Re: [PATCH] net/nfc: Fix A-B/B-A deadlock between nfc_unregister_device and rfkill_fop_write

[Krzysztof Kozlowski <krzk@kernel.org>]

On 15/08/2025 10:23, Yunseong Kim wrote:
> Hi Krzysztof,
> 
> Thank you for your review.
> 
> On 8/15/25 2:55 PM, Krzysztof Kozlowski wrote:
>> On 14/08/2025 19:31, Yunseong Kim wrote:
>>> A potential deadlock due to A-B/B-A deadlock exists between the NFC core
>>> and the RFKill subsystem, involving the NFC device lock and the
>>> rfkill_global_mutex.
>>>
>>> This issue is particularly visible on PREEMPT_RT kernels, which can
>>> report the following warning:
>>
>> Why are not you crediting syzbot and its report?
>>
>> there is clear INSTRUCTION in that email from Syzbot.
> 
> I wanted to clarify that this report did not originate from syzbot.
> 
> I found this issue by building and running syzkaller locally on my own
> Arm64 RADXA Orion6 board.
> 
> This is reproduction series on my local syzkaller.
> 
> WARNING in __rt_mutex_slowlock
> 
> #	Log	Report	Time	Tag
> 7	log	report	2025/08/14 20:01	
> 6	log	report	2025/08/14 05:55	
> 5	log	report	2025/08/14 02:31	
> 4	log	report	2025/08/12 09:38	
> 3	log	report	2025/07/30 07:09	
> 2	log	report	2025/07/27 23:29	
> 1	log	report	2025/07/26 04:18	
> 0	log	report	2025/07/26 04:17
> 
> The reason this is coming from syzbot recently is that I worked with Sebastian,
> the RT maintainer, to fix KCOV to be PREEMPT_RT-aware. This was merged recently:
> Link: https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git/commit/?h=usb-linus&id=9528d32873b38281ae105f2f5799e79ae9d086c2
> 
> So, syszbot now report it:
> https://syzkaller.appspot.com/bug?extid=535bbe83dfc3ae8d4be3


Syzbot reported it before you pasted patch, so it should also receive
the reported-by credit, even if you discovered it separately.

> 
>>> | rtmutex deadlock detected
>>> | WARNING: CPU: 0 PID: 22729 at kernel/locking/rtmutex.c:1674 rt_mutex_handle_deadlock+0x68/0xec kernel/locking/rtmutex.c:-1
>>> | Modules linked in:
>>> | CPU: 0 UID: 0 PID: 22729 Comm: syz.7.2187 Kdump: loaded Not tainted 6.17.0-rc1-00001-g1149a5db27c8-dirty #55 PREEMPT_RT
>>> | Hardware name: QEMU KVM Virtual Machine, BIOS 2025.02-8ubuntu1 06/11/2025
> 



Best regards,
Krzysztof