Re: [PATCH net] netpoll: run NAPI poll in softirq context to avoid rq->lock self-deadlock

[Petr Mladek]

On Wed 2026-06-17 13:19:58, Peter Zijlstra wrote:
> On Wed, Jun 17, 2026 at 12:37:30PM +0200, Petr Mladek wrote:
> > On Tue 2026-06-16 14:17:19, Jakub Kicinski wrote:
> > > On Tue, 16 Jun 2026 19:02:57 +0200 Peter Zijlstra wrote:
> > > > > So this is not an issue since commit 7eab73b18630e ("netconsole: convert
> > > > > to NBCON console infrastructure"). Because from here now on writes are
> > > > > deferred to the nbcon thread. So this purely about -stable in this case.  
> > > > 
> > > > Hmm, I thought netconsole had some reserved skbs and could to writes
> > > > 'atomic' like? That said, it was 2.6 era the last time I looked at
> > > > netconsole.
> > > 
> > > Yes, that part is fine. The problem is that netconsole tries
> > > to reap Tx completions if the Tx queue is full. We can't call
> > > skb destructor in irq context so we put the completed skbs on
> > > a queue and try to arm softirq to get to them later.
> > > Arming softirq causes a ksoftirq wake up.
> > > 
> > > We already skip the completion polling if we detect getting called
> > > from the same networking driver. It's best effort, anyway.
> > > Networking-side fix would be to toss another OR condition into
> > > the skip. But we don't have one that'd work cleanly :S
> > 
> > Alternative solution might be to offload the ksoftirq wake up
> > to an irq_work. It might make this part safe for the
> > console->write_atomic() call.
> > 
> > Well, my understanding is that there are more problems.
> > AFAIK, some drivers do not use an IRQ safe locking, see
> > https://lore.kernel.org/all/oth5t27z6acp7qxut7u45ekyil7djirg2ny3bnsvnzeqasavxb@nhwdxahvcosh/
> 
> But anything using locking is not ->write_atomic() and should be driven
> from a kthread, no?

Right. I am not sure where my head was this morning.

Best Regards,
Petr