linux-kernel - Re: NOHZ: local_softirq

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Message-ID: <20120608202701.GA26175@electric-eye.fr.zoreil.com>
Date:	Fri, 8 Jun 2012 22:27:01 +0200
From:	Francois Romieu <romieu@...zoreil.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Dave Jones <davej@...hat.com>,
	Marc Dionne <marc.c.dionne@...il.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: NOHZ: local_softirq_pending 08

Thomas Gleixner <tglx@...utronix.de> :
[...]
> I can't find the point where the warning is issued, but I think I
> found the cause of the problem.
> 
> static void rtl_slow_event_work(struct rtl8169_private *tp)
> {
> 	.....
> 	napi_schedule(&tp->napi);
> 	  --> __napi_schedule();
> 	      -->	list_add_tail(&napi->poll_list, &sd->poll_list);
> 	              __raise_softirq_irqoff(NET_RX_SOFTIRQ);
> 
> 		      This merily sets the softirq bit.
> 
> 
> So this code is really wrong. It's called from full preemptible
> context of the workqueue. And if the next thing is a context switch to
> idle then the pending softirq check will trigger.

void __napi_schedule(struct napi_struct *n)
{
        unsigned long flags;

        local_irq_save(flags);
        ____napi_schedule(&__get_cpu_var(softnet_data), n);
        local_irq_restore(flags);
}

Are you saying that this stuff should be considered "preemptible" ?

-- 
Ueimor
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