netdev - Re: [PATCH RFC] r8152: Ensure that napi

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Date:   Sat, 15 May 2021 15:09:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Jakub Kicinski <kuba@...nel.org>, linux-usb@...r.kernel.org,
        netdev@...r.kernel.org, Michal Svec <msvec@...e.com>,
        "David S. Miller" <davem@...emloft.net>,
        Hayes Wang <hayeswang@...ltek.com>,
        Thierry Reding <treding@...dia.com>,
        Lee Jones <lee.jones@...aro.org>,
        Borislav Petkov <bp@...en8.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH RFC] r8152: Ensure that napi_schedule() is handled

On Sat, May 15, 2021 at 01:23:02AM +0200, Thomas Gleixner wrote:
> We can make that work but sure I'm not going to argue when you decide to
> just go for raise_softirq_irqsoff().
> 
> I just hacked that check up which is actually useful beyond NAPI. It's
> straight forward except for that flush_smp_call_function_from_idle()
> oddball, which immeditately triggered that assert because block mq uses
> __raise_softirq_irqsoff() in a smp function call...
> 
> See below. Peter might have opinions though :)

Yeah, lovely stuff :-)


> +#define lockdep_assert_softirq_raise_ok()				\
> +do {									\
> +	WARN_ON_ONCE(__lockdep_enabled &&				\
> +		     !current->softirq_raise_safe &&			\
> +		     !(softirq_count() | hardirq_count()));		\
> +} while (0)

> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -691,7 +691,9 @@ void flush_smp_call_function_from_idle(v
>  	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->idle, CFD_SEQ_NOCPU,
>  		      smp_processor_id(), CFD_SEQ_IDLE);
>  	local_irq_save(flags);
> +	lockdep_set_softirq_raise_safe();
>  	flush_smp_call_function_queue(true);
> +	lockdep_clear_softirq_raise_safe();
>  	if (local_softirq_pending())
>  		do_softirq();

I think it might make more sense to raise hardirq_count() in/for
flush_smp_call_function_queue() callers that aren't already from hardirq
context. That's this site and smpcfd_dying_cpu().

Then we can do away with this new special case.