linux-kernel - Re: [bug-report] task info hung problem in fb_deferred_io

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Message-ID: <878r19voks.wl-tiwai@suse.de>
Date: Fri, 19 Apr 2024 08:44:35 +0200
From: Takashi Iwai <tiwai@...e.de>
To: Helge Deller <deller@....de>
Cc: Takashi Iwai <tiwai@...e.de>,
	Nam Cao <namcao@...utronix.de>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>,
	Daniel Vetter <daniel@...ll.ch>,
	linux-fbdev@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	bigeasy@...utronix.de,
	patrik.r.jakobsson@...il.com,
	LKML <linux-kernel@...r.kernel.org>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	George Kennedy <george.kennedy@...cle.com>,
	Darren Kenny <darren.kenny@...cle.com>,
	chuansheng.liu@...el.com
Subject: Re: [bug-report] task info hung problem in fb_deferred_io_work()

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