Re: Re: [PATCH 11/11] arch: xtensa: platforms: Fix deadlock in rs_close()

[duoming@zju.edu.cn]

Hello,

On Thu, 7 Apr 2022 12:42:31 +0300 Sergey Shtylyov wrote:

> > There is a deadlock in rs_close(), which is shown
> > below:
> > 
> >    (Thread 1)              |      (Thread 2)
> >                            | rs_open()
> > rs_close()                 |  mod_timer()
> >  spin_lock_bh() //(1)      |  (wait a time)
> >  ...                       | rs_poll()
> >  del_timer_sync()          |  spin_lock() //(2)
> >  (wait timer to stop)      |  ...
> > 
> > We hold timer_lock in position (1) of thread 1 and
> > use del_timer_sync() to wait timer to stop, but timer handler
> > also need timer_lock in position (2) of thread 2.
> > As a result, rs_close() will block forever.
> > 
> > This patch extracts del_timer_sync() from the protection of
> > spin_lock_bh(), which could let timer handler to obtain
> > the needed lock.
> > 
> > Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
> > ---
> >  arch/xtensa/platforms/iss/console.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/xtensa/platforms/iss/console.c b/arch/xtensa/platforms/iss/console.c
> > index 81d7c7e8f7e..d431b61ae3c 100644
> > --- a/arch/xtensa/platforms/iss/console.c
> > +++ b/arch/xtensa/platforms/iss/console.c
> > @@ -51,8 +51,10 @@ static int rs_open(struct tty_struct *tty, struct file * filp)
> >  static void rs_close(struct tty_struct *tty, struct file * filp)
> >  {
> >  	spin_lock_bh(&timer_lock);
> > -	if (tty->count == 1)
> > +	if (tty->count == 1) {
> > +		spin_unlock_bh(&timer_lock);
> >  		del_timer_sync(&serial_timer);
> > +	}
> >  	spin_unlock_bh(&timer_lock);
> 
>    Double unlock iff tty->count == 1?

Yes, Thanks a lot for your timer and advice. I found there is no race condition
between rs_close and rs_poll(timer handler), I think we could remove the timer_lock
in rs_close(), rs_open() and rs_poll().

Best regards,
Duoming Zhou