linux 5.12 - fails to boot - soft lockup - CPU#0 stuck for 23s! - RIP smp_call_function_single

I re-ran my git bisect, this time with a full power-down and cold boot, and more thorough testing, running a web browser.  My second bisect went from good to bad.

So now, instead, git bisect ended here:

4f432e8bb15b352da72525144da025a46695968f is the first bad commit
commit 4f432e8bb15b352da72525144da025a46695968f
Author: Borislav Petkov <[email protected]>
Date:   Thu Jan 7 13:23:34 2021 +0100

    x86/mce: Get rid of mcheck_intel_therm_init()

    Move the APIC_LVTTHMR read which needs to happen on the BSP, to
    intel_init_thermal(). One less boot dependency.

    No functional changes.

    Signed-off-by: Borislav Petkov <[email protected]>
    Tested-by: Srinivas Pandruvada <[email protected]>
    Link: https://lkml.kernel.org/r/[email protected]

 arch/x86/include/asm/mce.h            |  6 ------
 arch/x86/kernel/cpu/mce/core.c        |  1 -
 arch/x86/kernel/cpu/mce/therm_throt.c | 15 ++++-----------
 3 files changed, 4 insertions(+), 18 deletions(-)


Please let me know if that makes more sense.

Again:

Arch Linux
linux 5.12.arch1-1

Intel Core2 T7200
Mobile Intel 945PM Express Chipset
ICH7-M
Mobility Radeon X1600

Generally, on failure, the system will not boot past "Loading initial ramdisk...", or, when it does, the boot process will hang, and the console will eventually show:

watchdog: BUG: soft lockup - CPU#0 stuck for 23s! [systemd-udevd: 241]
...
RIP: 0010:smp_call_function_single+0xf7/0x140

The top of the call trace variously shows either "__flush_tlb_all" or "tlbflush_read_file", with the "soft lockup" repeating indefinitely.

If this is some race/timing issue on boot, I have to go back and re-test every "good" bisect, re-booting many times to see if there is *ever* a failure - and that is supposing that there is no interaction between whatever is causing the problem and all the other patches being added.  Any insight would be appreciated.


James