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

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 4 May 2015 11:45:41 -0700
Cong Wang <cwang@twopensource.com> wrote:

> On Mon, May 4, 2015 at 6:39 AM, Dominick Grift <dac.override@gmail.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.