Re: [PATCH 1/2] net: Assert proper context while calling napi_schedule()

Frederic Weisbecker
2025-02-15T21:16:52Z

Le Fri, Feb 14, 2025 at 02:00:45PM -0800, Jakub Kicinski a écrit :
> On Thu, 13 Feb 2025 21:38:12 +0100 Frederic Weisbecker wrote:
> > > > Just to make sure I follow the netpoll issue. What would you like to fix
> > > > in netpoll exactly?
> > >
> > > Nothing in netpoll, the problem is that netdevsim calls napi_schedule
> > > from the xmit path. That's incompatible with netpoll. We should fix
> > > netdevsim instead (unless more real drivers need napi-from-xmit to
> > > work).
> >
> > Let me clarify, because I don't know much this area. If the problem is that xmit
> > can't call napi_schedule() by design, then I defer to you. But if the problem is that
> > napi_schedule() may or may not be called from an interrupt, please note that
> > local_bh_enable() won't run softirqs from a hardirq and will instead defer to
> > IRQ tail. So it's fine to do an unconditional pair of local_bh_disable() / local_bh_enable().
>
> I don't know where this is in the code TBH, but my understanding is
> that HW IRQs - yes, as you say it'd be safe; the problem is that
> we have local_irq_save() all over the place. And that is neither
> protected from local_bh_enable(), not does irq_restore execute softirqs.

Yeah actually checking local_bh_enable() again, it's not safe to call within
a hardirq. Ok I've been thinking some more and how about this instead?

diff --git a/net/core/dev.c b/net/core/dev.c
index c0021cbd28fc..2419cc558a64 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4692,7 +4692,7 @@ static inline void ____napi_schedule(struct softnet_data *sd,
* we have to raise NET_RX_SOFTIRQ.
*/
if (!sd->in_net_rx_action)
- __raise_softirq_irqoff(NET_RX_SOFTIRQ);
+ raise_softirq_irqoff(NET_RX_SOFTIRQ);
}

#ifdef CONFIG_RPS

This will simply wake up ksoftirqd if called from a non-IRQ. I expect such
callers to be rare enough to not impact performances and it has the advantage
to work for everyone.

Thanks.