netdev - Re: [PATCH] net: stmmac: Don't call _irqoff() with hardirqs enabled

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Message-ID: <20201011090620.48afafd7@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Sun, 11 Oct 2020 09:06:20 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Heiner Kallweit <hkallweit1@...il.com>
Cc:     John Keeping <john@...anate.com>, netdev@...r.kernel.org,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S. Miller" <davem@...emloft.net>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH] net: stmmac: Don't call _irqoff() with hardirqs enabled

On Sun, 11 Oct 2020 11:24:41 +0200 Heiner Kallweit wrote:
> >> qeth_qdio_poll
> >> netvsc_channel_cb
> >> napi_watchdog  
> > 
> > This one runs from a hrtimer, which I believe will be a hard irq
> > context on anything but RT. I could be wrong.
> >   
> 
> A similar discussion can be found e.g. here:
> https://lore.kernel.org/netdev/20191126222013.1904785-1-bigeasy@linutronix.de/
> However I don't see any actual outcome.

Interesting, hopefully Eric will chime in. I think the hrtimer issue
was solved. But I'm not actually seeing a lockdep_assert_irqs_disabled()
in __raise_softirq_irqoff() in net, so IDK what that's for?

In any case if NAPI thinks it has irqs off while they're not, and
interacts with other parts of the kernel we may be in for a game of
whack-a-mole. 

Perhaps a way around touching force_irqthreads directly in net/ would 
be some form of a helper like "theaded_local_irq_save" or such that'd
disable IRQs only if force_irqthreads == 1? Is that cheating? :)