linux-kernel - Re: [PATCHv2] serial: samsung: drop the spinlock around uart_write

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Date:	Mon, 22 Feb 2016 09:08:20 +0900
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	Anand Moon <linux.amoon@...il.com>
Cc:	Peter Hurley <peter@...leysoftware.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>, linux-serial@...r.kernel.org,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robert Bałdyga <r.baldyga@...sung.com>
Subject: Re: [PATCHv2] serial: samsung: drop the spinlock around
 uart_write_wakeup

On 21.02.2016 11:59, Anand Moon wrote:
> Hi Krzysztof,
> 
> On 21 February 2016 at 07:07, Krzysztof Kozlowski
> <k.kozlowski@...sung.com> wrote:
>> 2016-02-20 4:30 GMT+09:00 Peter Hurley <peter@...leysoftware.com>:
>>> [ +cc Krzysztof Kozlowski ]
>>>
>>> On 02/18/2016 09:15 PM, Anand Moon wrote:
>>>> From: Anand Moon <linux.amoon@...il.com>
>>>>
>>>> drop the spin_unlock/lock around uart_write_wakeup to protect
>>>> write wakeup for uart port.
>>>
>>> What Krzysztof was saying wrt v1 of this patch is that the
>>> changelog should provide as much information as possible to
>>> the maintainer(s) and driver author(s), and that you should
>>> test that outcome.
>>>
>>> Here's what I would have written for a commit message:
>>>
>>>
>>>   Remove deadlock workaround for line disciplines that invoke
>>>   the tty driver's write() method directly from their write_wakeup()
>>>   method. As documented for the write_wakeup() line discipline method
>>>   in tty_ldisc.h, line disciplines must not attempt i/o directly
>>>   from write_wakeup() as this will deadlock. Reviews of in-tree line
>>>   disciplines confirm all defer i/o.
>>>
>>>   NB: This workaround was added in commit c15c3747ee32
>>>   ("serial: samsung: fix potential soft lockup during uart write")
>>>   which notes both slip and bluetooth hci attempt i/o directly from
>>>   write_wakeup(). These issues were fixed in commits 661f7fda21b1
>>>   ("slip: Fix deadlock in write_wakeup") and da64c27d3c93
>>>   ("bluetooth: hci_ldisc: fix deadlock condition"), respectively.
>>
>> Thanks Peter for thorough analysis. It shouldn't be done by you but by
>> the patch submitter... and I have big worries that Anand did not
>> perform that analysis.
>>
>> Anand, could you at least test that this lockup does not happen
>> anymore? You will need board with Bluetooth for that (and not USB
>> Bluetooth...). If you cannot test it, maybe guys from Polish R&D could
>> help you (Cc-ed), because they were working on DMA for serial used in
>> Bluetooth.
>>
>> Best regards,
>> Krzysztof
> 
> I have looked into the history of the changes,
> commit c15c3747ee32 (serial: samsung: fix potential soft lockup during
> uart write)
> was added long time ago, that's why have missed this.
> 
> I don't have on-board Bluetooth enable boards with me,
> so if their is potential lockup you people observer
> do not consider this patch.

Which means that you cannot test it... Find someone who can test it with
Bluetooth (or SLIP) and will provide you a Tested-by tag.

Best regards,
Krzysztof