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From:   Xiao Ni <xni@redhat.com>
Date:   Wed, 13 Oct 2021 15:57:57 +0800
Message-ID: <CALTww29m_RQHbZY6owX2y1_rmOeCEL9rSWg4=shnnitA1RpWQg@mail.gmail.com>
Subject: Re: [PATCH RESEND] md: allow to set the fail_fast on RAID1/RAID10
To:     Li Feng <fengli@smartx.com>
Cc:     Song Liu <song@kernel.org>,
        "open list:SOFTWARE RAID (Multiple Disks) SUPPORT" 
        <linux-raid@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
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On Tue, Oct 12, 2021 at 5:14 PM Li Feng <fengli@smartx.com> wrote:
> >          sysfs_notify_dirent_safe(rdev->sysfs_state);
> > +    if (need_update_sb)
> > +        if (mddev->pers) {
> > +            set_bit(MD_SB_CHANGE_DEVS, &mddev->sb_flags);
> > +            md_wakeup_thread(mddev->thread);
> > +        }
> When will mddev->pers is NULL?

The process of creating a raid device is:
1. md_ioctl->md_add_new_disk
2. md_ioctl->do_md_run->md_run->(pers->run)->(mddev->pers = pers)
In md_add_new_disk it creates the per device sysfs files. It can
read/write these
files before setting mddev->pers.

> If it is NULL, this change will not on disk.

I did a test. You are right. Someone can change the per device state
before ADD_NEW_DISK
and RUN_ARRAY ioctl. Please note, the mdadm --create command doesn't
return until RUN_ARRAY
ioctl finishes. Even though the are small, we can try to set MD_SB_CHANGE_DEVS.

Best regards
Xiao