[PATCH 3/3] serial: 8250_dw: Fix clk-notifier/port suspend deadlock

[Jonathan Liu]

On Wed, 23 Sep 2020 at 16:19, Serge Semin
<Sergey.Semin at baikalelectronics.ru> wrote:
>
> It has been discovered that there is a potential deadlock between
> the clock-change-notifier thread and the UART port suspending one:
>
>    CPU0 (suspend CPU/UART)   CPU1 (update clock)
>             ----                    ----
>    lock(&port->mutex);
>                              lock((work_completion)(&data->clk_work));
>                              lock(&port->mutex);
>    lock((work_completion)(&data->clk_work));
>
>    *** DEADLOCK ***
>
> The best way to fix this is to eliminate the CPU0
> port->mutex/work-completion scenario. So we suggest to register and
> unregister the clock-notifier during the DW APB UART port probe/remove
> procedures, instead of doing that at the points of the port
> startup/shutdown.
>
> Link: https://lore.kernel.org/linux-serial/f1cd5c75-9cda-6896-a4e2-42c5bfc3f5c3@redhat.com
>
> Fixes: cc816969d7b5 ("serial: 8250_dw: Fix common clocks usage race condition")
> Reported-by: Hans de Goede <hdegoede at redhat.com>
> Signed-off-by: Serge Semin <Sergey.Semin at baikalelectronics.ru>

Tested-by: Jonathan Liu <net147 at gmail.com>

Fixes hang while closing the serial port on RK3399 that I was
experiencing often with Linux 5.9.
After applying this patch, it no longer hangs while closing the serial port.
No problems while rebooting either.

Thanks.

Regards,
Jonathan