Re: [PATCH v4 0/4] printk: reduce deadlocks during panic

[Stephen Brennan]

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@xxxxxxxx>
>
> 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