linux-kernel - Re: [PATCH v5 3/4] serial: 8250_dw: warning on entering dw8250_force

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Message-ID: <71a295db-72ea-bf2a-338f-416b178f5305@linux.intel.com>
Date: Tue, 6 May 2025 15:06:18 +0300 (EEST)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Yunhui Cui <cuiyunhui@...edance.com>
cc: arnd@...db.de, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, 
    benjamin.larsson@...exis.eu, 
    Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
    heikki.krogerus@...ux.intel.com, Jiri Slaby <jirislaby@...nel.org>, 
    jkeeping@...usicbrands.com, john.ogness@...utronix.de, 
    LKML <linux-kernel@...r.kernel.org>, 
    linux-serial <linux-serial@...r.kernel.org>, markus.mayer@...aro.org, 
    matt.porter@...aro.org, namcao@...utronix.de, paulmck@...nel.org, 
    pmladek@...e.com, schnelle@...ux.ibm.com, sunilvl@...tanamicro.com, 
    tim.kryger@...aro.org
Subject: Re: [PATCH v5 3/4] serial: 8250_dw: warning on entering dw8250_force_idle
 unlocked

On Tue, 6 May 2025, Yunhui Cui wrote:

> Read UART_RX and check UART_LSR_DR in critical section. Unsure if

Unsure if -> Ensure the

> caller of dw8250_force_idle() holds port->lock. Don't acquire it
> directly to avoid deadlock. Use lockdep_assert_held_once for warning.

Add (), although the last two sentences don't seem that useful, IMO.

> 
> Signed-off-by: Yunhui Cui <cuiyunhui@...edance.com>
> ---
>  drivers/tty/serial/8250/8250_dw.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
> index af24ec25d976..f41c4a9ed58b 100644
> --- a/drivers/tty/serial/8250/8250_dw.c
> +++ b/drivers/tty/serial/8250/8250_dw.c
> @@ -13,6 +13,7 @@
>  #include <linux/delay.h>
>  #include <linux/device.h>
>  #include <linux/io.h>
> +#include <linux/lockdep.h>
>  #include <linux/mod_devicetable.h>
>  #include <linux/module.h>
>  #include <linux/notifier.h>
> @@ -112,6 +113,13 @@ static void dw8250_force_idle(struct uart_port *p)
>  	struct uart_8250_port *up = up_to_u8250p(p);
>  	unsigned int lsr;
>  
> +	/*
> +	 * The serial_in(p, UART_RX) should be under port->lock, but we can't add
> +	 * it to avoid AA deadlock as we're unsure if serial_out*(...UART_LCR)
> +	 * is under port->lock.

I'm left to wonder who/what "we" is here? Could you change it something 
more precise.

> +	 */
> +	lockdep_assert_held_once(&p->lock);
> +
>  	serial8250_clear_and_reinit_fifos(up);
>  
>  	/*
> 

-- 
 i.