Date:   Mon, 14 Feb 2022 14:54:10 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Stephen Brennan <stephen.s.brennan@...cle.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 Thu 2022-02-10 12:06:44, Stephen Brennan wrote:
> 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.

The patchset is committed in printk/linux.git, branch for-5.18-panic-deadlocks.

> 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.

Good question. If you saw these deadlocks on customer systems in
the real life then it might be worth it.

I newer saw them. But they hard to debug and report. Also they are
visible only when CPUs are stopped by NMI. And the default
smp_send_stop() tries to stop CPUs using normal IRQ first.

Anyway, the patches 1,2,4 are pretty straightforward and should be
safe. Feel free to send them to stable.

3rd patch is a heuristic. It tries to prevent livelock and the cost
is a possible loss of information. I am not 100% sure that it will
do the right thing in all situations. I would wait one or two release
cycles before we backport it to older stable releases.


> 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.

The main problem is that the locks can be safely re-initialized only
when the other CPUs were stopped using NMI. Otherwise, there is
a risk of double unlock. Such a patch would need to be arch-dependent.

Also stable people do not like much solutions that were not used
in the mainline. So, it might be a waste of time.

Best Regards,
Petr

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