linux-kernel - Re: Monthly md check == hung machine; how do I debug?

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Date:	Wed, 6 Feb 2008 07:27:56 +1100
From:	Neil Brown <neilb@...e.de>
To:	Robin Lee Powell <rlpowell@...italkingdom.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>, linux-kernel@...r.kernel.org
Subject: Re: Monthly md check == hung machine; how do I debug?

On Tuesday February 5, rlpowell@...italkingdom.org wrote:
> 
> I was able to solve the problem, however, like so:
> 
> 132c133
> < # CONFIG_PREEMPT_NONE is not set
> ---
> > CONFIG_PREEMPT_NONE=y
> 134,135c135,136
> < CONFIG_PREEMPT=y
> < CONFIG_PREEMPT_BKL=y
> ---
> > # CONFIG_PREEMPT is not set
> > # CONFIG_PREEMPT_BKL is not set
> 

This suggests that there is some sort of race.
Given that I've never hit it on SMP machines, it is probably a very
small window that opens immediately after some event that triggers
kernel preemption.

The only "mdadm --monitor" does in the kernel is read /proc/mdstat and
maybe make some GET_ARRAY_INFO/ GET_DISK_INFO ioctl calls.

They don't do much more than grab the reconfig_mutex.....

What sort of hardware do you have?  x86?  SMP or uni-processor?
Also, exactly what kernel are you running?

I might see if I can reproduce it... so if you can send me the broken
.config, that might help too.

Thanks,
NeilBrown
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