Re: [PATCH 0/3] serial: 8250_dw: Fix clk-notifier/port suspend deadlock

[Hans de Goede]

Hi,

On 9/23/20 6:19 PM, Serge Semin wrote:
> Hans has discovered that there is a potential deadlock between the ref
> clock change notifier and the port suspension procedures {see the link at
> the bottom of the letter}. Indeed the deadlock is possible if the port
> suspension is initiated during the ref clock rate change:
>
>      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 ***
>
> So the CPU performing the UART port shutdown procedure will wait until the
> ref clock change notifier is finished (worker is flushed), while the later
> will wait for a port mutex being released.
>
> A possible solution to bypass the deadlock is to move the worker flush out
> of the critical section protected by the TTY port mutex. For instance we
> can register and de-register the clock change notifier in the port probe
> and remove methods instead of having them called from the port
> startup/shutdown callbacks. But in order to do that we need to make sure
> that the serial8250_update_uartclk() method is safe to be used while the
> port is shutted down. Alas the current implementation doesn't provide that
> safety. The solution described above is introduced in the framework of
> this patchset. See individual patches for details.
>
> Link: https://lore.kernel.org/linux-serial/f1cd5c75-9cda-6896-a4e2-42c5bfc3f5c3@xxxxxxxxxx
>
> Hans, could you test the patchset out on your Cherry Trail (x86)-based
> devices? After that we can merge it in into the kernels 5.8 and 5.9 if
> there is no objections against the fix.

Done, I can confirm that this fixes the lockdep issue for me, so you
can add my:

Tested-by: Hans de Goede <hdegoede@xxxxxxxxxx>

To the entire series.

Regards,

Hans