linux-kernel - Re: [RFC] audit: avoid soft lockup in audit_log

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Date:	Wed, 28 Aug 2013 16:08:13 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Luiz Capitulino <lcapitulino@...hat.com>
Cc:	linux-kernel@...r.kernel.org, oleg@...hat.com, eparis@...hat.com,
	rgb@...hat.com
Subject: Re: [RFC] audit: avoid soft lockup in audit_log_start()

On Wed, 28 Aug 2013 18:54:36 -0400 Luiz Capitulino <lcapitulino@...hat.com> wrote:

> > Are you really sure that kauditd is stuck in schedule() and doesn't
> > come out?
> 
> No, that's a guess. Inferred from:
> 
> 1. I tried calling wake_up_interruptible(&kauditd_wait); right
>    before wait_for_auditd(). Nothing changes
> 
> 2. I added this debug printks:
> 
>   diff --git a/kernel/audit.c b/kernel/audit.c
>   index 91e53d0..27448ad 100644
>   --- a/kernel/audit.c
>   +++ b/kernel/audit.c
>   @@ -458,11 +458,14 @@ static int kauditd_thread(void *dummy)
>    		set_current_state(TASK_INTERRUPTIBLE);
>    		add_wait_queue(&kauditd_wait, &wait);
>    
>   +		pr_emerg_ratelimited("*** sleeping\n");
>   +
>    		if (!skb_queue_len(&audit_skb_queue)) {
>    			try_to_freeze();
>    			schedule();
>    		}
>    
>   +		pr_emerg_ratelimited("*** waking up\n");
>    		__set_current_state(TASK_RUNNING);
>    		remove_wait_queue(&kauditd_wait, &wait);
>    	}
> 
>  I get several pairs of sleeping/waking up strings right before the
>  system begins to shut down. Then it stops (even though we do
>  have SKBs queued)

Well.  I assume "*** sleeping" the last thing kauditd prints?  If the
last print is "waking up" then obviously kauditd is stuck somewhere
else, which makes more sense.  A sysrq-T (or echo t >
/proc/sysrq-trigger) should provide clues.

I'd remove the "_ratelimited" - you don't want your debug messages to be
ratelimited (ie: omitted).

> Now, the kauditd thread does exists when the soft lockup happens,
> I could verify that with crash.
> 
> What's interesting is that I can only get it in that first reboot
> done by the firstboot script in RHEL6. I didn't manage to get it
> in any other way.
> 
> I'm open to any debugging tips, but be gentle because I'm not
> very familiar with that stuff.
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