linux-kernel - Re: [PATCH] serial: samsung: fix the inconsistency in spinlock

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Date:	Fri, 19 Feb 2016 14:04:23 +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>
Subject: Re: [PATCH] serial: samsung: fix the inconsistency in spinlock

Hi Krzysztof,

On 19 February 2016 at 13:57, Krzysztof Kozlowski
<k.kozlowski@...sung.com> wrote:
> On 19.02.2016 17:23, Anand Moon wrote:
>> Hi Krzysztof,
>>
>> On 19 February 2016 at 13:14, Krzysztof Kozlowski
>> <k.kozlowski@...sung.com> wrote:
>>> On 19.02.2016 15:51, Anand Moon wrote:
>>>> Hi Krzysztof,
>>>>
>>>> On 19 February 2016 at 11:39, Krzysztof Kozlowski
>>>> <k.kozlowski@...sung.com> wrote:
>>>>> 2016-02-19 4:14 GMT+09:00 Anand Moon <linux.amoon@...il.com>:
>>>>>> Hi Peter,
>>>>>>
>>>>>> On 18 February 2016 at 23:18, Peter Hurley <peter@...leysoftware.com> wrote:
>>>>>>> Hi Anand,
>>>>>>>
>>>>>>> On 02/18/2016 09:40 AM, Anand Moon wrote:
>>>>>>>> From: Anand Moon <linux.amoon@...il.com>
>>>>>>>>
>>>>>>>> changes fix the correct order of the spin_lock_irqrestore/save.
>>>>>>>>
>>>>>>>> Signed-off-by: Anand Moon <linux.amoon@...il.com>
>>>>>>>> ---
>>>>>>>>  drivers/tty/serial/samsung.c | 4 ++--
>>>>>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/drivers/tty/serial/samsung.c b/drivers/tty/serial/samsung.c
>>>>>>>> index d72cd73..96fe14d 100644
>>>>>>>> --- a/drivers/tty/serial/samsung.c
>>>>>>>> +++ b/drivers/tty/serial/samsung.c
>>>>>>>> @@ -759,9 +759,9 @@ static irqreturn_t s3c24xx_serial_tx_chars(int irq, void *id)
>>>>>>>>       }
>>>>>>>>
>>>>>>>>       if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS) {
>>>>>>>> -             spin_unlock(&port->lock);
>>>>>>>> +             spin_unlock_irqrestore(&port->lock, flags);
>>>>>>>>               uart_write_wakeup(port);
>>>>>>>> -             spin_lock(&port->lock);
>>>>>>>> +             spin_lock_irqsave(&port->lock, flags);
>>>>>>>
>>>>>>> This driver shouldn't be dropping the spin lock at for write wakeup.
>>>>>>> If this is causing lock-ups in a line discipline, the line discipline
>>>>>>> needs fixed.
>>>>>>>
>>>>>>
>>>>>> Thanks for pointing out.
>>>>>> Their is no lock up, just the inconstancy of the spin_lock.
>>>>>> Then I will resend this patch dropping the spin_unlock/spin_lock
>>>>>> around uart_write_wakeup.
>>>>>> Is that ok with you.
>>>>>
>>>>> Anand, before doing that, can you check Peter's second sentence? I
>>>>> mean the "If this is causing lock-ups in a line discipline, the line
>>>>> discipline needs fixed.".
>>>>> Don't drop the spin-locks "just because". I would be happy to see more
>>>>> detailed explanation in changelog.
>>>>>
>>>>> Best regards,
>>>>> Krzysztof
>>>>
>>>> Yes I understood the meaning of the sentence. Already the
>>>> s3c24xx_serial_tx_chars function.
>>>> holds the lock port->lock for safe IRQ execution.
>>>
>>> I am sorry but I don't get your explanation. I mentioned Peter's
>>> thoughts about lockups after adding locking over uart_write(). However
>>> you are referring to s3c24xx_serial_tx_chars() holding the spin lock...
>>> I am missing the point...
>>>
>>> Best regards,
>>> Krzysztof
>>>
>>
>> I should be sorry I could not explain you in technical terms.
>> Interrupt routine already hold the port->lock
>>
>> s3c24xx_serial_tx_chars
>>      \
>>      spin_lock_irqsave(&port->lock, flags);
>>      \...
>>     spin_unlock(&port->lock);
>>      uart_write_wakeup(port);
>>      spin_lock(&port->lock);
>>      \
>>      spin_unlock_irqrestore(&port->lock, flags);
>>
>
> This is obvious.
>
>> In my next patch I have tried to remove the spin_unlock/spin_lock over
>> uart_write_wakeup(port);
>
> Which may create lockups. Previously there was no port locking around
> uart_write_wakeup. Now there will be. You are effectively adding locking
> over uart_write_wakeup().
> Again, we are back at the Peter's message - just check the damned lockups...
>
> BR,
> Krzysztof
>
> BR
>

Lets drop this patch. I have send new one earlier.
https://lkml.org/lkml/2016/2/19/2

If you have any comment on that.
Sorry for the confusion.

Best Regards.
-Anand Moon