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

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Date:   Fri, 26 Aug 2016 17:20:48 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Jan Kara <jack@...e.cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        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>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH v10 1/2] printk: Make printk() completely async

On (08/26/16 10:56), Sergey Senozhatsky wrote:


> but every lock we take is potentially dangerous as well.
...
> vprintk_emit()
> {
> 	alt_printk_enter();
> 	...
> 	log_store();
> 	...
> 	alt_printk_exit();
> 
> 	wakep_up_process() /* direct from async printk,
> 			      or indirect from console_unlock()->up() */
> 		alt_printk_enter();
> 		... enqueue task
> 		alt_printk_exit();
> }


OTOH, after a very quick thought, up() also takes a spin lock, which
may spindump. so I'll probably prefer to keep the entire alt-printk
thing entirely in printk(). something like this

vprintk_emit()
{
	alt_printk_enter()
	log_store()
	alt_printk_exit()

	if (async_printk)
	{
		alt_printk_enter()
		wake_up_process()
		alt_printk_exit()
	} else {
		if (console_trylock()) {
			console_unlock()
			{
				....
				alt_printk_enter()
				up()
				alt_printk_exit()
			}
		}
	}
}

this leaves console_trylock() `unprotected'. so I guess I'll do
something like this:

	} else {
		int ret;

		alt_printk_enter()
		ret = console_trylock();
		alt_printk_exit()

		if (ret)
			console_unlock();
	}

a bit ugly, but well, it is what it is. will think more about it.

	-ss