Subject: Re: [net,PATCH v2] net: ks8851: Reinstate disabling of BHs around IRQ handler

From: Jakub Kicinski <[email-masked]>
Date: Sun, 12 Apr 2026 09:01:41 -0700

On Wed,  8 Apr 2026 18:24:58 +0200 Marek Vasut wrote:
> If CONFIG_PREEMPT_RT=y is set AND the driver executes ks8851_irq() AND
> KSZ_ISR register bit IRQ_RXI is set AND ks8851_rx_pkts() detects that
> there are packets in the RX FIFO, then netdev_alloc_skb_ip_align() is
> called to allocate SKBs. If netdev_alloc_skb_ip_align() is called with
> BH enabled, local_bh_enable() at the end of netdev_alloc_skb_ip_align()
> will call __local_bh_enable_ip(), which will call __do_softirq(), which
> may trigger net_tx_action() softirq, which may ultimately call the xmit
> callback ks8851_start_xmit_par(). The ks8851_start_xmit_par() will try
> to lock struct ks8851_net_par .lock spinlock, which is already locked
> by ks8851_irq() from which ks8851_start_xmit_par() was called. This
> leads to a deadlock, which is reported by the kernel, including a trace
> listed below.

lock_par is a spinlock, and AFAIU softirqs run in their on thread on RT.
I'm not following.

The patch looks way to "advanced" for a driver. Something is going
very wrong here. Or the commit message must be updated to explain
it better to people like me. Or both.
-- 
pw-bot: cr