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

[John Ogness <john.ogness@linutronix.de>]

On 2026-06-16, Breno Leitao <leitao@debian.org> 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.
>
> Does the nbcon thread handle defer even for consoles that support atomic
> operations?

The all "printk deferred" variants have zero effect on nbcon
drivers. The "printk deferred" variants exist purely as duct tape for
legacy console drivers.

If nbcon drivers provide a safe write_atomic(), they will _always_ write
synchronously when the CPU is in an emergency state. Otherwise nbcon
drivers _always_ defer to their dedicated console printing kthread and
there they use the write_thread() callback.

> netconsole is marked with CON_NBCON_ATOMIC_UNSAFE, which means it rarely
> performs inline/direct printk and instead pushes to the thread, which
> flushes in a safe context.

CON_NBCON_ATOMIC_UNSAFE means it _never_ performs inline/direct printk
console writing. That flags means that in panic, at the _very_ end, just
before going into an infinite nop loop, the CON_NBCON_ATOMIC_UNSAFE
consoles will be flushed directly from the panic context.

> For drivers that behave correctly, I'd like to be able to drop
> CON_NBCON_ATOMIC_UNSAFE, potentially setting it at runtime based on the
> underlying driver capabilities. If netconsole is backed by a well-behaving
> network driver, we could eventually remove the flag (!?)
>
> Would that approach cause any issues?

Removing the flag means the driver can safely write from _any_ context
(including scheduler and NMI), regardless what locks that context may be
holding.

Note that the nbcon framework allows console drivers to mark unsafe
regions in themselves, where atomic writing would not be possible. In
such scenarios, it defers to the dedicated printing kthread (except
during panic, where more agressive tactics are used).

John Ogness