Here are some notes for anyone trying to reproduce this problem.
By "this problem" I mean failure to reboot after "reboot" command issued.

1. On my system (Slackware, no systemd) I am triggering a reboot by Ctl-Alt-Del
   and this line in /etc/inittab:
   ca::ctrlaltdel:/sbin/shutdown -t5 -r now

2. I am only booting up to the command line (no X).

3. The occurrence of BUG is intermittent. I am seeing it on about 2 reboots in
3.

4. If there is no BUG, the next reboot will be OK.

5. If a user logs in before Ctl-Alt-Del, reboot with BUG still works maybe 90%
   of the time.

6. With BUG present, Ctl-Alt-Del at the login prompt succeeds about 50% of the
   time (measured over 25 reboots). (I put a dmesg command in rc.local to do
this
   test).

With e1cb3e4801 reverted, reboot always works and BUG never shows. Were it not
for that, I would be suspecting a local hardware problem by now