linux-kernel - Re: [PATCH] tty: serial: rad-uart: replace spin_lock_irqsave by spin

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Message-ID: <X7d7HXBp01JN5It5@localhost>
Date:   Fri, 20 Nov 2020 09:15:25 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Tian Tao <tiantao6@...ilicon.com>
Cc:     gregkh@...uxfoundation.org, jirislaby@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-unisoc@...ts.infradead.org, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: serial: rad-uart: replace spin_lock_irqsave by
 spin_lock in hard IRQ

On Fri, Nov 20, 2020 at 09:26:53AM +0800, Tian Tao wrote:
> The code has been in a irq-disabled context since it is hard IRQ. There
> is no necessity to do it again.
> 
> Signed-off-by: Tian Tao <tiantao6@...ilicon.com>
> ---
>  drivers/tty/serial/rda-uart.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/serial/rda-uart.c b/drivers/tty/serial/rda-uart.c
> index 85366e0..d6705a0 100644
> --- a/drivers/tty/serial/rda-uart.c
> +++ b/drivers/tty/serial/rda-uart.c
> @@ -406,10 +406,9 @@ static void rda_uart_receive_chars(struct uart_port *port)
>  static irqreturn_t rda_interrupt(int irq, void *dev_id)
>  {
>  	struct uart_port *port = dev_id;
> -	unsigned long flags;
>  	u32 val, irq_mask;
>  
> -	spin_lock_irqsave(&port->lock, flags);
> +	spin_lock(&port->lock);

This will break with forced irq threading (i.e. "threadirqs") since the
console code can still end up being called from interrupt context (which
can result in a deadlock).

>  
>  	/* Clear IRQ cause */
>  	val = rda_uart_read(port, RDA_UART_IRQ_CAUSE);
> @@ -426,7 +425,7 @@ static irqreturn_t rda_interrupt(int irq, void *dev_id)
>  		rda_uart_send_chars(port);
>  	}
>  
> -	spin_unlock_irqrestore(&port->lock, flags);
> +	spin_unlock(&port->lock);
>  
>  	return IRQ_HANDLED;
>  }

Johan