netdev - Re: [vlan_device_event] BUG: unable to handle kernel paging request at 6b6b6ccf

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Date:   Thu, 9 Nov 2017 16:16:01 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Girish Moodalbail <girish.moodalbail@...cle.com>
Cc:     Fengguang Wu <fengguang.wu@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        Network Development <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>
Subject: Re: [vlan_device_event] BUG: unable to handle kernel paging request
 at 6b6b6ccf

On Thu, Nov 9, 2017 at 7:51 AM, Girish Moodalbail
<girish.moodalbail@...cle.com> wrote:
>
> Upon receiving NETDEV_DOWN event, we are calling
>
>         vlan_vid_del(dev, htons(ETH_P_8021Q), 0);
>
> which in turn calls call_rcu() to queue vlan_info_free_rcu() to be called at
> some point. This free function frees the array[]
> (vlan_info.vlan_grp.vn_devices_array).  My guess is that
> vlan_info_free_rcu() is being called first and then the array[] is being
> accessed in vlan_device_event().
>

Well yes and no.

No, RCU itself is not broken and we clearly unpublish the RCU pointer
before calling call_rcu().

Yes, I see where it is broken: the grp pointer still points to old
dev->vlan_info, we should re-fetch it after vlan_vid_del().

I will send a fix.

Thanks!