linux-kernel - Re: [PATCH -next v2 1/6] md: remove useless debug code to print configuration

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Message-ID: <CAPhsuW4d4mHQ9C=3gGKOG=QuXVH11ZtksQV3WV3CbYANgq7FYw@mail.gmail.com>
Date:   Fri, 24 Nov 2023 08:49:07 -0800
From:   Song Liu <song@...nel.org>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, yangerkun@...wei.com,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH -next v2 1/6] md: remove useless debug code to print configuration

On Fri, Nov 24, 2023 at 1:12 AM Yu Kuai <yukuai1@...weicloud.com> wrote:
>
> Hi,
>
> 在 2023/11/24 16:17, Song Liu 写道:
> > On Fri, Oct 20, 2023 at 7:25 PM Yu Kuai <yukuai1@...weicloud.com> wrote:
> >>
> >> From: Yu Kuai <yukuai3@...wei.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@...wei.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.

I think for most of these cases, we can already do print_conf()
safely (holding mutex/lock), no? We can grab active_io when it is
really necessary.

Thanks,
Song