Re: [PATCH] printk: rebalance printk

From: Peter Hurley
Date: Wed Aug 12 2015 - 08:31:51 EST

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@xxxxxxxxx>
>> 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.
>> 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).

Peter Hurley
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