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

[Yu Kuai]

Hi,

在 2023/11/24 16:17, Song Liu 写道:
> On Fri, Oct 20, 2023 at 7:25 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
>
> 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
>
> [...]
> .