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

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Date: Thu, 18 Apr 2024 16:34:22 +0200
From: Patrik Jakobsson <patrik.r.jakobsson@...il.com>
To: Nam Cao <namcao@...utronix.de>
Cc: Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>, jayalk@...works.biz, 
	Daniel Vetter <daniel@...ll.ch>, deller@....de, linux-fbdev@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, tiwai@...e.de, bigeasy@...utronix.de, 
	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, Apr 18, 2024 at 4:05 PM Nam Cao <namcao@...utronix.de> wrote:
>
> On 2024-04-18 Patrik Jakobsson wrote:
> > This sounds similar to the SUSE bug [1]. We fixed it by reverting [2]
> > in the SUSE kernel. The problem seems to be that flush_delayed_work()
> > kills the timer and re-queues the work but doesn't guarantee that it
> > is finished when returning. So when the device is closed, the
> > fb_deferred_io_work() function can still be queued and tries to access
> > memory that's been freed.
>
> flush_delayed_work() *does* guarantee the work is finished before
> returning.

Right, flush_work() does guarantee that the work is finished, but it
doesn't guarantee that the queue is idle if work has been requeued
since flush started. So fb_deferred_io_work() should be able to happen
after the device is closed. Or am I missing something?

>
> > Patch [2] tries to solve the problem of not throwing away pending data
> > when closing the device. Perhaps calling cancel_delayed_work_sync()
> > and then follow up with a manual call to fb_deferred_io_work() would
> > be enough to flush the remaining data?
> >
> > -Patrik
> >
> > [1] https://bugzilla.suse.com/show_bug.cgi?id=1221814
> > [2] 33cd6ea9c067 fbdev: flush deferred IO before closing
> >
> > >
> > >
> > > Thanks,
> > > Harshit
>