Re: [RFC PATCH] serial: core: Use pm_runtime_get_sync() in uart_start()

[Xuewen Yan]

Hi John

On Mon, Nov 27, 2023 at 12:17 AM John Ogness <john.ogness@xxxxxxxxxxxxx> wrote:
>
> [Added printk maintainers CC.]
>
> On 2023-11-24, Xuewen Yan <xuewen.yan@xxxxxxxxxx> wrote:
> > The commit 84a9582fd203("serial: core: Start managing serial
> > controllers to enable runtime PM") use the pm_runtime_get() after
> > uart_port_lock() which would close the irq and disable preement.  At
> > this time, pm_runtime_get may cause the following two problems:
> >
> > (1) deadlock in try_to_wake_up:
> >
> > uart_write()
> > uart_port_lock() <<< get lock
> > __uart_start
> > __pm_runtime_resume
> > rpm_resume
> > queue_work_on
> > try_to_wake_up
> > _printk
> > uart_console_write
> > ...
> > uart_port_lock() <<< wait forever
>
> I suppose you got this because of the lockdep message generated by
> #2. It probably would make sense to call __printk_safe_enter() inside
> uart_port_lock(). This would allow printk() to automatically defer the
> printing for that CPU until the port lock is released.

Thanks for the suggestion, I would use printk_deferred in our tree to
retest the case.

And I also notice the warning was reported by syzbot:
https://lore.kernel.org/all/0000000000006f01f00608a16cea@xxxxxxxxxx/
https://lore.kernel.org/all/000000000000e7765006072e9591@xxxxxxxxxx/

>
> > (2) scheduling while atomic:
> > uart_write()
> > uart_port_lock() <<< get lock
> > __uart_start
> > __pm_runtime_resume
> > rpm_resume
> > schedule() << sleep
>
> rpm_resume() is a fascinating function. It requires the caller to hold a
> spin_lock (dev->power.lock) with interrupts disabled. But it seems to
> believe that this is the *only* spin_lock held so that it can
> temporarily spin_unlock and call might_sleep() functions. In the case of
> uart_write(), it certainly is not the only spin_lock held.
>
> I do not know enough about the internals of RPM to suggest a proper
> solution. But it looks like rpm_resume() cannot assume dev->power.lock
> is the only spin_lock held by the caller.

I would also be very grateful if could give us more suggestions.

Thanks!

BR
---
xuewen
>
> John Ogness