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

[Breno Leitao <leitao@debian.org>]

On Wed, Jun 17, 2026 at 01:19:58PM +0200, 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?

Good point. If that's the case, netconsole might not ever be able to drop
CON_NBCON_ATOMIC_UNSAFE for any network-based console driver at all. 

As far as I can tell, there isn't a network driver today whose transmit
path is completely lockless, so, even if we make netpoll lockless.

It's unlikely any NIC will ever achieve this, given that NIC TX
fundamentally relies on a shared DMA ring and doorbell register, which
inherently cannot be made lockless.

So, is it correct to state that CON_NBCON_ATOMIC_UNSAFE will be part of
netconsole forever-ish?