Re: "Dead loop on virtual device" error without softirq-BKL on PREEMPT_RT

[Daniel Vacek]

On Thu, 2 Apr 2026 at 09:03, Sebastian Andrzej Siewior
<bigeasy@xxxxxxxxxxxxx> wrote:
> On 2026-04-01 18:55:43 [+0200], Daniel Vacek wrote:
> > > > > The above was me thinking and does not even compile for !RT. Commit
> > > > > b824c3e16c190 ("net: Provide a PREEMPT_RT specific check for
> > > > > netdev_queue::_xmit_lock") is what was merged in the end.
> >
> > Thinking about it again, wouldn't it be better to have one generic
> > solution rather then special-casing for PREEMPT_RT vs. !PREEMPT_RT?
>
> PREEMPT_RT and !PREEMPT_RT is fundamentally different here. The one is
> not preemptible and records the CPU of the lock owner to detect a
> recursive deadlock.
> The other is preemptible, uses a different locking type/ class which
> records the lock owner which can be utilised for this purpose.

I understand that (or at least I think I do).
My idea was that the non-preemptible one can record `current` task
instead of the CPU to detect the deadlock. And that would also work
for the preemptible case (it would actually match the lock owner
approach as you did for the PREEMPT_RT case).
One code for both configurations, no special-casing. I'd argue that's
a better result. Am I missing something?

The size of the netdev_queue structure would grow by 8 bytes for !RT
case, but that's not a big deal, IMO. For RT case it would just fill
the hole.

--nX


> A generic thing would be to remove this and rely on lockdep. This could
> work if it is only a devel thing and never "I setup something and make a
> loop" sort of thing.
>
> > --nX
>
> Sebastian