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

[Max Filippov]

Hi Duoming,

On Wed, Apr 6, 2022 at 11:38 PM Duoming Zhou <duoming@xxxxxxxxxx> 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.

I agree with this.

> This patch extracts del_timer_sync() from the protection of
> spin_lock_bh(), which could let timer handler to obtain
> the needed lock.

Looking at the timer_lock I don't really understand what it protects.
It looks like it is not needed at all.

Also, I see that rs_poll rewinds the timer regardless of whether del_timer_sync
was called or not, which violates del_timer_sync requirements.

> 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);

Now in case tty->count == 1 the timer_lock would be unlocked twice.

-- 
Thanks.
-- Max