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

[Jiri Slaby]

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);
> 	}
>
> 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.

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@xxxxxxxxxxxxx/

thanks,
--
js
suse labs