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

From: Jiri Slaby <jirislaby@kernel.org>
To: Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	John Ogness <john.ogness@linutronix.de>
Cc: Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Paul Cercueil <paul@crapouillou.net>,
	Matthias Brugger <matthias.bgg@gmail.com>,
	Andrew Jeffery <andrew@aj.id.au>,
	Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
	kuldip dwivedi <kuldip.dwivedi@puresoftware.com>,
	Wang Qing <wangqing@vivo.com>, Andrij Abyzov <aabyzov@slb.com>,
	Johan Hovold <johan@kernel.org>,
	Eddie Huang <eddie.huang@mediatek.com>,
	Claire Chang <tientzu@chromium.org>,
	Hsin-Yi Wang <hsinyi@chromium.org>,
	Zhang Qilong <zhangqilong3@huawei.com>,
	"Maciej W. Rozycki" <macro@orcam.me.uk>,
	Guenter Roeck <linux@roeck-us.net>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Serge Semin <Sergey.Semin@baikalelectronics.ru>,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	Al Cooper <alcooperx@gmail.com>,
	linux-serial@vger.kernel.org, linux-mips@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-mediatek@lists.infradead.org
Subject: Re: [PATCH printk v1 10/10] serial: 8250: implement write_atomic
Date: Thu, 5 Aug 2021 09:47:12 +0200	[thread overview]
Message-ID: <931ebc1a-3cc9-677c-44c3-7cbd645eb4f4@kernel.org> (raw)
In-Reply-To: <YQlNtr7TNAWtB8XF@smile.fi.intel.com>

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@linutronix.de/

thanks,
-- 
js
suse labs