Re: [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@xxxxxxx/
> >
> > 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@xxxxxxxxxx/

Thanks,
Harshit
> Takashi