netdev - Re: Softirq error with mcp251xfd driver

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Date:   Wed, 10 Mar 2021 08:13:51 +0100
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Daniel Glöckner <dg@...ix.com>
Cc:     netdev@...r.kernel.org, linux-can@...r.kernel.org
Subject: Re: Softirq error with mcp251xfd driver

On 10.03.2021 07:46:26, Daniel Glöckner wrote:
> the mcp251xfd driver uses a threaded irq handler to queue skbs with the
> can_rx_offload_* helpers. I get the following error on every packet until
> the rate limit kicks in:
> 
> NOHZ tick-stop error: Non-RCU local softirq work is pending, handler
> #08!!!

That's a known problem. But I had no time to investigate it.

> Adding local_bh_disable/local_bh_enable around the can_rx_offload_* calls
> gets rid of the error, but is that the correct way to fix this?
> Internally the can_rx_offload code uses spin_lock_irqsave to safely
> manipulate its queue.

The problem is not the queue handling inside of rx_offload, but the call
to napi_schedule(). This boils down to raising a soft IRQ (the NAPI)
from the threaded IRQ handler of the mcp251xfd driver.

The local_bh_enable() "fixes" the problem running the softirq if needed.

https://elixir.bootlin.com/linux/v5.11/source/kernel/softirq.c#L1913

I'm not sure how to properly fix the problem, yet.

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |

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