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

From: Jakub Kicinski <[email-masked]>
Date: Tue, 17 Jan 2023 11:10:19 -0800

On Tue, 17 Jan 2023 10:00:56 +0100 Paolo Abeni wrote:
> 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 didn't repro this myself, TBH, but there is u64_stats_update_begin() 
inside _bstats_update(). Pretty sure it will trigger the warning that
preemption is not disabled on non-SMP systems.

> 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.

Yup, I'm not really sure if we're fixing the right splat for the bug.
But I am fairly confident we should be holding a lock while writing
bstats from the dump path, enqueue/dequeue may run concurrently.

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