netdev - Re: [PATCH] net: stmmac: Enable NAPI before interrupts go live

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Message-ID: <20220218104504.62sfwbc4yoaceqdw@pengutronix.de>
Date:   Fri, 18 Feb 2022 11:45:04 +0100
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Vincent Whitchurch <vincent.whitchurch@...s.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>, kernel@...s.com,
        Lars Persson <larper@...s.com>,
        Srinivas Kandagatla <srinivas.kandagatla@...com>,
        netdev@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: stmmac: Enable NAPI before interrupts go live

On 17.02.2022 20:36:04, Jakub Kicinski wrote:
> On Thu, 17 Feb 2022 15:55:26 +0100 Vincent Whitchurch wrote:
> > From: Lars Persson <larper@...s.com>
> > 
> > The stmmac_open function has a race window between enabling the RX
> > path and its interrupt to the point where napi_enabled is called.
> > 
> > A chatty network with plenty of broadcast/multicast traffic has the
> > potential to completely fill the RX ring before the interrupt handler
> > is installed. In this scenario the single interrupt taken will find
> > napi disabled and the RX ring will not be processed. No further RX
> > interrupt will be delivered because the ring is full.
> > 
> > The RX stall could eventually clear because the TX path will trigger a
> > DMA interrupt once the tx_coal_frames threshold is reached and then
> > NAPI becomes scheduled.
> 
> LGTM, although now the ndo_open and ndo_stop paths are not symmetrical.
> Is there no way to mask the IRQs so that they don't fire immediately?
> More common flow (IMO) would be:
>  - request irq
>  - mask irq

I think you can merge these, to avoid a race condition, see:

| cbe16f35bee6 genirq: Add IRQF_NO_AUTOEN for request_irq/nmi()

>  - populate rings
>  - start dma
>  - enable napi
>  - unmask irq
> Other than the difference in flow between open/stop there may also be
> some unpleasantness around restarting tx queues twice with the patch
> as is.

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
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