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

[Peter Zijlstra]

On Tue, Jun 16, 2026 at 12:35:29PM +0200, Sebastian Andrzej Siewior 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.

> Now. The scheduler usually does printk_deferred() because of the rq lock
> so it does not deadlock for various reasons. It is kind of a pity that
> the various WARN macros don't do that.

People have tried, last time was here:

  https://lkml.kernel.org/r/20260611074344.GG48970@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

and I hate deferred with a passion. It means you'll never see the
message when you wreck the machine.

> We could add printk_deferred_enter/exit() to all the rq_lock() variants.
> I think PeterZ loves this the most. And Greg will appreciate it too
> while backporting because of all the context changes.

No, not going to happen, ever, sorry. Instead printk should delete
console sem and have printk() itself be atomic safe.

As stated, printk deferred is an abomination and needs to die a horrible
painful death.

As described here:

  https://lkml.kernel.org/r/20260611191922.GK187714@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

"So printk should:

 - stick msg in buffer (lockless)
 - print to atomic consoles (lockless)
 - use irq_work to wake console kthreads (lockless)
 - each kthread then tries to flush buffer to its own non-atomic console
   in non-atomic context."