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

From: duoming
Date: Thu Apr 07 2022 - 10:04:03 EST


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@xxxxxxxxxx>
> > ---
> > 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@xxxxxxxxxxxxxxxxxxxxxx>
> 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