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

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Date:	Fri, 19 Feb 2016 13:53:19 +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 13:14, 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);

In my next patch I have tried to remove the spin_unlock/spin_lock over
uart_write_wakeup(port);

Best Regards.
-Anand Moon