linux-kernel - Re: [RFC][PATCH -next 1/2] printk: move can_use_console out of console_trylock_for

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Message-ID: <20160120100909.GA3305@pathway.suse.cz>
Date:	Wed, 20 Jan 2016 11:09:09 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Jan Kara <jack@...e.com>,
	Kyle McMartin <kyle@...nel.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Calvin Owens <calvinowens@...com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH -next 1/2] printk: move can_use_console out of
 console_trylock_for_printk

On Wed 2016-01-20 13:18:04, Sergey Senozhatsky wrote:
> On (01/19/16 17:16), Petr Mladek wrote:
> > 	do_cond_resched = console_may_schedule;
> > 	console_may_schedule = 0;
> > 
> > +again:
> > +	if (!can_use_console()) {
> > +		console_locked = 0;
> > +		up_console_sem();
> > +		return;
> > 	}
> > 
> > 	/* flush buffered message fragment immediately to console */
> > 	console_cont_flush(text, sizeof(text));
> > -again:
> > 	for (;;) {
> > 		struct printk_log *msg;
> > 
> 
> looks better. we do extra IRQ disable/enable (spin lock irq) when we jump
> to `again' label, but I don't think this introduces any significant overhead.
> however, if it does, we always can avoid extra console_cont_flush() by simply
> checking @retry -- it's `false' only once, when we execute this part for
> the first time in the current console_unlock() call; all goto-jumps imply
> that @retry is `true'.

IMHO, the extra spin_lock/unlock does not harm much. We do this
for each line in the for(;;) cycle anyway. But the check for retry
value is clever. We could use this if others would want to preserve
the existing behavior and call console_cont_flush() only once.

Best Regards,
Petr