Re: [PATCH net-next 1/2] net: napi: wake up ksoftirqd if needed after scheduling NAPI

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2022-02-03 17:09:01 [-0800], Jakub Kicinski wrote:
> Let's be clear that the problem only exists when switching to threaded
> IRQs on _non_ PREEMPT_RT kernel (or old kernels). We already have a
> check in __napi_schedule_irqoff() which should handle your problem on
> PREEMPT_RT.

It does not. The problem is the missing bh-off/on around the call. The
forced-threaded handler has this. His explicit threaded-handler does not
and needs it.

> We should slap a lockdep warning for non-irq contexts in
> ____napi_schedule(), I think, it was proposed by got lost.

Something like this perhaps?:

diff --git a/net/core/dev.c b/net/core/dev.c
index 1baab07820f65..11c5f003d1591 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4217,6 +4217,9 @@ static inline void ____napi_schedule(struct softnet_data *sd,
 {
 	struct task_struct *thread;
 
+	lockdep_assert_once(hardirq_count() | softirq_count());
+	lockdep_assert_irqs_disabled();
+
 	if (test_bit(NAPI_STATE_THREADED, &napi->state)) {
 		/* Paired with smp_mb__before_atomic() in
 		 * napi_enable()/dev_set_threaded().

Be aware that this (the first assert) will trigger in dev_cpu_dead() and
needs a bh-off/on around. I should have something in my RT tree :)

Sebastian