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

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Message-ID: <YfzhioY0Mj3M1v4S@linutronix.de>
Date:   Fri, 4 Feb 2022 09:19:22 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Yannick Vignon <yannick.vignon@....nxp.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S. Miller" <davem@...emloft.net>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Antoine Tenart <atenart@...nel.org>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Paolo Abeni <pabeni@...hat.com>, Wei Wang <weiwan@...gle.com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Yunsheng Lin <linyunsheng@...wei.com>,
        Arnd Bergmann <arnd@...db.de>, netdev <netdev@...r.kernel.org>,
        Vladimir Oltean <olteanv@...il.com>,
        Xiaoliang Yang <xiaoliang.yang_1@....com>, mingkai.hu@....com,
        Joakim Zhang <qiangqing.zhang@....com>,
        sebastien.laveze@....com, Yannick Vignon <yannick.vignon@....com>
Subject: Re: [PATCH net-next 1/2] net: napi: wake up ksoftirqd if needed
 after scheduling NAPI

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