Aha, the task doesn't hang, it has exited (as expected), please see below...
Great. Thanks a lot Laurent!
So what happens is:
We have the very old bug (bugs, actually) with the global init && signals
which I tried to fix many times but can't find a simple solution. The fatal
signal sent to init doesn't really kill it (we have the check in
get_signal_to_deliver) but it sets SIGNAL_GROUP_EXIT. This is wrong, now
init can't exec, this has other bad implications, and this is just insane.
With the signals-send_signal-factor-out-signal_group_exit-checks.patch the
task with SIGNAL_GROUP_EXIT doesn't recieve the signals. While this change
itself is (I hope) correct, the "killed" /sbin/init now can't see SIGCHLD
and the system hangs on boot.
Not a kernel problem, but this looks a bit strange to me.
init has SIG_DFL for SIGUSR1, and someone does kill(1, SIGUSR1).
Note that init was explicitly targeted, the signal was not sent
to prgp or -1.
Most likely Ubuntu knows what it does, and I can't find any email
at ubuntu.com to cc...
Oleg.
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