Re: [PATCH net] net: sched: gred: prevent races when adding offloads to stats

[Cong Wang <xiyou.wangcong@gmail.com>]

On Tue, Jan 17, 2023 at 10:00:56AM +0100, Paolo Abeni wrote:
> Hello,
> 
> On Sat, 2023-01-14 at 18:20 -0800, Cong Wang wrote:
> > On Thu, Jan 12, 2023 at 08:41:37PM -0800, Jakub Kicinski wrote:
> > > Naresh reports seeing a warning that gred is calling
> > > u64_stats_update_begin() with preemption enabled.
> > > Arnd points out it's coming from _bstats_update().
> > 
> > 
> > The stack trace looks confusing to me without further decoding.
> > 
> > Are you sure we use sch->qstats/bstats in __dev_queue_xmit() there
> > not any netdev stats? It may be a false positive one as they may end up
> > with the same lockdep class.
> 
> I'm unsure I read you comment correctly. Please note that the
> referenced message includes several splats. The first one - arguably
> the most relevant - points to the lack of locking in the gred control
> path.

I think it means lack of BH disable, not necessarily locking.

> 
> The posted patch LGTM, could you please re-phrase your doubts?

Yes, because the key in lockdep ("&syncp->seq#14") may be same for other
dev stats (aka, not qdisc stats).

Also, HTB does not accquire this lock when dumping
(htb_dump_class_stats()) stats. Why don't we have a problem there? ;)

Thanks.