linux-kernel - Re: [PATCH V2 11/11] arch: xtensa: platforms: Fix deadlock in rs

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Date:   Thu, 7 Apr 2022 15:08:12 -0700
From:   Max Filippov <jcmvbkbc@...il.com>
To:     Duoming Zhou <duoming@....edu.cn>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "open list:TENSILICA XTENSA PORT (xtensa)" 
        <linux-xtensa@...ux-xtensa.org>, Jiri Slaby <jirislaby@...nel.org>,
        alexander.deucher@....com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Chris Zankel <chris@...kel.net>, s.shtylyov@....ru
Subject: Re: [PATCH V2 11/11] arch: xtensa: platforms: Fix deadlock in rs_close()

On Thu, Apr 7, 2022 at 8:44 AM Duoming Zhou <duoming@....edu.cn> 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 deletes the redundant timer_lock in order to
> prevent the deadlock. Because there is no race condition
> between rs_close, rs_open and rs_poll.
>
> Signed-off-by: Duoming Zhou <duoming@....edu.cn>
> ---
> Changes in V2:
>   - Remove the timer_lock.
>
>  arch/xtensa/platforms/iss/console.c | 8 --------
>  1 file changed, 8 deletions(-)

Thanks. Applied to my xtensa tree.

-- Max