Re: port lock: was: Re: [PATCH printk v1 11/18] printk: nobkl: Introduce printer threads

[John Ogness <john.ogness@linutronix.de>]

On 2023-04-20, Petr Mladek <pmladek@suse.com> wrote:
>> OK, let's first define what the two locks are supposed to synchronize.
>> My understanding is that this patchset uses them the following way:
>> 
>>     + The new lock (atomic_state) is used to serialize emiting
>>       messages between different write contexts. It replaces
>>       the functionality of console_lock.

It replaces the functionality of console_lock, but operates at a finer
level. It is serializing all access to the hardware registers involved
in outputting. For the 8250 driver, this is the IER register.

>>       It is a per-console sleeping lock, allows voluntary and has
>>       hand-over using priorities and spinning with a timeout.

It is not a sleeping lock. It is used as a trylock or spinning with
timeout. It has the special feature that it can be handed over to or
stolen by another context with a higher ownership priority.

>>     + The port_lock is used to synchronize various operations
>>       of the console driver/device, like probe, init, exit,
>>       configuration update.
>> 
>>       It is typically a per-console driver/device spin lock.
>> 
>> 
>> I guess that we would want to keep both locks:

I agree because the port_lock has a much larger scope and is fully
preemptible under PREEMPT_RT.

> I forgot to check how these two locks are supposed to be used
> in write_atomic().
>
> It seems that cons_atomic_flush_con() takes only the new lock
> (atomic_state) and ignores the port_lock(). It should be safe
> against write_kthread(). But it is not safe against other
> operations with the console device that are synchronized
> only by the port_lock().

Yes, it is because the console drivers will also take the atomic_state
lock when needed. You can see this in the POC patch I posted [0].

For example, a new function serial8250_enter_unsafe() is used by the
serial drivers to mark the beginning of an unsafe section. To use this
function, the port_lock must be held. This function additionally takes
the atomic_state lock. Then the driver is allowed to touch hardware
registers related to outputting (IER).

But typically the driver will use a new higher level function, for
example serial8250_in_IER(), which will enter unsafe, read the register,
and exit unsafe. This provides the necessary synchronization against
write_atomic() (for the 8250 driver).

Please also remember that hostile takeovers of drivers in unsafe
sections are done as a last resort in panic, after all other nbcon
consoles have safely flushed their buffers. So we should not spend too
many brain cycles on "what if the atomic_state lock is stolen while in
an unsafe section" questions. The answer is: then you are in "hope and
pray" mode.

John

[0] https://lore.kernel.org/lkml/877cv1geo4.fsf@jogness.linutronix.de