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Message-ID: <20160210094407.7a953c68@gandalf.local.home>
Date:	Wed, 10 Feb 2016 09:44:07 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>, linux-kernel@...r.kernel.org,
	srostedt@...hat.com, Tejun Heo <tj@...nel.org>,
	Peter Hurley <peter@...leysoftware.com>,
	Jan Kara <jack@...e.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kyle McMartin <kyle@...nel.org>,
	KY Srinivasan <kys@...rosoft.com>,
	Dave Jones <davej@...emonkey.org.uk>,
	Calvin Owens <calvinowens@...com>
Subject: Re:

On Wed, 10 Feb 2016 15:36:49 +0100
Petr Mladek <pmladek@...e.com> wrote:

> Bcc: 
> Subject: Re: [PATCH] printk: avoid livelock if another CPU printks
>  continuously
> Reply-To: 
> In-Reply-To: <1454963703-20433-1-git-send-email-dvlasenk@...hat.com>
> 

Hmm, playing with mail headers?

> > +	if (cnt == 0) {
> > +		/*
> > +		 * Other CPU(s) printk like crazy, filling log_buf[].
> > +		 * Try to get rid of the "honor" of servicing their data:
> > +		 * give _them_ time to grab console_sem and start working.
> > +		 */
> > +		cnt = 9999;
> > +		while (--cnt != 0) {
> > +			cpu_relax();
> > +			if (console_seq == log_next_seq) {  
> 
> This condition is true when all available messages are printed to
> the console. It means that there is nothing to do at all. It is
> quite late. A much better solution would be to store console_seq
> to a local variable and check it is being modified by an other CPU.
> 

Yep, I recommended the same thing.

> 
> > +				/* Good, other CPU entered "for(;;)" loop */
> > +				goto out;
> > +			}
> > +		}
> > +		/* No one seems to be willing to take it... */
> > +		if (console_trylock())
> > +			goto again; /* we took it */
> > +		/* Nope, someone else holds console_sem! Good */  
> 
> The cycle gives a big chance other CPUs to enter console_unlock().
> It means that more CPUs might end up in the above busy cycle.
> 
> It gives a chance to move the printing to another CPU. It likely
> slows down the flood of messages because the producer end up
> here as well.
> 
> So, it probably works but the performance is far from optimal.
> Many CPUs might end up doing nothing. I am afraid that this is
> not the right way to go.

Note, it's not that performance critical, and the loop only happens if
someone else is adding to the console, which hopefully, should be rare.

-- Steve