Hi Jann,

On Fri, Jun 26, 2020 at 04:25:59PM +0200, Jann Horn wrote:
>On Fri, Jun 26, 2020 at 3:41 PM Greg KH <gre...@linuxfoundation.org> wrote:
>> On Fri, Jun 26, 2020 at 12:35:58PM +0100, Steve McIntyre wrote:


>> > Considering I'm running strace build tests to provoke this bug,
>> > finding the failure in a commit talking about ptrace changes does look
>> > very suspicious...!
>> >
>> > Annoyingly, I can't reproduce this on my disparate other machines
>> > here, suggesting it's maybe(?) timing related.
>Does "hard lockup" mean that the HARDLOCKUP_DETECTOR infrastructure
>prints a warning to dmesg? If so, can you share that warning?

I mean the machine locks hard - X stops updating, the mouse/keyboard
stop responding. No pings, etc. When I reboot, there's nothing in the

>If you don't have any way to see console output, and you don't have a
>working serial console setup or such, you may want to try re-running
>those tests while the kernel is booted with netconsole enabled to log
>to a different machine over UDP (see

ACK, will try that now for you.

>You may want to try setting the sysctl kernel.sysrq=1 , then when the
>system has locked up, press ALT+PRINT+L (to generate stack traces for
>all active CPUs from NMI context), and maybe also ALT+PRINT+T and
>ALT+PRINT+W (to collect more information about active tasks).


>(If you share stack traces from these things with us, it would be
>helpful if you could run them through scripts/decode_stacktrace.pl
>from the kernel tree first, to add line number information.)


>Trying to isolate the problem:
>__end_current_label_crit_section and end_current_label_crit_section
>are aliases of each other (via #define), so that line change can't
>have done anything.
>That leaves two possibilities AFAICS:
> - the might_sleep() call by itself is causing issues for one of the
>remaining users of begin_current_label_crit_section() (because it
>causes preemption to happen more often where it didn't happen on
>PREEMPT_VOLUNTARY before, or because it's trying to print a warning
>message with the runqueue lock held, or something like that)
> - the lack of "if (aa_replace_current_label(label) == 0)
>aa_put_label(label);" in __begin_current_label_crit_section() is
>somehow causing issues
>You could try to see whether just adding the might_sleep(), or just
>replacing the begin_current_label_crit_section() call with
>__begin_current_label_crit_section(), triggers the bug.
>If you could recompile the kernel with CONFIG_DEBUG_ATOMIC_SLEEP - if
>that isn't already set in your kernel config -, that might help track
>down the problem, unless it magically makes the problem stop
>triggering (which I guess would be conceivable if this indeed is a

OK, will try that second...

Steve McIntyre, Cambridge, UK.                                st...@einval.com
"I can't ever sleep on planes ... call it irrational if you like, but I'm
 afraid I'll miss my stop" -- Vivek Das Mohapatra

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