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Sender: Tejun Heo <htejun@gmail.com>
Date:   Thu, 5 Jan 2023 08:34:17 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Yu Kuai <yukuai1@huaweicloud.com>
Cc:     hch@infradead.org, josef@toxicpanda.com, axboe@kernel.dk,
        cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, yi.zhang@huawei.com,
        yangerkun@huawei.com, "yukuai (C)" <yukuai3@huawei.com>
Subject: Re: [PATCH -next 3/4] block/rq_qos: use a global mutex to protect
 rq_qos apis
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Hello,

On Thu, Jan 05, 2023 at 09:35:21AM +0800, Yu Kuai wrote:
> > Can you please take a look at the following patchset I just posted:
> > 
> >    https://lkml.kernel.org/r/20230105002007.157497-1-tj@kernel.org
> > 
> > After that, all these configuration operations are wrapped between
> > blkg_conf_init() and blkg_conf_exit() which probably are the right place to
> > implement the synchronization.
> 
> I see that, blkg_conf_init() and blkg_conf_exit() is good, however there
> are some details I want to confirm:
> 
> 1) rq_qos_add() can be called from iocost/iolatency, where
> blkg_conf_init() will be called first, while rq_qos_add() can also be
> called from wbt, where there is no blkg_conf_init(). Hence it seems to
> me we need two locks here, one to protect rq_qos apis; one to
> synchronize policy configuration and device removal.

wbt's lazy init is tied to one of the block device sysfs files, right? So,
it *should* already be protected against device removal.

Thanks.

-- 
tejun