[+Cc clang-built-linux FYI]
On Mon, 18 May 2020 at 12:11, Marco Elver <el...@google.com> wrote: > > On Sun, 17 May 2020 at 05:47, Paul E. McKenney <paul...@kernel.org> 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.c...@intel.com> > > > > > > > > > [ 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. > > Thanks, > -- Marco