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 <bp@xxxxxxx> 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 <bp@xxxxxxx> Tested-by: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx> Link: https://lkml.kernel.org/r/20210201142704.12495-2-bp@xxxxxxxxx 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