
+	lockdep_assert_not_held(&q->rq_qos_mutex);
+	lockdep_assert_not_held(&q->blkcg_mutex);
+
 	if (IS_ERR_OR_NULL(parent))
 		return;
 
@@ -624,7 +638,7 @@ void blk_mq_debugfs_register(struct request_queue *q)
 	struct blk_mq_hw_ctx *hctx;
 	unsigned long i;
 
-	debugfs_create_files(q->debugfs_dir, q, blk_mq_debugfs_queue_attrs);
+	debugfs_create_files(q, q->debugfs_dir, q, blk_mq_debugfs_queue_attrs);
 
 	queue_for_each_hw_ctx(q, hctx, i) {
 		if (!hctx->debugfs_dir)
@@ -650,7 +664,8 @@ static void blk_mq_debugfs_register_ctx(struct blk_mq_hw_ctx *hctx,
 	snprintf(name, sizeof(name), "cpu%u", ctx->cpu);
 	ctx_dir = debugfs_create_dir(name, hctx->debugfs_dir);
 
-	debugfs_create_files(ctx_dir, ctx, blk_mq_debugfs_ctx_attrs);
+	debugfs_create_files(hctx->queue, ctx_dir, ctx,
+			     blk_mq_debugfs_ctx_attrs);
 }
 
 void blk_mq_debugfs_register_hctx(struct request_queue *q,
@@ -666,7 +681,8 @@ void blk_mq_debugfs_register_hctx(struct request_queue *q,
 	snprintf(name, sizeof(name), "hctx%u", hctx->queue_num);
 	hctx->debugfs_dir = debugfs_create_dir(name, q->debugfs_dir);
 
-	debugfs_create_files(hctx->debugfs_dir, hctx, blk_mq_debugfs_hctx_attrs);
+	debugfs_create_files(q, hctx->debugfs_dir, hctx,
+			     blk_mq_debugfs_hctx_attrs);
 
 	hctx_for_each_ctx(hctx, ctx, i)
 		blk_mq_debugfs_register_ctx(hctx, ctx);
@@ -717,7 +733,7 @@ void blk_mq_debugfs_register_sched(struct request_queue *q)
 
 	q->sched_debugfs_dir = debugfs_create_dir("sched", q->debugfs_dir);
 
-	debugfs_create_files(q->sched_debugfs_dir, q, e->queue_debugfs_attrs);
+	debugfs_create_files(q, q->sched_debugfs_dir, q, e->queue_debugfs_attrs);
 }
 
 void blk_mq_debugfs_unregister_sched(struct request_queue *q)
@@ -766,7 +782,8 @@ void blk_mq_debugfs_register_rqos(struct rq_qos *rqos)
 							 q->debugfs_dir);
 
 	rqos->debugfs_dir = debugfs_create_dir(dir_name, q->rqos_debugfs_dir);
-	debugfs_create_files(rqos->debugfs_dir, rqos, rqos->ops->debugfs_attrs);
+	debugfs_create_files(q, rqos->debugfs_dir, rqos,
+			     rqos->ops->debugfs_attrs);
 }
 
 void blk_mq_debugfs_register_sched_hctx(struct request_queue *q,
@@ -789,7 +806,7 @@ void blk_mq_debugfs_register_sched_hctx(struct request_queue *q,
 
 	hctx->sched_debugfs_dir = debugfs_create_dir("sched",
 						     hctx->debugfs_dir);
-	debugfs_create_files(hctx->sched_debugfs_dir, hctx,
+	debugfs_create_files(q, hctx->sched_debugfs_dir, hctx,
 			     e->hctx_debugfs_attrs);
 }
 
-- 
2.39.2