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

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Message-ID: <20150812020406.GA3383@kroah.com>
Date:	Tue, 11 Aug 2015 19:04:06 -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 Wed, Aug 12, 2015 at 09:53:58AM +0800, Pan Xinhui wrote:
> Hi, Greg
> 	thanks for your reply!
> On 2015年08月12日 02:16, Greg Kroah-Hartman wrote:
> > 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.
> > 
> 
> Sorry for misleading. these loops are just an example. In real world,
> there are many pr_info/pr_err when some drivers hit device errors to
> dump debug information.

That's not good, they should use dev_dbg() for that instead.

> There is camera and sound test(multimedia test) in our side, we enable
> dynamic_debug to collect logs to fix some weird issues.  sound driver
> will dump many reg/memory values, and camera will output a few logs
> with irqs disabled. So we hit this printk issues very frequently, and
> it blocks our tests indeed.

Maybe those memory value dumps should be done through debugfs instead of
printk?

> As printk is announced that it could be used in most context, So I
> prefer to enhance the stability of printk to handle such case.

Stability is nice, but using printk for things it was never ment to be
used for, isn't good.  Especially if you are forced to add additional
complexity to the printk logic just for these corner cases that no one
uses :)

thanks,

greg k-h
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