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

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Date:   Mon, 12 Apr 2021 07:52:24 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     dillon.minfei@...il.com
Cc:     jirislaby@...nel.org, mcoquelin.stm32@...il.com,
        alexandre.torgue@...s.st.com, linux-serial@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] serial: stm32: optimize spin lock usage

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

thanks,

greg k-h