netdev - Re: [PATCH net 1/3] slip/slcan: added locking in wakeup function

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Date:	Thu, 19 Sep 2013 12:35:02 +0200
From:	Marc Kleine-Budde <mkl@...gutronix.de>
To:	Andre Naujoks <nautsch2@...il.com>
CC:	davem@...emloft.net, Wolfgang Grandegger <wg@...ndegger.com>,
	linux-can@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 1/3] slip/slcan: added locking in wakeup function

On 09/19/2013 12:29 PM, Andre Naujoks wrote:
> On 19.09.2013 11:36, schrieb Marc Kleine-Budde:
>> On 09/13/2013 07:37 PM, Andre Naujoks wrote:
>>> The locking is needed, since the the internal buffer for the CAN
>>> frames is changed during the wakeup call. This could cause buffer
>>> inconsistencies under high loads, especially for the outgoing
>>> short CAN packet skbuffs.
>>>
>>> The needed locks led to deadlocks before commit 
>>> "5ede52538ee2b2202d9dff5b06c33bfde421e6e4 tty: Remove extra
>>> wakeup from pty write() path", which removed the direct callback
>>> to the wakeup function from the tty layer.
>>
>> What does that mean for older kernels? (<
>> 5ede52538ee2b2202d9dff5b06c33bfde421e6e4)
> 
> It seems the slcan (and slip) driver is broken for older kernels. See
> this thread for a discussion about the patch in pty.c.
> 
> http://marc.info/?l=linux-kernel&m=137269017002789&w=2

Thanks for the info.

> The patch from Peter Hurley was actually already in the queue, when I
> ran into the problem, and is now in kernel 3.12.
> 
> Without the pty patch and slow CAN traffic, the driver works, because
> the wakeup is called directly from the pty driver. That is also the
> reason why there was no locking. It would just deadlock.
> 
> When the pty driver defers the wakeup, we ran into synchronisation
> problems (which should be fixed by the locking) and eventually into a
> kernel panic because of a recursive loop (which should be fixed by the
> pty.c patch).
> 
> Maybe it is possible to get both patches back into the stable branches?

Sounds reasonable. You might get in touch with Peter Hurley, if his
patch is scheduled for stable. Documentation/stable_kernel_rules.txt
suggests a procedure if your patch depends on others to be cherry picked.

Marc

-- 
Pengutronix e.K.                  | Marc Kleine-Budde           |
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