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

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