linux-kernel - Re: [PATCH 2/3 v2] fb: udlfb: fix hang at disconnect

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Date:	Mon, 04 Feb 2013 20:17:04 +0100
From:	Alexander Holler <holler@...oftware.de>
To:	Greg KH <gregkh@...uxfoundation.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fbdev@...r.kernel.org,
	Florian Tobias Schandinat <FlorianSchandinat@....de>,
	Bernie Thompson <bernie@...gable.com>,
	Steve Glendinning <steve.glendinning@...well.net>,
	stable@...r.kernel.org
Subject: Re: [PATCH 2/3 v2] fb: udlfb: fix hang at disconnect

Am 04.02.2013 13:05, schrieb Alexander Holler:
> Am 04.02.2013 02:14, schrieb Greg KH:
>
>> So you are right in that your driver will wait for forever for a
>> disconnect() to happen, as it will never be called.  I don't understand
>> the problem that this is causing when it happens.  What's wrong with
>> udlfb that having the cpu suddently reset as the powerdown happened
>> without it knowing about it?
>
> There is nothing wrong with that. I've just explained why a problem
> doesn't occur on shutdown but on disconnect (of the device).

Maybe my explanation before was just to long and I try to explain it a 
bit shorter:

If a device gets disconnected, the disconnect in udlfb might wait 
forever in down_interruptible() (because it waits for an urb it never 
receives). This even prevents a shutdown afterwards, because that 
down_interruptible() never receives a signal (at shutdown, because 
kernel threads don't get such).

So the change from down_timeout() to down_interruptible() in 
dlfb_free_urb_list() with commit 
33077b8d3042e01da61924973e372abe589ba297 only results in that the 
following code (thus the break there) will never be reached if an urb 
got missed (because of a disconnect).

And the accompanying comment (... at shutdown) is misleading, because on 
shutdown, the kernel thread which calls dlfb_free_urb_list() never gets 
a signal, so the interruption just never happens.

As I've experienced the "missing urb on disconnect" problem quiet often, 
I've changed that down_interruptible() to down_timeout() (in v1 and in 
v2 to down_timeout_interruptible, because I wasn't aware that no signal 
arrives on shutdown).

Hmm, ok, that explanation isn't much shorter. ;)

Regards,

Alexander
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