linux-kernel - Re: [PATCH] printk: rebalance printk

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Date:	Tue, 11 Aug 2015 11:16:08 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Pan Xinhui <xinhuix.pan@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, pmladek@...e.cz,
	rostedt@...dmis.org, Tejun Heo <tj@...nel.org>,
	Peter Hurley <peter@...leysoftware.com>,
	Joe Perches <joe@...ches.com>, hch@....de,
	viro@...iv.linux.org.uk, Vasily Averin <vvs@...tuozzo.com>
Subject: Re: [PATCH] printk: rebalance printk

On Tue, Aug 11, 2015 at 07:23:01PM +0800, Pan Xinhui wrote:
> From: Pan Xinhui <xinhuix.pan@...el.com>
> 
> printk can be called in any context, It's very useful to output debug
> info.
> 
> But it might cause very bad issues on some special cases. For example,
> some driver hit errors, and it dumps many messages like reg values, etc. 
> 
> Sometimes, printk is called when irqs disabled. This is OKay if there is
> a few messages. But What would happen if many messages outputted by other
> drivers at same time.
> 
> Here is the scenario.
> CPUA					CPUB
> 					local_irq_save(flags);
> 					printk()
> while(..) {					--> console_unlock
> 	printk(...);			
> 	//hundreds or thousands loops
> }				//all messages flushed out to consoles
> 					local_irq_restore(flags);
> 

Where are you seeing this type of scenario "in the wild"?  Or is this
just a "debug/bringup hardware" issue?  We shouldn't be ever stuck in a
printk that prints hundreds or thousands of loops, if so, we need to fix
the kernel code that does that, as we do have control over this.

thanks,

greg k-h
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