Re: DSA: Suspicious RCU usage (via rtnl_bridge_getlink)

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Tue, Sep 20, 2016 at 03:38:33PM +0200, Andrew Lunn wrote:
> On Tue, Sep 20, 2016 at 11:26:12AM +0100, Russell King - ARM Linux wrote:
> > Issuing "bridge vlan show" on clearfog provokes a "suspicious RCU usage"
> > warning from the kernel (see below).
> > 
> > As it's illegal to schedule while holding the RCU read lock, there's the
> > possibility for this happening much earlier in the call sequence -
> > mv88e6xxx_port_vlan_dump() takes a mutex, and if that mutex were already
> > held, we'd schedule at that point.  The RCU read lock was taken by
> > rtnl_bridge_getlink().
> > 
> > It looks horrible to fix - mvmdio.c as well as DSA locking are involved.
> 
> Hi Russell
> 
> I would say this needs fixing higher up, in the bridge code. DSA has
> to be able to sleep, since the switch can be on any arbitrary bus,
> MDIO, SPI, etc. This will affect pure switchdev devices as well, since
> they often need to send a request to the switch and wait for a reply.

Hmm, okay, so looking around, other rtnl operations in there just
use for_each_netdev() or for_each_netdev_safe() without taking
any locks, apart from the rtnl mutex which we can see was already
taken.

Why does rtnl_bridge_getlink use RCU?  Can we drop the RCU read lock
and switch to using for_each_netdev() here?  Adding Dave and Eric,
as I guess they're more knowledgeable of the core rtnl code.

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