[bug-report] task info hung problem in fb_deferred_io_work()

[Harshit Mogalapalli]

Hi Takashi,

On 19/04/24 12:14, Takashi Iwai wrote:
> On Thu, 18 Apr 2024 21:29:57 +0200,
> Helge Deller wrote:
>>
>> On 4/18/24 16:26, Takashi Iwai wrote:
>>> On Thu, 18 Apr 2024 16:06:52 +0200,
>>> Nam Cao wrote:
>>>>
>>>> On 2024-04-18 Harshit Mogalapalli wrote:
>>>>> While fuzzing 5.15.y kernel with Syzkaller, we noticed a INFO: task hung
>>>>> bug in fb_deferred_io_work()
>>>>
>>>> Which framebuffer device are you using exactly? It is possible that
>>>> the problem is with the device driver, not core framebuffer.
>>>
>>> Note that it was already known that using flush_delayed_work() caused
>>> a problem.  See the thread of the fix patch:
>>>     https://lore.kernel.org/all/20230129082856.22113-1-tiwai@suse.de/
>>
>> Harshit reported the hung tasks with kernel v5.15-stable, and can even reproduce
>> that issue with kernel v6.9-rc4 although it has all of your patches from
>> that referenced mail thread applied.
>> So, what does your statement that "it was already known that it causes problems" exactly mean?
>> Can it be fixed? Is someone looking into fixing it?
> 
> My original fix was intentionally with cancel_delayed_work_sync()
> because flush_delayed_work() didn't work.  We knew that it'd miss some
> last-minute queued change, but it's better than crash, so it was
> applied in that way.
> 

Thanks for sharing these details.

> Then later on, the commit 33cd6ea9c067 changed cancel_*() to
> flush_delayed_work() blindly, and the known problem resurfaced again.
> 

I have reverted that commit, but still could see some other task hung 
message as shared here on other reply:

https://lore.kernel.org/all/d2485cb9-277d-4b8e-9794-02f1efababc9@oracle.com/

Thanks,
Harshit
> 
> Takashi