[PATCH printk v1 10/10] serial: 8250: implement write_atomic

[John Ogness]

On 2021-08-05, Jiri Slaby <jirislaby at kernel.org> wrote:
> On 03. 08. 21, 16:07, Andy Shevchenko wrote:
>> On Tue, Aug 03, 2021 at 03:19:01PM +0206, John Ogness wrote:
>>> Implement an NMI-safe write_atomic() console function in order to
>>> support synchronous console printing.
>>>
>>> Since interrupts need to be disabled during transmit, all usage of
>>> the IER register is wrapped with access functions that use the
>>> printk cpulock to synchronize register access while tracking the
>>> state of the interrupts. This is necessary because write_atomic()
>>> can be called from an NMI context that has preempted write_atomic().
>> 
>> ...
>> 
>>> +static inline void serial8250_set_IER(struct uart_8250_port *up,
>>> +				      unsigned char ier)
>>> +{
>>> +	struct uart_port *port = &up->port;
>>> +	unsigned long flags;
>>> +	bool is_console;
>> 
>>> +	is_console = uart_console(port);
>>> +
>>> +	if (is_console)
>>> +		console_atomic_cpu_lock(flags);
>>> +
>>> +	serial_out(up, UART_IER, ier);
>>> +
>>> +	if (is_console)
>>> +		console_atomic_cpu_unlock(flags);
>> 
>> I would rewrite it as
>> 
>> 	if (uart_console()) {
>> 		console_atomic_cpu_lock(flags);
>> 		serial_out(up, UART_IER, ier);
>> 		console_atomic_cpu_unlock(flags);
>> 	} else {
>> 		serial_out(up, UART_IER, ier);
>> 	}

Some locations have more than just 1 line of code in between
lock/unlock. I agree this looks better, but am unsure how much
copy/paste code is acceptable.

>> No additional variable, easier to get the algorithm on the first
>> glance, less error prone.
>
> Yes, the original is terrible.
>
> Another option:
>
> bool locked = console_atomic_cpu_lock(flags, uart_console());
> serial_out(up, UART_IER, ier);
> console_atomic_cpu_unlock(flags, locked);
>
> Which makes console_atomic_cpu_lock to lock only if second parameter
> is true and return its value too.

I am not sure how common such semantics for lock/unlock functions
are. But since this pattern, using uart_console(), will most likely be a
common pattern for atomic consoles, I can see how this will be useful.

I will choose one of these 2 suggestions for v2. Thanks.

> BTW I actually don't know what console_atomic_cpu_lock does to think 
> about it more as I was not CCed, and neither lore sees the other patches:
> https://lore.kernel.org/linux-mips/20210803131301.5588-1-john.ogness@linutronix.de/

Only the lkml mailing list saw the full series:

https://lore.kernel.org/lkml/20210803131301.5588-1-john.ogness@linutronix.de/

John Ogness