linux-kernel - Re: [RFC][PATCH v8 1/2] printk: Make printk() completely async

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Date:	Thu, 31 Mar 2016 13:52:50 +0200
From:	Jan Kara <jack@...e.cz>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.com>, Tejun Heo <tj@...nel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org,
	Byungchul Park <byungchul.park@....com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Jan Kara <jack@...e.cz>
Subject: Re: [RFC][PATCH v8 1/2] printk: Make printk() completely async

On Thu 31-03-16 13:12:29, Petr Mladek wrote:
> >  #if defined CONFIG_PRINTK
> > +static int printk_kthread_func(void *data)
> > +{
> > +	while (1) {
> > +		set_current_state(TASK_INTERRUPTIBLE);
> > +		if (!need_flush_console)
> > +			schedule();
> > +
> > +		__set_current_state(TASK_RUNNING);
> 
> 
> We still must do here:
> 
> 		need_flush_console = false;
> 
> Othrerwise, we might start "busy" cycling. cosole_unlock()
> sometimes returns earlly, e.g. when console_suspended is set
> or !can_use_console() returns true.
> 
> Sigh, the handling of "need_flush_console" is a bit strange.
> Part of the logic depends on logbuf_lock and the other part
> must be lockless.

Frankly, I think we are overcomplicating this. What we really need to detect
in printk_kthread_func() is whether someone appended something to the console
since we woken up. Sure, console_unlock() may have already printed that
and we would unnecessarily make one more loop over console_lock() and
console_unlock() but who cares...

So what about having printk_kthread_func() like:

	while (1) {
		set_current_state(TASK_INTERRUPTIBLE);
		if (!need_flush_console)
			schedule();
		__set_current_state(TASK_RUNNING);
		need_flush_console = false;
		console_lock();
		console_unlock();
	}

In vprintk_emit() we do:

	if (!in_panic && printk_kthread) {
		/* Offload printing to a schedulable context. */
		need_flush_console = true;
		wake_up_process(printk_kthread);
	} else {
		...

This guarantees that after message was appended to the buffer in
vprintk_emit(), the message got either printed by console_unlock() or
printk_kthread is in TASK_RUNNING state and will call console_unlock() once
scheduled. It also guarantees that printk_kthread_func() won't loop forever
when there's nothing to print. And that is all we need...

I think the simplicity of this is worth the possible extra loops in
printk_kthread_func().

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR