linux-kernel - Re: [PATCH 0/4] blk-rq-qos: fix possible deadlock

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Message-ID: <d4fe218b-9fc5-4466-ac56-0d4c5a8ccd96@linux.ibm.com>
Date: Tue, 14 Oct 2025 16:28:18 +0530
From: Nilay Shroff <nilay@...ux.ibm.com>
To: Yu Kuai <yukuai3@...wei.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, yukuai1@...weicloud.com,
        yi.zhang@...wei.com, yangerkun@...wei.com, johnny.chenyi@...wei.com
Subject: Re: [PATCH 0/4] blk-rq-qos: fix possible deadlock



On 10/14/25 7:51 AM, 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
>   delaying after wbt_init();
> - For other policies, they can only be initialized by blkg configuration,
>   fix it by delaying to blkg_conf_end();
> 
> Noted this set is cooked on the top of my other thread:
> https://lore.kernel.org/all/20251010091446.3048529-1-yukuai@kernel.org/
> 
> And the deadlock can be reporduced with above thead, by running blktests
> throtl/001 with wbt enabled by default. While the deadlock is really a
> long term problem.
> 
While freezing the queue we also mark GFP_NOIO scope, so doesn't that
help avoid fs-reclaim? Or maybe if you can share the lockdep splat 
encountered running throtl/001?

Thanks,
--Nilay