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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <55CB3CAC.6000003@hurleysoftware.com>
Date:	Wed, 12 Aug 2015 08:31:40 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	Pan Xinhui <xinhuix.pan@...el.com>,
	"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>,
	Joe Perches <joe@...ches.com>, hch@....de,
	viro@...iv.linux.org.uk, Vasily Averin <vvs@...tuozzo.com>
Subject: Re: [PATCH] printk: rebalance printk

On 08/11/2015 02:16 PM, 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?

There have been problem reports of big machines getting soft-lockup/RCU stall
warnings with serial console attached. I think SLES is carrying patches
from Jan Kara to try to workaround this 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.

The loop referred to here is the loop in console_unlock(). Essentially
what happens is one cpu can get trapped in the console_unlock() output
loop; printk()'s from other cpus are only appending to the logbuf since
they can't acquire the console_lock (which is owned by the one cpu trapped
in the output loop).

Regards,
Peter Hurley
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/