linux-kernel - Re: [LKP] [tty] c96cf923a9: WARNING:possible_circular_locking_dependency

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Date:   Wed, 12 Dec 2018 10:07:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Dmitry Safonov <dima@...sta.com>, Jiri Slaby <jslaby@...e.cz>,
        kernel test robot <rong.a.chen@...el.com>, lkp@...org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Waiman Long <longman@...hat.com>
Subject: Re: [LKP] [tty] c96cf923a9:
 WARNING:possible_circular_locking_dependency_detected

On Wed, Dec 12, 2018 at 12:42:52PM +0900, Sergey Senozhatsky wrote:
> Something like this (not tested):
> 
> ---
> 
>  drivers/tty/serial/serial_core.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index c439a5a1e6c0..64050f506348 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -268,6 +268,7 @@ static void uart_shutdown(struct tty_struct *tty, struct uart_state *state)
>  	struct uart_port *uport = uart_port_check(state);
>  	struct tty_port *port = &state->port;
>  	unsigned long flags = 0;
> +	char *xmit_buf = NULL;
>  
>  	/*
>  	 * Set the TTY IO error marker
> @@ -297,15 +298,16 @@ static void uart_shutdown(struct tty_struct *tty, struct uart_state *state)
>  	 */
>  	tty_port_set_suspended(port, 0);
>  
> +	uart_port_lock(state, flags);
> +	xmit_buf = state->xmit.buf;
> +	state->xmit.buf = NULL;
> +	uart_port_unlock(uport, flags);
> +
>  	/*
>  	 * Free the transmit buffer page.
>  	 */
> -	uart_port_lock(state, flags);
> -	if (state->xmit.buf) {
> -		free_page((unsigned long)state->xmit.buf);
> -		state->xmit.buf = NULL;
> -	}
> -	uart_port_unlock(uport, flags);
> +	if (xmit_buf)
> +		free_page((unsigned long)xmit_buf);
>  }

That looks sensible anyhow. One should strive to not do alloc or free
under locks as much as possible anyway.