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

[Marek Vasut]

On 4/13/26 6:10 PM, Sebastian Andrzej Siewior wrote:

> > TBH changing the driver feels like a workaround / invitation for a
> > whack-a-mole game. I'd prefer to fix the skb allocation.
>
> The problem is that _irq() implicitly disables bh processing but this
> does not happen. Forcing this is possible but expensive.
> However, I did remove lock from bh_disable() on RT.
>
> Marek: from which kernel version was this backtrace?
That was v6.12.79-rt17-rebase where it was easy to trigger , here is a 
fresh one from next-20260413 with b44596ffe1b4 ("ARM: Allow to enable 
RT") from stable-rt added on top:

"
rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu:     Tasks blocked on level-0 rcu_node (CPUs 0-1): P127/5:b..l
rcu:     (detected by 1, t=2102 jiffies, g=-503, q=1 ncpus=2)
task:irq/68-eth1     state:D stack:0     pid:127   tgid:127   ppid:2 
 task_flags:0x4208040 flags:0x00000000
Call trace:
 __schedule from schedule_rtlock+0x1c/0x34
 schedule_rtlock from rtlock_slowlock_locked+0x548/0x904
 rtlock_slowlock_locked from rt_spin_lock+0x60/0x9c
 rt_spin_lock from ks8851_start_xmit_par+0x74/0x1a8
 ks8851_start_xmit_par from netdev_start_xmit+0x20/0x44
 netdev_start_xmit from dev_hard_start_xmit+0xd0/0x188
 dev_hard_start_xmit from sch_direct_xmit+0xb8/0x25c
 sch_direct_xmit from __qdisc_run+0x1f8/0x4ec
 __qdisc_run from qdisc_run+0x1c/0x28
 qdisc_run from net_tx_action+0x1f0/0x268
 net_tx_action from handle_softirqs+0x1a4/0x270
 handle_softirqs from __local_bh_enable_ip+0xcc/0xe0
 __local_bh_enable_ip from __alloc_skb+0xd8/0x128
 __alloc_skb from __netdev_alloc_skb+0x3c/0x19c
 __netdev_alloc_skb from ks8851_irq+0x388/0x4d4
 ks8851_irq from irq_thread_fn+0x24/0x64
 irq_thread_fn from irq_thread+0x178/0x28c
 irq_thread from kthread+0x12c/0x138
 kthread from ret_from_fork+0x14/0x28
Exception stack(0xf4c99fb0 to 0xf4c99ff8)
9fa0:                                     00000000 00000000 00000000 
00000000
9fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
00000000
9fe0: 00000000 00000000 00000000 00000000 00000013 00000000
"