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

[Jia-Ju Bai]

On 2022/2/1 17:07, Hillf Danton wrote:
> On Tue, 1 Feb 2022 14:51:09 +0800 Jia-Ju Bai wrote:
> > 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
> Hey Jia-Ju
>
> Thank you for reporting it.
>
> In uart_flush_buffer(), uart_port_unlock() pairs with uart_port_lock()
> which bumps refcount up. OTOH no wakep is needed without refcount
> incremented, so the wakeup above in the hangup path is not waited for
> in the remove path.

Hi Hillf,

Thanks for the explanation :)
So I wonder which wait_event() can be paired with 
wake_up(&uport->state->remove_wait) in uart_port_deref()?


Best wishes,
Jia-Ju Bai