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Subject: Re: [PATCH 11/11] arch: xtensa: platforms: Fix deadlock in rs_close()
To:     Duoming Zhou <duoming@zju.edu.cn>, <linux-kernel@vger.kernel.org>
CC:     <chris@zankel.net>, <jcmvbkbc@gmail.com>,
        <mustafa.ismail@intel.com>, <shiraz.saleem@intel.com>,
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References: <cover.1649310812.git.duoming@zju.edu.cn>
 <9ca3ab0b40c875b6019f32f031c68a1ae80dd73a.1649310812.git.duoming@zju.edu.cn>
From:   Sergey Shtylyov <s.shtylyov@omp.ru>
Organization: Open Mobile Platform
Message-ID: <1195e776-328d-12fe-d1f8-22085dc77b44@omp.ru>
Date:   Thu, 7 Apr 2022 12:42:31 +0300
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Hello!

On 4/7/22 9:37 AM, Duoming Zhou 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?

[...]

MBR, Sergey