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

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Message-ID: <0351f07e-4ac4-a1e2-b4ee-08e49e7d0b6e@huaweicloud.com>
Date: Tue, 14 Oct 2025 16:42:30 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Ming Lei <ming.lei@...hat.com>, Yu Kuai <yukuai1@...weicloud.com>
Cc: nilay@...ux.ibm.com, tj@...nel.org, josef@...icpanda.com,
 axboe@...nel.dk, cgroups@...r.kernel.org, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, yi.zhang@...wei.com, yangerkun@...wei.com,
 johnny.chenyi@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH 3/4] blk-rq-qos: fix possible deadlock

Hi,

在 2025/10/14 16:37, Ming Lei 写道:
> On Tue, Oct 14, 2025 at 04:24:23PM +0800, Yu Kuai wrote:
>> Hi,
>>
>> 在 2025/10/14 16:13, Ming Lei 写道:
>>> 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@...wei.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.
>>>
>>
>> I think we can't grab rq_qos_mutex to create debugfs entries, right?
> 
> It depends on the finalized order between rq_qos_mutex and freezing queue.
> 
>> With the respect of this, perhaps we can grab debugfs_mutex to protect
>> insering rq_qos list instead?
> 
> No, debugfs_mutex shouldn't protect rq_qos list, and rq_qos_mutex is
> supposed to do the job at least from naming viewpoint.

Ok, then we'll have to make sure the order is rq_qos_mutex before
freezing queue, I was thinking the inverse order because of the helper
blkg_conf_open_bdev_frozen().

I'll check first if this is possible.

Thanks,
Kuai
> 
> 
> Thanks,
> Ming
> 
> 
> .
>