Re: [rcu] 2f08469563: BUG:kernel_reboot-without-warning_in_boot_stage

[Marco Elver]

On Mon, 18 May 2020 at 14:44, Marco Elver <elver@xxxxxxxxxx> wrote:
>
> [+Cc clang-built-linux FYI]
>
> On Mon, 18 May 2020 at 12:11, Marco Elver <elver@xxxxxxxxxx> wrote:
> >
> > On Sun, 17 May 2020 at 05:47, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > >
> > > On Sun, May 17, 2020 at 09:17:32AM +0800, kernel test robot wrote:
> > > > Greeting,
> > > >
> > > > FYI, we noticed the following commit (built with clang-11):
> > > >
> > > > commit: 2f08469563550d15cb08a60898d3549720600eee ("rcu: Mark rcu_state.ncpus to detect concurrent writes")
> > > > https://git.kernel.org/cgit/linux/kernel/git/paulmck/linux-rcu.git dev.2020.05.14c
> > > >
> > > > in testcase: boot
> > > >
> > > > on test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 8G
> > > >
> > > > caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace):
> > > >
> > > >
> > > >
> > > >
> > > > If you fix the issue, kindly add following tag
> > > > Reported-by: kernel test robot <rong.a.chen@xxxxxxxxx>
> > > >
> > > >
> > > > [    0.054943] BRK [0x05204000, 0x05204fff] PGTABLE
> > > > [    0.061181] BRK [0x05205000, 0x05205fff] PGTABLE
> > > > [    0.062403] BRK [0x05206000, 0x05206fff] PGTABLE
> > > > [    0.065200] RAMDISK: [mem 0x7a247000-0x7fffffff]
> > > > [    0.067344] ACPI: Early table checksum verification disabled
> > > > BUG: kernel reboot-without-warning in boot stage
> > >
> > > I am having some difficulty believing that this commit is at fault given
> > > that the .config does not list CONFIG_KCSAN=y, but CCing Marco Elver
> > > for his thoughts.  Especially given that I have never built with clang-11.
> > >
> > > But this does invoke ASSERT_EXCLUSIVE_WRITER() in early boot from
> > > rcu_init().  Might clang-11 have objections to early use of this macro?
> >
> > The macro is a noop without KCSAN. I think the bisection went wrong.
> >
> > I am able to reproduce a reboot-without-warning when building with
> > Clang 11 and the provided config. I did a bisect, starting with v5.6
> > (good), and found this:
> > - Since v5.6, first bad commit is
> > 20e2aa812620439d010a3f78ba4e05bc0b3e2861 (Merge tag
> > 'perf-urgent-2020-04-12' of
> > git://git.kernel.org/pub/scm/linux/kernel//git/tip/tip)
> > - The actual commit that introduced the problem is
> > 2b3b76b5ec67568da4bb475d3ce8a92ef494b5de (perf/x86/intel/uncore: Add
> > Ice Lake server uncore support) -- reverting it fixes the problem.

Some more clues:

1. I should have noticed that this uses CONFIG_KASAN=y.

2. Something about function icx_uncore_mmio_init(). Making it a noop
also makes the issue go away.

3. Leaving icx_uncore_mmio_init() a noop but removing the 'static'
from icx_mmio_uncores also presents the issue. So this seems to be
something about how/where icx_mmio_uncores is allocated.

Thanks,
-- Marco