linux-kernel - Re: [PATCH printk v2 30/38] printk: console_device: use srcu console list iterator

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Date:   Tue, 25 Oct 2022 17:35:08 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v2 30/38] printk: console_device: use srcu console
 list iterator

On Wed 2022-10-19 17:01:52, John Ogness wrote:
> Use srcu console list iteration for console list traversal.
> 
> Document why the console_lock is still necessary.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> ---
>  kernel/printk/printk.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index e478cb92e7ba..410ad9d5649c 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3025,15 +3025,24 @@ struct tty_driver *console_device(int *index)
>  {
>  	struct console *c;
>  	struct tty_driver *driver = NULL;
> +	int cookie;
>  
> +	/*
> +	 * Stop console printing because the device() callback may
> +	 * assume the console is not within its write() callback.

Again, I would like to know more details about the possible races
with the write() callback. It is not that obvious.

> +	 */
>  	console_lock();
> -	for_each_console(c) {
> +
> +	cookie = console_srcu_read_lock();
> +	for_each_console_srcu(c) {
>  		if (!c->device)
>  			continue;
>  		driver = c->device(c, index);
>  		if (driver)
>  			break;
>  	}
> +	console_srcu_read_unlock(cookie);
> +
>  	console_unlock();
>  	return driver;

Otherwise, the change looks good.

Best Regards,
Petr