Subject: Re: [PATCH 01/11] drivers: tty: serial: Fix deadlock in sa1100_set_termios()

From: Jiri Slaby <[email-masked]>
Date: Thu, 7 Apr 2022 09:02:05 +0200

On 07. 04. 22, 8:33, Duoming Zhou wrote:
> There is a deadlock in sa1100_set_termios(), which is shown
> below:
> 
>     (Thread 1)              |      (Thread 2)
>                             | sa1100_enable_ms()
> sa1100_set_termios()       |  mod_timer()
>   spin_lock_irqsave() //(1) |  (wait a time)
>   ...                       | sa1100_timeout()
>   del_timer_sync()          |  spin_lock_irqsave() //(2)
>   (wait timer to stop)      |  ...
> 
> We hold sport->port.lock in position (1) of thread 1 and
> use del_timer_sync() to wait timer to stop, but timer handler
> also need sport->port.lock in position (2) of thread 2. As a result,
> sa1100_set_termios() will block forever.
> 
> This patch extracts del_timer_sync() from the protection of
> spin_lock_irqsave(), which could let timer handler to obtain
> the needed lock.
> 
> Signed-off-by: Duoming Zhou <[email-masked]>
> ---
>   drivers/tty/serial/sa1100.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/tty/serial/sa1100.c b/drivers/tty/serial/sa1100.c
> index 5fe6cccfc1a..3a5f12ced0b 100644
> --- a/drivers/tty/serial/sa1100.c
> +++ b/drivers/tty/serial/sa1100.c
> @@ -476,7 +476,9 @@ sa1100_set_termios(struct uart_port *port, struct ktermios *termios,
>   				UTSR1_TO_SM(UTSR1_ROR);
>   	}
>   
> +	spin_unlock_irqrestore(&sport->port.lock, flags);

Unlocking the lock at this point doesn't look safe at all. Maybe moving 
the timer deletion before the lock? There is no current maintainer to 
ask. Most of the driver originates from rmk. Ccing him just in case.

FWIW the lock was moved by this commit around linux 2.5.55 (from 
full-history-linux [1])
commit f38aef3e62c26a33ea360a86fde9b27e183a3748
Author: Russell King <[email-masked]>
Date:   Fri Jan 3 15:42:09 2003 +0000

     [SERIAL] Convert change_speed() to settermios()

[1] 
https://archive.org/download/git-history-of-linux/full-history-linux.git.tar

>   	del_timer_sync(&sport->timer);
> +	spin_lock_irqsave(&sport->port.lock, flags);
>   
>   	/*
>   	 * Update the per-port timeout.

thanks,
-- 
js
suse labs