linux-kernel - Re: [PATCH 11/11] arch: xtensa: platforms: Fix deadlock in rs

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Message-ID: <1195e776-328d-12fe-d1f8-22085dc77b44@omp.ru>
Date:   Thu, 7 Apr 2022 12:42:31 +0300
From:   Sergey Shtylyov <s.shtylyov@....ru>
To:     Duoming Zhou <duoming@....edu.cn>, <linux-kernel@...r.kernel.org>
CC:     <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>,
        <jirislaby@...nel.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>
Subject: Re: [PATCH 11/11] arch: xtensa: platforms: Fix deadlock in rs_close()

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@....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