netdev - Re: Suspicious RCU usage in bridge with Linux v4.0-9362-g1fc149933fd4

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Date:	Mon, 4 May 2015 14:35:27 -0700
From:	Cong Wang <cwang@...pensource.com>
To:	Stephen Hemminger <stephen@...workplumber.org>
Cc:	Dominick Grift <dac.override@...il.com>,
	netdev <netdev@...r.kernel.org>,
	Vlad Yasevich <vyasevic@...hat.com>
Subject: Re: Suspicious RCU usage in bridge with Linux v4.0-9362-g1fc149933fd4

On Mon, May 4, 2015 at 1:27 PM, Stephen Hemminger
<stephen@...workplumber.org> wrote:
> On Mon, 4 May 2015 11:45:41 -0700
> Cong Wang <cwang@...pensource.com> wrote:
>
>> On Mon, May 4, 2015 at 6:39 AM, Dominick Grift <dac.override@...il.com> wrote:
>> > On Thu, Apr 23, 2015 at 01:07:45PM -0400, Josh Boyer wrote:
>> >> Hi All,
>> >>
>> >> We've had a user report the following backtrace from the bridge module
>> >> with a recent Linus' tree.  Has anything like this been reported yet?
>> >> If you have any questions on setup, the user is CC'd.
>> >>
>> >> josh
>> >>
>> >> [   29.382235] br0: port 1(tap0) entered forwarding state
>> >>
>> >> [   29.382286] ===============================
>> >> [   29.382315] [ INFO: suspicious RCU usage. ]
>> >> [   29.382344] 4.1.0-0.rc0.git11.1.fc23.x86_64 #1 Not tainted
>> >> [   29.382380] -------------------------------
>> >> [   29.382409] net/bridge/br_private.h:626 suspicious
>> >> rcu_dereference_check() usage!
>> >
>> > <snip>
>> >
>> > With 4.1.0-0.rc1.git1.1.fc23.x86_64 the situation seems to have slightly changed:
>> >
>>
>> Should be the same issue. Please give the attached patch a try,
>> it is compile-tested only.
>>
>> Thanks!
>
> Good analysis in identifying the issue. But the proposed patch
> doesn't seem right.
>
> The br->lock protects against changes to the bridge port state.
> vlan_info should be treated as part of the bridge state.
>
> The correct fix is to get vlan_info out of depending on RTNL
> and use br->lock to control modifications.

It _looks like_ we only retrieve vlan info to fill netlink
messages in timer context, so it doesn't seem we need to
hold br->lock here.

But I never look into br vlan code of course.
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