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Subject: Re: [Linux-stm32] [PATCH] serial: stm32: optimize spin lock usage
To:     dillon min <dillon.minfei@gmail.com>,
        Greg KH <gregkh@linuxfoundation.org>
CC:     <linux-serial@vger.kernel.org>,
        Alexandre TORGUE <alexandre.torgue@foss.st.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Maxime Coquelin <mcoquelin.stm32@gmail.com>,
        <jirislaby@kernel.org>, <linux-stm32@st-md-mailman.stormreply.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Fabrice Gasnier <fabrice.gasnier@foss.st.com>,
        Valentin Caron <valentin.caron@foss.st.com>
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From:   Erwan LE RAY <erwan.leray@foss.st.com>
Message-ID: <f3985d70-4f00-7442-de4e-e382b19e3e50@foss.st.com>
Date:   Mon, 12 Apr 2021 15:19:27 +0200
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Hi Dillon,

Thanks for your patch.

Could you please elaborate the use case in your commit message ?

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@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@linuxfoundation.org> wrote:
>>>>
>>>> On Mon, Apr 12, 2021 at 12:34:21PM +0800, dillon.minfei@gmail.com wrote:
>>>>> From: dillon min <dillon.minfei@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@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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