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Message-ID: <2ad5aaaf.3fa1d.1800455f764.Coremail.duoming@zju.edu.cn>
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
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