linux-kernel - Re: [PATCH v2] serial: stm32: optimize spin lock usage

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Date:   Mon, 12 Apr 2021 15:08:14 +0200
From:   Johan Hovold <johan@...nel.org>
To:     dillon.minfei@...il.com
Cc:     gregkh@...uxfoundation.org, jirislaby@...nel.org,
        mcoquelin.stm32@...il.com, alexandre.torgue@...s.st.com,
        lkp@...el.com, linux-serial@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kbuild-all@...ts.01.org, clang-built-linux@...glegroups.com,
        Gerald Baeza <gerald.baeza@...s.st.com>,
        Erwan Le Ray <erwan.leray@...s.st.com>
Subject: Re: [PATCH v2] serial: stm32: optimize spin lock usage

On Mon, Apr 12, 2021 at 05:31:38PM +0800, dillon.minfei@...il.com wrote:
> From: dillon min <dillon.minfei@...il.com>
> 
> To avoid potential deadlock in spin_lock usage, use spin_lock_irqsave,
> spin_trylock_irqsave(), spin_unlock_irqrestore() in process context.

This doesn't make much sense as console_write can be called in any
context. And where's the deadlock you claim to be fixing here?
 
> remove unused local_irq_save/restore call.
> 
> Cc: Alexandre Torgue <alexandre.torgue@...s.st.com>
> Cc: Maxime Coquelin <mcoquelin.stm32@...il.com>
> Cc: Gerald Baeza <gerald.baeza@...s.st.com>
> Cc: Erwan Le Ray <erwan.leray@...s.st.com>
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: dillon min <dillon.minfei@...il.com>
> ---
> v2: remove unused code from stm32_usart_threaded_interrupt() according from
>     Greg's review.
> 
>  drivers/tty/serial/stm32-usart.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
> index b3675cf25a69..b1ba5e36e36e 100644
> --- a/drivers/tty/serial/stm32-usart.c
> +++ b/drivers/tty/serial/stm32-usart.c
> @@ -1354,13 +1354,12 @@ static void stm32_usart_console_write(struct console *co, const char *s,
>  	u32 old_cr1, new_cr1;
>  	int locked = 1;
>  
> -	local_irq_save(flags);
>  	if (port->sysrq)
>  		locked = 0;
>  	else if (oops_in_progress)
> -		locked = spin_trylock(&port->lock);
> +		locked = spin_trylock_irqsave(&port->lock, flags);
>  	else
> -		spin_lock(&port->lock);
> +		spin_lock_irqsave(&port->lock, flags);
>  
>  	/* Save and disable interrupts, enable the transmitter */
>  	old_cr1 = readl_relaxed(port->membase + ofs->cr1);
> @@ -1374,8 +1373,7 @@ static void stm32_usart_console_write(struct console *co, const char *s,
>  	writel_relaxed(old_cr1, port->membase + ofs->cr1);
>  
>  	if (locked)
> -		spin_unlock(&port->lock);
> -	local_irq_restore(flags);
> +		spin_unlock_irqrestore(&port->lock, flags);
>  }
>  
>  static int stm32_usart_console_setup(struct console *co, char *options)

Johan