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

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Message-ID: <CANAwSgQ2GeLaWcW5zRWPcNZ1Pk-Yq-NY4z5yqo6DXyyMRC2y3g@mail.gmail.com>
Date:	Fri, 19 Feb 2016 12:21:11 +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 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.

http://lxr.free-electrons.com/source/drivers/tty/serial/samsung.c#L701

Best Regards
-Anand Moon