On Thu, 18 Apr 2024 21:29:57 +0200,
Helge Deller wrote:

On 4/18/24 16:26, Takashi Iwai wrote:

quoted

On Thu, 18 Apr 2024 16:06:52 +0200,
Nam Cao wrote:

quoted

On 2024-04-18 Harshit Mogalapalli wrote:

quoted

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/ (local)

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