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

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

On 2026-06-17, Breno Leitao <leitao@debian.org> wrote:
> On Wed, Jun 17, 2026 at 01:19:58PM +0200, Peter Zijlstra wrote:
>> 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. 

It depends on what it needs to synchronize against. For example, the
UART consoles cannot write if the port lock is taken by another
context. And the port lock is the sole lock for writing to the UART. To
deal with this, we added wrappers [0] for acquiring/releasing the port
lock. The wrappers acquire the nbcon hardware after taking the port
lock.

The write_atomic() implementations for UART consoles do not take the
port lock. Only the nbcon hardware is acquired (which can be done from
any context). This automatically provides the synchronization based on
the port lock.

> 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?

Is there some lock that can be taken to synchronize all writing of
packets to the network? If yes, the netconsole can use a similar
solution.

That is an example of a general solution, but individual drivers may be
able to provide unique solutions, such as dedicated tx-channels for
netconsole. (Sorry, I am not a network guy.)

John Ogness

[0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/serial_core.h?h=v7.1#n715