Re: [PATCH 3/4] blk-rq-qos: fix possible deadlock

[Ming Lei <ming.lei@redhat.com>]

On Tue, Oct 14, 2025 at 10:21:48AM +0800, Yu Kuai wrote:
> Currently rq-qos debugfs entries is created from rq_qos_add(), while
> rq_qos_add() requires queue to be freezed. This can deadlock because
> creating new entries can trigger fs reclaim.
> 
> Fix this problem by delaying creating rq-qos debugfs entries until
> it's initialization is complete.
> 
> - For wbt, it can be initialized by default of by blk-sysfs, fix it by
>   calling blk_mq_debugfs_register_rq_qos() after wbt_init;
> - For other policies, they can only be initialized by blkg configuration,
>   fix it by calling blk_mq_debugfs_register_rq_qos() from
>   blkg_conf_end();
> 
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>  block/blk-cgroup.c | 6 ++++++
>  block/blk-rq-qos.c | 7 -------
>  block/blk-sysfs.c  | 4 ++++
>  block/blk-wbt.c    | 7 ++++++-
>  4 files changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index d93654334854..e4ccabf132c0 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -33,6 +33,7 @@
>  #include "blk-cgroup.h"
>  #include "blk-ioprio.h"
>  #include "blk-throttle.h"
> +#include "blk-mq-debugfs.h"
>  
>  static void __blkcg_rstat_flush(struct blkcg *blkcg, int cpu);
>  
> @@ -746,6 +747,11 @@ void blkg_conf_end(struct blkg_conf_ctx *ctx)
>  	mutex_unlock(&q->elevator_lock);
>  	blk_mq_unfreeze_queue(q, ctx->memflags);
>  	blkdev_put_no_open(ctx->bdev);
> +
> +	mutex_lock(&q->debugfs_mutex);
> +	blk_mq_debugfs_register_rq_qos(q);
> +	mutex_unlock(&q->debugfs_mutex);
> +
>  }
>  EXPORT_SYMBOL_GPL(blkg_conf_end);
>  
> diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
> index 654478dfbc20..d7ce99ce2e80 100644
> --- a/block/blk-rq-qos.c
> +++ b/block/blk-rq-qos.c
> @@ -347,13 +347,6 @@ int rq_qos_add(struct rq_qos *rqos, struct gendisk *disk, enum rq_qos_id id,
>  	blk_queue_flag_set(QUEUE_FLAG_QOS_ENABLED, q);
>  
>  	blk_mq_unfreeze_queue(q, memflags);
> -
> -	if (rqos->ops->debugfs_attrs) {
> -		mutex_lock(&q->debugfs_mutex);
> -		blk_mq_debugfs_register_rqos(rqos);
> -		mutex_unlock(&q->debugfs_mutex);
> -	}
> -
>  	return 0;
>  ebusy:
>  	blk_mq_unfreeze_queue(q, memflags);
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 76c47fe9b8d6..52bb4db25cf5 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -688,6 +688,10 @@ static ssize_t queue_wb_lat_store(struct gendisk *disk, const char *page,
>  	mutex_unlock(&disk->rqos_state_mutex);
>  
>  	blk_mq_unquiesce_queue(q);
> +
> +	mutex_lock(&q->debugfs_mutex);
> +	blk_mq_debugfs_register_rq_qos(q);
> +	mutex_unlock(&q->debugfs_mutex);
>  out:
>  	blk_mq_unfreeze_queue(q, memflags);
>  
> diff --git a/block/blk-wbt.c b/block/blk-wbt.c
> index eb8037bae0bd..a120b5ba54db 100644
> --- a/block/blk-wbt.c
> +++ b/block/blk-wbt.c
> @@ -724,8 +724,13 @@ void wbt_enable_default(struct gendisk *disk)
>  	if (!blk_queue_registered(q))
>  		return;
>  
> -	if (queue_is_mq(q) && enable)
> +	if (queue_is_mq(q) && enable) {
>  		wbt_init(disk);
> +
> +		mutex_lock(&q->debugfs_mutex);
> +		blk_mq_debugfs_register_rq_qos(q);
> +		mutex_unlock(&q->debugfs_mutex);
> +	}

->debugfs_mutex only may be not enough, because blk_mq_debugfs_register_rq_qos()
has to traverse rq_qos single list list, you may have to grab q->rq_qos_mutex
for protect the list.


Thanks,
Ming