On many different machines, when I come across a lockup, I often use Alt+SysRq+
REISUB to reboot without too much losses. But I often noticed that although
REISU commands work even when I just hold Alt+SysRq and enter them without releasing Alt+SysRq, the last one,
B, seems to be very "lazy": I have to repeat it many times, and in fact it doesn't trigger until I do the cycle "press Alt+SysRq, type B, release Alt+SysRq" multiple times (and not always the same number of times).
At first occurrences of this problem I thought it's the kernel which has locked too hard that it doesn't "see" my
B command, but when I realized that multiple repetitions of it do allow me to trigger reboot, it now seems that it's something general. Even on a working system (be it Debian, Ubuntu, LFS etc.), I can easily reproduce this. In fact, I can even load the kernel with
init=/bin/bash and reproduce this from this bash prompt.
Looking at serial console output, I see all the feedback on
REISU, but only one feedback print on multiple
B commands — when the kernel finally is convinced to do a reboot.
Why is this? Is it some kernel feature which prevents unintended reset, or maybe it's just a bug (quite strange one)?
Note that I'm using plain keyboard with no Fn or multimedia keys, so this question isn't a duplicate of this one.