Hello,
My static analysis tool reports a possible deadlock in the wfx driver in
Linux 5.16:
wfx_conf_tx()
mutex_lock(&wdev->conf_mutex); --> Line 225 (Lock A)
wfx_update_pm()
wait_for_completion_timeout(&wvif->set_pm_mode_complete, ...); -->
Line 3019 (Wait X)
wfx_add_interface()
mutex_lock(&wdev->conf_mutex); --> Line 737 (Lock A)
complete(&wvif->set_pm_mode_complete); --> Line 758 (Wake X)
When wfx_conf_tx() is executed, "Wait X" is performed by holding "Lock
A". If wfx_add_interface() is executed at this time, "Wake X" cannot be
performed to wake up "Wait X" in wfx_conf_tx(), because "Lock A" has
been already hold by wfx_conf_tx(), causing a possible deadlock.
I find that "Wait X" is performed with a timeout, to relieve the
possible deadlock; but I think this timeout can cause inefficient execution.
I am not quite sure whether this possible problem is real and how to fix
it if it is real.
Any feedback would be appreciated, thanks :)
Best wishes,
Jia-Ju Bai
On 2022/2/1 19:33, Hillf Danton wrote:
> On Tue, 1 Feb 2022 15:09:34 +0800 Jia-Ju Bai wrote:
>> Hello,
>>
>> My static analysis tool reports a possible deadlock in the wfx driver in
>> Linux 5.16:
>>
>> wfx_conf_tx()
>> mutex_lock(&wdev->conf_mutex); --> Line 225 (Lock A)
>> wfx_update_pm()
>> wait_for_completion_timeout(&wvif->set_pm_mode_complete, ...); -->
>> Line 3019 (Wait X)
>>
>> wfx_add_interface()
>> mutex_lock(&wdev->conf_mutex); --> Line 737 (Lock A)
>> complete(&wvif->set_pm_mode_complete); --> Line 758 (Wake X)
>>
>> When wfx_conf_tx() is executed, "Wait X" is performed by holding "Lock
>> A". If wfx_add_interface() is executed at this time, "Wake X" cannot be
>> performed to wake up "Wait X" in wfx_conf_tx(), because "Lock A" has
>> been already hold by wfx_conf_tx(), causing a possible deadlock.
>> I find that "Wait X" is performed with a timeout, to relieve the
>> possible deadlock; but I think this timeout can cause inefficient execution.
>>
>> I am not quite sure whether this possible problem is real and how to fix
>> it if it is real.
>> Any feedback would be appreciated, thanks :)
>>
>>
>> Best wishes,
>> Jia-Ju Bai
> Hey Jia-Ju
>
> Thank you for reporting it.
>
> Given the init_completion() prior to complete() in wfx_add_interface(),
> no waiter is waken up by the complete(), so it has nothing to do with
> the waiter in the conf path.
Hi Hillf,
Thanks for your reply and detailed explanation :)
>
> BTW if the unusual wfx init is a real use case then we can add a new helper.
>
> Mind introducing your toy to LKML? How much time have you been put in it?
> Its current status and future works?
Do you mean my static analysis tool that generated the report?
In fact, I spent 3-4 months of my part time on implementing this tool,
which can detect deadlocks caused by locking cycles and improper
waiting/waking operations.
This tool still reports some false positives, due to missing some
special patterns in the kernel code, such as "init_completion() prior to
complete()" in this false bug.
Thus, I am improving the tool to reduce false positives now.
Any suggestion on deadlock detection or the tool would be appreciated,
thanks :)
Best wishes,
Jia-Ju Bai