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

[Takashi Iwai]

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.

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


Takashi