linux-kernel - Re: [PATCH v3] printk: Relocate wake_klogd check close to the end of console

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Date:   Mon, 26 Feb 2018 15:37:35 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v3] printk: Relocate wake_klogd check close to the end of
 console_unlock()

On (02/19/18 17:01), Petr Mladek wrote:
[..]
> -	raw_spin_lock(&logbuf_lock);
>  	retry = console_seq != log_next_seq;
> +	/*
> +	 * Check whether userland needs notification. Do this only when really
> +	 * leaving to avoid race with console_trylock_spinning().
> +	 */
> +	if (seen_seq != log_next_seq && !retry) {
> +		wake_klogd = true;
> +		seen_seq = log_next_seq;
> +	}

Let's add the "why" part. This "!retry" might be hard to understand. We
are looking at

- CPUa is about to leave console_unlock()
- printk on CPUb appends a new message
- CPUa detects that `console_seq != log_next_seq', updates `seen_seq'
- printk on CPUb is getting preempted
- CPUa re-takes the console_sem via retry path
- printk CPUb is becoming TASK_RUNNING again - it now spins for console_sem,
  since we have an active console_sem owner
- CPUa detects that there is a console_sem waiter, so it offloads the
  printing task, without ever waking up klogd


Either we can have that complex "seen_seq != log_next_seq && !retry"
check - or we simply can add

	if (console_lock_spinning_disable_and_check()) {
		printk_safe_exit_irqrestore(flags);
		if (wake_klogd)
			wake_up_klogd();
	}

to the offloading return path.

The later is *may be* simpler to follow. The rule is: every
!console_suspend and !cant-use-consoles return path from console_unlock()
must wake_up_klogd() [if needed].

	-ss