[PATCH] blk-iocost: fix the lock order in ioc_cost_model_write()

[PATCH] blk-iocost: fix the lock order in ioc_cost_model_write()

[Tao Cui]

ioc_cost_model_write() holds rq_qos_mutex, which is acquired by
blkg_conf_open_bdev(), across blk_mq_freeze_queue(). ioc_qos_write()
freezes the queue before taking the mutex instead.

The inversion cannot currently deadlock because blk_mq_freeze_queue()
is in a noio scope since 1e1a9cecfa ("block: force noio scope in
blk_mq_freeze_queue"), so a caller freezing a queue cannot recurse
into the frozen queue through reclaim. Holding the mutex across
the freeze needlessly serializes config writers behind it and
leaves two different orderings in the same file. Release the mutex
around the freeze to match ioc_qos_write(), mirroring the fix there
in 9730763f4756 ("block: correct locking order for protecting
blk-wbt parameters").

Reported-by: sashiko-bot@kernel.org
Link: https://sashiko.dev/#/patchset/20261103132613.1952975-1-cui.tao@linux.dev?part=1
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
 block/blk-iocost.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 1c1025157d8e..7fe0a6fa146e 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -3605,7 +3605,14 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
 		ioc = q_to_ioc(q);
 	}
 
+	/*
+	 * Nothing in the cost model is protected yet; release rq_qos_mutex
+	 * around the freeze to match ioc_qos_write().
+	 */
+	mutex_unlock(&q->rq_qos_mutex);
 	memflags = blk_mq_freeze_queue(q);
+	mutex_lock(&q->rq_qos_mutex);
+
 	blk_mq_quiesce_queue(q);
 
 	spin_lock_irq(&ioc->lock);
-- 
2.53.0

Re: [PATCH] blk-iocost: fix the lock order in ioc_cost_model_write()

[Christoph Hellwig]

On Thu, Nov 12, 2026 at 12:21:37AM +0800, Tao Cui wrote:
> ioc_cost_model_write() holds rq_qos_mutex, which is acquired by
> blkg_conf_open_bdev(), across blk_mq_freeze_queue(). ioc_qos_write()
> freezes the queue before taking the mutex instead.
> 
> The inversion cannot currently deadlock because blk_mq_freeze_queue()
> is in a noio scope since 1e1a9cecfa ("block: force noio scope in
> blk_mq_freeze_queue"), so a caller freezing a queue cannot recurse
> into the frozen queue through reclaim. Holding the mutex across
> the freeze needlessly serializes config writers behind it and
> leaves two different orderings in the same file. Release the mutex
> around the freeze to match ioc_qos_write(), mirroring the fix there
> in 9730763f4756 ("block: correct locking order for protecting
> blk-wbt parameters").
> 
> Reported-by: sashiko-bot@kernel.org
> Link: https://sashiko.dev/#/patchset/20261103132613.1952975-1-cui.tao@linux.dev?part=1
> Signed-off-by: Tao Cui <cuitao@kylinos.cn>
> ---
>  block/blk-iocost.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
> index 1c1025157d8e..7fe0a6fa146e 100644
> --- a/block/blk-iocost.c
> +++ b/block/blk-iocost.c
> @@ -3605,7 +3605,14 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
>  		ioc = q_to_ioc(q);
>  	}
>  
> +	/*
> +	 * Nothing in the cost model is protected yet; release rq_qos_mutex
> +	 * around the freeze to match ioc_qos_write().
> +	 */
> +	mutex_unlock(&q->rq_qos_mutex);
>  	memflags = blk_mq_freeze_queue(q);
> +	mutex_lock(&q->rq_qos_mutex);

blkg_conf_open_bdev takes rq_qos_mutex aronud the disk_live
call.  So you'll have to redo it here.  Or maybe we should just move
that check and the locking into the callers of blkg_conf_open_bdev
in general to better support the need for freezing and other
potentially nested locking?