[Linux-stm32] [PATCH] serial: stm32: optimize spin lock usage

[dillon min]

On 4/12/21, Erwan LE RAY <erwan.leray at foss.st.com> wrote:
> Hi Dillon,
>
> Thanks for your patch.
>
> Could you please elaborate the use case in your commit message ?

Sorry, local_irq_save() plus spin_lock() same to spin_lock_irqsave()
There is no deadlock . Please ignore this patch.

Thanks

Dillon
>
> Best Regards, Erwan.
>
> On 4/12/21 10:54 AM, dillon min wrote:
>> Hi Greg,
>>
>> On Mon, Apr 12, 2021 at 4:25 PM Greg KH <gregkh at linuxfoundation.org>
>> wrote:
>>>
>>> On Mon, Apr 12, 2021 at 02:50:20PM +0800, dillon min wrote:
>>>> Hi Greg，
>>>>
>>>> Thanks for the quick response, please ignore the last private mail.
>>>>
>>>> On Mon, Apr 12, 2021 at 1:52 PM Greg KH <gregkh at linuxfoundation.org>
>>>> wrote:
>>>>>
>>>>> On Mon, Apr 12, 2021 at 12:34:21PM +0800, dillon.minfei at gmail.com
>>>>> wrote:
>>>>>> From: dillon min <dillon.minfei at gmail.com>
>>>>>>
>>>>>> To avoid potential deadlock in spin_lock usage, change to use
>>>>>> spin_lock_irqsave(), spin_unlock_irqrestore() in process(thread_fn)
>>>>>> context.
>>>>>> spin_lock(), spin_unlock() under handler context.
>>>>>>
>>>>>> remove unused local_irq_save/restore call.
>>>>>>
>>>>>> Signed-off-by: dillon min <dillon.minfei at gmail.com>
>>>>>> ---
>>>>>> Was verified on stm32f469-disco board. need more test on stm32mp
>>>>>> platform.
>>>>>>
>>>>>>   drivers/tty/serial/stm32-usart.c | 27 +++++++++++++++++----------
>>>>>>   1 file changed, 17 insertions(+), 10 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/tty/serial/stm32-usart.c
>>>>>> b/drivers/tty/serial/stm32-usart.c
>>>>>> index b3675cf25a69..c4c859b34367 100644
>>>>>> --- a/drivers/tty/serial/stm32-usart.c
>>>>>> +++ b/drivers/tty/serial/stm32-usart.c
>>>>>> @@ -214,7 +214,7 @@ static void stm32_usart_receive_chars(struct
>>>>>> uart_port *port, bool threaded)
>>>>>>        struct tty_port *tport = &port->state->port;
>>>>>>        struct stm32_port *stm32_port = to_stm32_port(port);
>>>>>>        const struct stm32_usart_offsets *ofs =
>>>>>> &stm32_port->info->ofs;
>>>>>> -     unsigned long c;
>>>>>> +     unsigned long c, flags;
>>>>>>        u32 sr;
>>>>>>        char flag;
>>>>>>
>>>>>> @@ -276,9 +276,17 @@ static void stm32_usart_receive_chars(struct
>>>>>> uart_port *port, bool threaded)
>>>>>>                uart_insert_char(port, sr, USART_SR_ORE, c, flag);
>>>>>>        }
>>>>>>
>>>>>> -     spin_unlock(&port->lock);
>>>>>> +     if (threaded)
>>>>>> +             spin_unlock_irqrestore(&port->lock, flags);
>>>>>> +     else
>>>>>> +             spin_unlock(&port->lock);
>>>>>
>>>>> You shouldn't have to check for this, see the other patches on the
>>>>> list
>>>>> recently that fixed this up to not be an issue for irq handlers.
>>>> Can you help to give more hints, or the commit id of the patch which
>>>> fixed this. thanks.
>>>>
>>>> I'm still confused with this.
>>>>
>>>> The stm32_usart_threaded_interrupt() is a kthread context, once
>>>> port->lock holds by this function, another serial interrupts raised,
>>>> such as USART_SR_TXE,stm32_usart_interrupt() can't get the lock,
>>>> there will be a deadlock. isn't it?
>>>>
>>>>   So, shouldn't I use spin_lock{_irqsave} according to the caller's
>>>> context ?
>>>
>>> Please see 81e2073c175b ("genirq: Disable interrupts for force threaded
>>> handlers") for when threaded irq handlers have irqs disabled, isn't that
>>> the case you are trying to "protect" from here?
>>>
>>> Why is the "threaded" flag used at all?  The driver should not care.
>>>
>>> Also see 9baedb7baeda ("serial: imx: drop workaround for forced irq
>>> threading") in linux-next for an example of how this was fixed up in a
>>> serial driver.
>>>
>>> does that help?
>>>
>> Yes, it's really helpful. and 81e2073c175b should be highlighted in a
>> doc.
>> In my past knowledge, we should care about hard irq & thread_fn lock
>> conflict.
>> This patch has totally avoided patching code in the separate driver side.
>> thanks.
>>
>> I will just keep the changes in stm32_usart_console_write(), remove
>> these code in
>> thread_fn. update version 2 for you.
>>
>> thanks.
>>
>> Dillon,
>>> thanks,
>>>
>>> greg k-h
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