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

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Message-ID: <CANAwSgSVHPVWBwaO6b4QvUEopCFSKXrFbHkY837+gG62wpc-Jg@mail.gmail.com>
Date:	Sun, 21 Feb 2016 08:29:36 +0530
From:	Anand Moon <linux.amoon@...il.com>
To:	Krzysztof Kozlowski <k.kozlowski@...sung.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

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.

My change's were on the perception of the locking/unlocking.

Thanks for your inputs.

Best regards,
-Anand Moon