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

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Date:	Fri, 19 Feb 2016 16:44:35 +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>
Subject: Re: [PATCH] serial: samsung: fix the inconsistency in spinlock

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