linux-kernel - Re: [RFC 2/2] printk: Add more information about the printk caller

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Message-ID: <20200924021756.GD577@jagdpanzerIV.localdomain>
Date:   Thu, 24 Sep 2020 11:17:56 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Prarit Bhargava <prarit@...hat.com>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        Orson Zhai <orsonzhai@...il.com>,
        Changki Kim <changki.kim@...sung.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 2/2] printk: Add more information about the printk caller

On (20/09/23 15:56), Petr Mladek wrote:
[..]
>  	/*
>  	 * To reduce unnecessarily reopening, first check if the descriptor
> -	 * state and caller ID are correct.
> +	 * state and caller infromation are correct.
>  	 */
> -	d_state = desc_read(desc_ring, id, &desc, NULL, &cid);
> -	if (d_state != desc_committed || cid != caller_id)
> +	d_state = desc_read(desc_ring, id, &desc, NULL, &cal);
> +	if (d_state != desc_committed ||
> +	    cal.pid != caller->pid ||
> +	    cal.cpu_ctx != caller->cpu_ctx) {

You probably might want to factor out ctx check into a static
inline helper. Since you use this check in several places, and
we may check more context fields in the future.

[..]
> +/* Information about the process and context that adds the message */
> +struct printk_caller {
> +	pid_t pid;	/* thread id */
> +	u32 cpu_ctx;	/* processor id and interrupt context */
> +};

A question. Suppose we have a task which does

	CPU0

	pr_err(...);

	preempt_disable();
	pr_err(...);
	preempt_enable();

	pr_err(...);

	rcu_read_lock();
	pr_info(...);
	rcu_read_unlock();

Should we distinguish those as 3 different contexts?

- normal printk
- printk under disabled preemption (affects scheduling)
- printk under RCU read side lock (affects RCU grace periods)

	-ss