Re: [PATCH net-next v2 12/14] net: macb: re-read ISR inside IRQ handler locked section

[Jakub Kicinski]

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---
net: macb: re-read ISR inside IRQ handler locked section

This commit adds a second read of the ISR register inside the lock in
macb_interrupt() to handle a race condition where the status might have
been updated by concurrent hardware resets.

> diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
> index 2633234012bae..3deab87c565b5 100644
> --- a/drivers/net/ethernet/cadence/macb_main.c
> +++ b/drivers/net/ethernet/cadence/macb_main.c
> @@ -2190,6 +2190,13 @@ static irqreturn_t macb_interrupt(int irq, void *dev_id)
>  
>  	spin_lock(&bp->lock);
>  
> +	/* `status` stack variable might be stalled => re-read it */
> +	status = queue_readl(queue, ISR);

Will this second read discard interrupt events on older MACB variants that
use clear-on-read for the ISR?

If the hardware does not have the MACB_CAPS_ISR_CLEAR_ON_WRITE capability,
the first queue_readl call before the lock already cleared the active bits.

Overwriting the local status variable here sets it to 0, which permanently
drops the initially captured Rx/Tx completions and stalls the network
interface.

> +	if (unlikely(!status)) {
> +		spin_unlock(&bp->lock);
> +		return IRQ_NONE;

If the function reaches this point, the initial status read before the lock
was non-zero, meaning the device legitimately asserted the interrupt.

If a concurrent operation cleared the hardware state while this thread
waited for the lock, will returning IRQ_NONE falsely flag this as a
spurious interrupt?

Since the interrupt was actually raised by the device, doing this repeatedly
can cause the core IRQ subsystem to permanently disable the IRQ line.

Could this return IRQ_HANDLED instead?

> +	}
> +
>  	while (status) {
>  		/* close possible race with dev_close */
>  		if (unlikely(!netif_running(netdev))) {
-- 
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