linux-kernel - Re: [PATCH next v2 3/3] printk: remove logbuf_lock, add syslog

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Date:   Fri, 4 Dec 2020 15:41:43 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH next v2 3/3] printk: remove logbuf_lock, add syslog_lock

On (20/12/01 21:59), John Ogness wrote:
>  
> +#ifdef CONFIG_PRINTK_NMI
> +#define NUM_RECURSION_CTX 2
> +#else
> +#define NUM_RECURSION_CTX 1
> +#endif
> +
> +struct printk_recursion {
> +	char	count[NUM_RECURSION_CTX];
> +};
> +
> +static DEFINE_PER_CPU(struct printk_recursion, percpu_printk_recursion);
> +static char printk_recursion_count[NUM_RECURSION_CTX];
> +
> +static char *get_printk_count(void)

A nit: I think get_foo() has some sort of special meaning and one would
expect that there should be put_foo() as well, and that those have
something to do with the ref counting.

> +{
> +	struct printk_recursion *rec;
> +	char *count;
> +
> +	if (!printk_percpu_data_ready()) {
> +		count = &printk_recursion_count[0];
> +	} else {
> +		rec = this_cpu_ptr(&percpu_printk_recursion);
> +
> +		count = &rec->count[0];
> +	}
> +
> +#ifdef CONFIG_PRINTK_NMI
> +	if (in_nmi())
> +		count++;
> +#endif
> +
> +	return count;
> +}
> +
> +static bool printk_enter(unsigned long *flags)

This better explicitly state that it disables local IRQs

	printk_enter_irqsave()

I'm not very certain that printk_enter/printk_exit are best names:

	if (!printk_enter())
		return;

Maybe it can spell out why we couldn't enter printk so the function
name can contain recursion_limit or something.

> +{
> +	char *count;
> +
> +	local_irq_save(*flags);
> +	count = get_printk_count();
> +	/* Only 1 level of recursion allowed. */
> +	if (*count > 1) {
> +		local_irq_restore(*flags);
> +		return false;
> +	}
> +	(*count)++;
> +
> +	return true;
> +}
> +
> +static void printk_exit(unsigned long flags)

This enables local IRQs, so

	printk_exit_irqrestore()

> +{
> +	char *count;
> +
> +	count = get_printk_count();
> +	(*count)--;
> +	local_irq_restore(flags);
> +}

	-ss