linux-kernel - Re: [PATCH v10 1/2] printk: Make printk() completely async

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Date:   Fri, 2 Sep 2016 17:15:49 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Jan Kara <jack@...e.cz>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        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" <linux-kernel@...r.kernel.org>,
        Byungchul Park <byungchul.park@....com>, vlevenetz@...sol.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v10 1/2] printk: Make printk() completely async

On Fri 2016-09-02 16:58:08, Sergey Senozhatsky wrote:
> On (09/01/16 10:58), Petr Mladek wrote:
> > On Wed 2016-08-31 21:52:24, Sergey Senozhatsky wrote:
> > > a console_unlock() doing
> > > wake_up_process(printk_kthread) would make it better.
> > 
> > I am not sure what you mean by this.
> 
> I meant that this thing
> 
> 	local_irq_save() // or preempt_disable()
> 	...
> 	if (console_trylock())
> 		console_unlock();
> 	...
> 	local_irq_restore() // or preempt_enable()

I see.

> can easily lockup the system if console_trylock() was successful and there
> are enough messages to print. printk_kthread can't help, because here we
> basically enforce the `old' behavior. we have async printk, but not async
> console output. tweaking console_unlock() to offload the actual printing loop
> to printk_kthread would make the entire console output async:
> 
> 	static void console_sync_flush_and_unlock(void)
> 	{
> 		for (;;) {
> 		...
> 			call_console_drivers();
> 		...
> 		}
> 	}
> 
> 	void console_unlock(void)
> 	{
> 		if (!MOTORMOUTH && can_printk_async()) {
> 			up();
> 			wake_up_process(printk_kthread);
> 			return;
> 		}
> 		console_sync_flush_and_unlock();
> 	}

Something like this would make sense. But I would do it in a separate
patch(set). We need to go through all console_unlock() callers and
make sure that they are fine with the potential async behavior.
I would not complicate the async printk patchset by this.

Best Regards,
Petr