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

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Date: Fri, 2 Feb 2024 07:29:16 -1000
From: Tejun Heo <tj@...nel.org>
To: Helge Deller <deller@....de>
Cc: Helge Deller <deller@...nel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	linux-kernel@...r.kernel.org, linux-parisc@...r.kernel.org
Subject: Re: [PATCH][RFC] workqueue: Fix kernel panic on CPU hot-unplug

Hello, Helge.

On Fri, Feb 02, 2024 at 09:41:38AM +0100, Helge Deller wrote:
> In a second step I extended your patch to print the present
> and online CPUs too. Below is the relevant dmesg part.
> 
> Note, that on parisc the second CPU will be activated later in the
> boot process, after the kernel has the inventory.
> This I think differs vs x86, where all CPUs are available earlier
> in the boot process.
> ...
> [    0.000000] XXX workqueue_init_early: possible_cpus=ffff  present=0001  online=0001
..
> [    0.228080] XXX workqueue_init: possible_cpus=ffff  present=0001  online=0001
..
> [    0.263466] XXX workqueue_init_topology: possible_cpus=ffff  present=0001  online=0001

So, what's bothersome is that when the wq_dump.py script printing each cpu's
pwq, it's only printing for CPU 0 and 1. The for_each_possible_cpu() drgn
helper reads cpu_possible_mask from the kernel and iterates that, so that
most likely indicates at some point the cpu_possible_mask becomes 0x3
instead of the one used during boot - 0xffff, which is problematic.

Can you please sprinkle more printks to find out whether and when the
cpu_possible_mask changes during boot?

Thanks.

-- 
tejun