[BUG] tty: serial: possible deadlock in uart_remove_one_port() and uart_hangup()

[Jia-Ju Bai <baijiaju1990@gmail.com>]

Hello,

My static analysis tool reports a possible deadlock in the tty driver in 
Linux 5.10:

uart_remove_one_port()
   mutex_lock(&port->mutex); --> Line 3017 (Lock A)
   wait_event(state->remove_wait, ...); --> Line 3019 (Wait X)
   mutex_unlock(&port->mutex); --> Line 3021 (Unlock A)

uart_hangup()
   mutex_lock(&port->mutex); --> Line 1667 (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 1684 (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