linux-kernel - Re: [BUG] tty: serial: possible deadlock in uart_remove_one_port() and uart

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Date:   Tue, 1 Feb 2022 14:51:09 +0800
From:   Jia-Ju Bai <baijiaju1990@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     jirislaby@...nel.org, linux-serial@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] tty: serial: possible deadlock in uart_remove_one_port()
 and uart_hangup()

On 2022/1/29 17:57, Greg KH wrote:
> On Sat, Jan 29, 2022 at 05:34:05PM +0800, Jia-Ju Bai wrote:
>> Hello,
>>
>> My static analysis tool reports a possible deadlock in the tty driver in
>> Linux 5.10:
> 5.10 was released over a year ago and over 100 thousand changes ago.
> Please redo your check on 5.16 at the oldest.

My static analysis tool checks the tty driver in Linux 5.16, and also 
finds this possible deadlock:

uart_remove_one_port()
   mutex_lock(&port->mutex); --> Line 3032 (Lock A)
   wait_event(state->remove_wait, ...); --> Line 3034 (Wait X)
   mutex_unlock(&port->mutex); --> Line 3036 (Unlock A)

uart_hangup()
   mutex_lock(&port->mutex); --> Line 1669 (Lock A)
   uart_flush_buffer()
     uart_port_unlock()
       uart_port_deref()
         wake_up(&uport->state->remove_wait); --> Line 68 (Wake X)
   mutex_unlock(&port->mutex); --> Line 1686 (Unlock A)

When uart_remove_one_port() is executed, "Wait X" is performed by 
holding "Lock A". If uart_hangup() is executed at this time, "Wake X" 
cannot be performed to wake up "Wait X" in uart_remove_one_port(), 
because "Lock A" has been already hold by uart_remove_one_port(), 
causing a possible deadlock.

I am not quite sure whether this possible problem is real and how to fix 
it if it is real.
Maybe we can call wait_event() before mutex_lock() in 
uart_remove_one_port().
Any feedback would be appreciated, thanks :)

Best wishes,
Jia-Ju Bai