Re: [PATCH net-next] net: Treat __napi_schedule_irqoff() as __napi_schedule() on PREEMPT_RT

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, May 12 2021 at 23:43, Sebastian Andrzej Siewior wrote:
> __napi_schedule_irqoff() is an optimized version of __napi_schedule()
> which can be used where it is known that interrupts are disabled,
> e.g. in interrupt-handlers, spin_lock_irq() sections or hrtimer
> callbacks.
>
> On PREEMPT_RT enabled kernels this assumptions is not true. Force-
> threaded interrupt handlers and spinlocks are not disabling interrupts
> and the NAPI hrtimer callback is forced into softirq context which runs
> with interrupts enabled as well.
>
> Chasing all usage sites of __napi_schedule_irqoff() is a whack-a-mole
> game so make __napi_schedule_irqoff() invoke __napi_schedule() for
> PREEMPT_RT kernels.
>
> The callers of ____napi_schedule() in the networking core have been
> audited and are correct on PREEMPT_RT kernels as well.
>
> Reported-by: Juri Lelli <juri.lelli@redhat.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

> ---
> Alternatively __napi_schedule_irqoff() could be #ifdef'ed out on RT and
> an inline provided which invokes __napi_schedule().
>
> This was not chosen as it creates #ifdeffery all over the place and with
> the proposed solution the code reflects the documentation consistently
> and in one obvious place.

Blame me for that decision.

No matter which variant we end up with, this needs to go into all stable
RT kernels ASAP.

Thanks,

        tglx