netdev - Re: Re: [PATCH 01/11] drivers: tty: serial: Fix deadlock in sa1100_set

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Date:   Thu, 7 Apr 2022 22:03:04 +0800 (GMT+08:00)
From:   duoming@....edu.cn
To:     "Jiri Slaby" <jirislaby@...nel.org>
Cc:     linux-kernel@...r.kernel.org, chris@...kel.net, jcmvbkbc@...il.com,
        mustafa.ismail@...el.com, shiraz.saleem@...el.com, jgg@...pe.ca,
        wg@...ndegger.com, mkl@...gutronix.de, davem@...emloft.net,
        kuba@...nel.org, pabeni@...hat.com, jes@...ined-monkey.org,
        gregkh@...uxfoundation.org, alexander.deucher@....com,
        linux-xtensa@...ux-xtensa.org, linux-rdma@...r.kernel.org,
        linux-can@...r.kernel.org, netdev@...r.kernel.org,
        linux-hippi@...site.dk, linux-staging@...ts.linux.dev,
        linux-serial@...r.kernel.org, linux-usb@...r.kernel.org,
        "Russell King - ARM Linux" <linux@...linux.org.uk>,
        linma@....edu.cn, rmk@...nt.arm.linux.org.uk
Subject: Re: Re: [PATCH 01/11] drivers: tty: serial: Fix deadlock in
 sa1100_set_termios()

Hello,

On Thu, 7 Apr 2022 09:02:05 +0200 Jiri Slaby 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 <duoming@....edu.cn>
> > ---
> >   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.

Thanks a lot for your time and advice. I think moving the del_timer_sync()
before the lock is good. Because we may use "sa1100_enable_ms(&sport->port)"
to start the timer after we have set termios.

> FWIW the lock was moved by this commit around linux 2.5.55 (from 
> full-history-linux [1])
> commit f38aef3e62c26a33ea360a86fde9b27e183a3748
> Author: Russell King <rmk@...nt.arm.linux.org.uk>
> 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.


Best regards,
Duoming Zhou