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Subject: Re: [PATCH -next v2 1/6] md: remove useless debug code to print
 configuration
To:     Song Liu <song@kernel.org>, Yu Kuai <yukuai1@huaweicloud.com>
Cc:     linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org,
        yi.zhang@huawei.com, yangerkun@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20231021102059.3198284-1-yukuai1@huaweicloud.com>
 <20231021102059.3198284-2-yukuai1@huaweicloud.com>
 <CAPhsuW5=fDpsAofik+4jHObFkRMcTTeQPbtXSBG_KAes0YgQGA@mail.gmail.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <1f3080ca-cde6-2473-4679-a79fa744eb70@huaweicloud.com>
Date:   Fri, 24 Nov 2023 17:12:30 +0800
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Hi,

在 2023/11/24 16:17, Song Liu 写道:
> On Fri, Oct 20, 2023 at 7:25 PM Yu Kuai <yukuai1@huaweicloud.com> wrote:
>>
>> From: Yu Kuai <yukuai3@huawei.com>
>>
>> One the one hand, print_conf() can be called without grabbing
>> 'reconfig_mtuex' and current rcu protection to access rdev through 'conf'
>> is not safe. Fortunately, there is a separate rcu protection to access
>> rdev from 'mddev->disks', and rdev is always removed from 'conf' before
>> 'mddev->disks'.
>>
>> On the other hand, print_conf() is just used for debug,
>> and user can always grab such information(/proc/mdstat and mdadm).
>>
>> There is no need to always enable this debug and try to fix misuse rcu
>> protection for accessing rdev from 'conf', hence remove print_conf().
>>
>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> 
> I wouldn't call these debug functions useless. There is probably some
> users who use them for debugging (or even in some automations).
> How hard is it to keep these functions? Can we just add some lockdep
> to these functions to make sure they are called from safe places?

Okay, I can keep these debug code, and since these code are
dereferencing rdev from conf, and they need new syncronization:

1) dereference rdev from mddev->disks instead of conf, and use
rdev->raid_disk >= 0 to judge if this rdev is in conf. There might
be a race window that rdev can be removed from conf, however, I think
this dones't matter. Or:

2) grab 'active_io' before print_conf(), to make sure rdev won't be
removed from conf.

Thanks,
Kuai

> 
> Thanks,
> Song
> 
> [...]
> .
>