Re: [PATCH][RFC] workqueue: Fix kernel panic on CPU hot-unplug

[Tejun Heo]

Hello,

On Wed, Jan 31, 2024 at 08:27:45PM +0100, Helge Deller wrote:
> When hot-unplugging a 32-bit CPU on the parisc platform with
> "chcpu -d 1", I get the following kernel panic. Adding a check
> for !pwq prevents the panic.
> 
>  Kernel Fault: Code=26 (Data memory access rights trap) at addr 00000000
>  CPU: 1 PID: 21 Comm: cpuhp/1 Not tainted 6.8.0-rc1-32bit+ #1291
>  Hardware name: 9000/778/B160L
>  
>  IASQ: 00000000 00000000 IAOQ: 10446db4 10446db8
>   IIR: 0f80109c    ISR: 00000000  IOR: 00000000
>   CPU:        1   CR30: 11dd1710 CR31: 00000000
>   IAOQ[0]: wq_update_pod+0x98/0x14c
>   IAOQ[1]: wq_update_pod+0x9c/0x14c
>   RP(r2): wq_update_pod+0x80/0x14c
>  Backtrace:
>   [<10448744>] workqueue_offline_cpu+0x1d4/0x1dc
>   [<10429db4>] cpuhp_invoke_callback+0xf8/0x200
>   [<1042a1d0>] cpuhp_thread_fun+0xb8/0x164
>   [<10452970>] smpboot_thread_fn+0x284/0x288
>   [<1044d8f4>] kthread+0x12c/0x13c
>   [<1040201c>] ret_from_kernel_thread+0x1c/0x24
>  Kernel panic - not syncing: Kernel Fault
> 
> Signed-off-by: Helge Deller <deller@xxxxxx>
> 
> ---
> 
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 76e60faed892..dfeee7b7322c 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -4521,6 +4521,8 @@ static void wq_update_pod(struct workqueue_struct *wq, int cpu,
>  	wq_calc_pod_cpumask(target_attrs, cpu, off_cpu);
>  	pwq = rcu_dereference_protected(*per_cpu_ptr(wq->cpu_pwq, cpu),
>  					lockdep_is_held(&wq_pool_mutex));
> +	if (!pwq)
> +		return;

Hmm... I have a hard time imagining a scenario where some CPUs don't have
pwq installed on wq->cpu_pwq. Can you please run `drgn
tools/workqueue/wq_dump.py` before triggering the hotplug event and paste
the output along with full dmesg?

Thanks.

-- 
tejun