Re: [syzbot] [btrfs?] [netfilter?] BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low! (2)

[Paolo Abeni]

On Wed, 2023-07-19 at 20:30 -0700, Jakub Kicinski wrote:
> On Thu, 20 Jul 2023 01:12:07 +0200 Florian Westphal wrote:
> > I don't see any netfilter involvement here.
> > 
> > The repro just creates a massive amount of team devices.
> > 
> > At the time it hits the LOCKDEP limits on my test vm it has
> > created ~2k team devices, system load is at +14 because udev
> > is also busy spawing hotplug scripts for the new devices.
> > 
> > After reboot and suspending the running reproducer after about 1500
> > devices (before hitting lockdep limits), followed by 'ip link del' for
> > the team devices gets the lockdep entries down to ~8k (from 40k),
> > which is in the range that it has on this VM after a fresh boot.
> > 
> > So as far as I can see this workload is just pushing lockdep
> > past what it can handle with the configured settings and is
> > not triggering any actual bug.
> 
> The lockdep splat because of netdevice stacking is one of our top
> reports from syzbot. Is anyone else feeling like we should add 
> an artificial but very high limit on netdev stacking? :(

We already have a similar limit for xmit: XMIT_RECURSION_LIMIT. I guess
stacking more then such devices will be quite useless/non functional.
We could use such value to limit the device stacking, too.

Cheers,

Paolo