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

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Date:   Thu, 17 Feb 2022 20:36:04 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc:     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 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
 - 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.