Date:   Thu, 10 Feb 2022 12:06:44 -0800
From:   Stephen Brennan <stephen.s.brennan@...cle.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>
Subject: Re: [PATCH v4 0/4] printk: reduce deadlocks during panic

On 2/10/22 01:22, Petr Mladek wrote:
> On Wed 2022-02-02 09:18:17, Stephen Brennan wrote:
>> When a caller writes heavily to the kernel log (e.g. writing to
>> /dev/kmsg in a loop) while another panics, there's currently a high
>> likelihood of a deadlock (see patch 2 for the full description of this
>> deadlock).
>>
>> The principle fix is to disable the optimistic spin once panic_cpu is
>> set, so the panic CPU doesn't spin waiting for a halted CPU to hand over
>> the console_sem.
>>
>> However, this exposed us to a livelock situation, where the panic CPU
>> holds the console_sem, and another CPU could fill up the log buffer
>> faster than the consoles could drain it, preventing the panic from
>> progressing and halting the other CPUs. To avoid this, patch 3 adds a
>> mechanism to suppress printk (from non-panic-CPU) during panic, if we
>> reach a threshold of dropped messages.
>>
>> A major goal with all of these patches is to try to decrease the
>> likelihood that another CPU is holding the console_sem when we halt it
>> in panic(). This reduces the odds of needing to break locks and
>> potentially encountering further deadlocks with the console drivers.
>>
>> To test, I use the following script, kmsg_panic.sh:
>>
>>      #!/bin/bash
>>      date
>>      # 991 chars (based on log buffer size):
>>      chars="$(printf 'a%.0s' {1..991})"
>>      while :; do
>>          echo $chars > /dev/kmsg
>>      done &
>>      echo c > /proc/sysrq-trigger &
>>      date
>>      exit
>>
>> I defined a hang as any time the system did not reboot to a login prompt
>> on the serial console within 60 seconds. Here are the statistics on
>> hangs using this script, before and after the patch.
>>
>> before:  776 hangs / 1484 trials - 52.3%
>> after :    0 hangs /  15k trials -  0.0%
>>
>> Stephen Brennan (4):
>>    printk: Add panic_in_progress helper
>>    printk: disable optimistic spin during panic
>>    printk: Avoid livelock with heavy printk during panic
>>    printk: Drop console_sem during panic
>>
>>   kernel/printk/printk.c | 55 +++++++++++++++++++++++++++++++++++++++++-
>>   1 file changed, 54 insertions(+), 1 deletion(-)
> 
> For the entire patchset:
> 
> Reviewed-by: Petr Mladek <pmladek@...e.com>
> 
> It looks ready for linux-next from my POV. I am going to push it early
> next week unless anyone complains in the meantime.
Thank you Petr! It occurs to me that some of this could be 
stable-worthy, depending on your feelings on it. Patches 1-3 resolve 
real bugs on customer systems, and they'd apply back a decent way. 1-2 
apply all the way back to 4.14, and 3 would apply with some minor 
changes. I suppose the question is whether they are simple enough. Patch 
4 is useful but I don't have a real reproducer for a bug it fixes, so I 
wouldn't say it's stable worthy.

Of course we have the logbuf_lock in 5.10 and previous, and if a CPU is 
halted holding that lock, then printk hangs even before the optimistic 
spinning. I have patches which reinitialize those locks after the CPUs 
are halted if necessary. I think they are reasonable for stable - printk 
is guaranteed to hang without doing this, so in the worst case you trade 
a hang during a panic, with some other sort of printk log buffer bug 
during a panic. But in the common case, you eliminate the hang. I can 
send that patch to linux-stable as well.

What do you think about these patches and stable?

Thanks again,
Stephen

> 
> Best Regards,
> Petr Mladek

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