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@xxxxxxxxxx>
> > ---
> > 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
