Re: [RFC] audit: avoid soft lockup in audit_log_start()

[Konstantin Khlebnikov <khlebnikov@openvz.org>]

Luiz Capitulino wrote:
> On Mon, 09 Sep 2013 18:32:13 +0400
> Konstantin Khlebnikov<khlebnikov@openvz.org>  wrote:
>
>> Luiz Capitulino wrote:
>>> I'm getting the following soft lockup:
>>>
>>> CPU: 6 PID: 2278 Comm: killall5 Tainted: GF            3.11.0-rc7+ #1
>>> Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
>>>    0000000000000099 ffff88011fd83de8 ffffffff815324df 0000000000002800
>>>    ffffffff817d48f9 ffff88011fd83e68 ffffffff8152e669 ffff88011fd83e68
>>>    ffffffff00000008 ffff88011fd83e78 ffff88011fd83e18 0000004081dac040
>>> Call Trace:
>>>    <IRQ>    [<ffffffff815324df>] dump_stack+0x46/0x58
>>>    [<ffffffff8152e669>] panic+0xbb/0x1c4
>>>    [<ffffffff810d03c3>] watchdog_timer_fn+0x163/0x170
>>>    [<ffffffff8106c691>] __run_hrtimer+0x81/0x1c0
>>>    [<ffffffff810d0260>] ? watchdog+0x30/0x30
>>>    [<ffffffff8106cea7>] hrtimer_interrupt+0x107/0x240
>>>    [<ffffffff8102f61b>] local_apic_timer_interrupt+0x3b/0x60
>>>    [<ffffffff81542465>] smp_apic_timer_interrupt+0x45/0x60
>>>    [<ffffffff8154124a>] apic_timer_interrupt+0x6a/0x70
>>>    <EOI>    [<ffffffff810c2f5f>] ? audit_log_start+0xbf/0x430
>>>    [<ffffffff810c2fe7>] ? audit_log_start+0x147/0x430
>>>    [<ffffffff81079030>] ? try_to_wake_up+0x2a0/0x2a0
>>>    [<ffffffff810c86be>] audit_log_exit+0x6ae/0xc30
>>>    [<ffffffff81188662>] ? __alloc_fd+0x42/0x100
>>>    [<ffffffff810c98e7>] __audit_syscall_exit+0x257/0x2b0
>>>    [<ffffffff81540794>] sysret_audit+0x17/0x21
>>>
>>> The reproducer is somewhat unusual:
>>>
>>>    1. Install RHEL6.5 (maybe a similar older user-space will do)
>>>    2. Boot the just installed system
>>>    3. In this first boot you'll meet the firstboot script, which
>>>       will do some setup and (depending on your answers) it will
>>>       reboot the machine
>>>    4. During that first reboot the system hangs while terminating
>>>       all processes:
>>>
>>>          Sending all processes the TERM signal...
>>>
>>>       It's when the soft lockup above happens. And yes, I managed
>>>       to get this with latest upstream kernel (HEAD fa8218def1b1)
>>>
>>> I'm reproducing it on a VM, but the first report was on bare-metal.
>>>
>>> This is what is happening:
>>>
>>>    1. audit_log_start() is called
>>>    2. As we have SKBs waiting in audit_skb_queue and all conditions
>>>       evaluate to true, we sleep in wait_for_auditd()
>>>    3. Go to 2, until sleep_time gets negative and audit_log_start()
>>>       just busy-waits
>>>
>>> Now, *why* this is happening is a mistery to me. I tried debugging
>>> it, but all I could find is that at some point the kauditd thread
>>> never wakes up after having called schedule(). I even tried waking
>>> it up before calling wait_for_auditd(), but it didn't.
>>
>> We run into the same problem in rhel6 kernel.
>>
>> "readahead-collector" uses audit interface and sometimes stuck in 'stopped' state.
>
> Yes, please also see:
>
>   http://marc.info/?l=linux-kernel&m=137818375024600&w=2
>
>> After commit 829199197a430dade2519d54f5545c4a094393b8
>> (which was backported by RH into their kernel)
>> audit emiters will block forever if userspace daemon cannot handle backlog.
>> That commit just breaks timeout condition, after timeout waiting loop turns
>> into busy loop until deamon dies or returns back to work.
>>
>> this trivial patch should fix that problem
>>
>> --- a/kernel/audit.c
>> +++ b/kernel/audit.c
>> @@ -1215,9 +1215,10 @@ struct audit_buffer *audit_log_start(struct audit_context *ctx, gfp_t gfp_mask,
>>
>>                           sleep_time = timeout_start + audit_backlog_wait_time -
>>                                           jiffies;
>> -                       if ((long)sleep_time>  0)
>> +                       if ((long)sleep_time>  0) {
>>                                   wait_for_auditd(sleep_time);
>> -                       continue;
>> +                               continue;
>> +                       }
>>                   }
>
>
> Chuck Anderson posted a similar fix:
>
>   http://marc.info/?l=linux-kernel&m=137817994623832&w=2
>
> I still get a hang for around a minute with Chuck's fix, I believe I'll
> get the same thing with yours.

Yep, this is normal behaviour -- default audit_backlog_wait_time is 60 seconds.

But this logic is really strange, I don't see any interface for tuning that timeout
and seems like kernel set it to zero after first disaster and never recovers it back.