For delivering a signal, Mach forwards an msg_sig_post message from the invoker of kill to the target process. The target process' signal thread job is it to listen to such messages and to set up signal handler contexts in other threads.

IRC, freenode, #hurd, 2011-04-20

<braunr> bugs around signals are very tricky
<braunr> signals are actually the most hairy part of the hurd
<braunr> and the reason they're aynchronous is that they're handled by a
  second thread
<braunr> (so yes, every process on the hurd has at least two threads)
<svante_> braunr: How to solve the asynch problem then if every process has
  two threads?
<braunr> the easiest method would be to align ourselves on what most other
  Unices do
<braunr> establish a "signal protocol" between kernel and userspace
<braunr> with a set of signal info in a table, most likely at the top of
  the stack
<braunr> but this is explicitely what the original Mach developers didn't
  want, and they were right IMO
<braunr> having two threads is very clean, but it creates incompatibilites
  with what POSIX requires
<braunr> so there might be a radical choice to make here
<braunr> and i doubt we have the resources to make it happen
<svante_> What is the advantage of having two threads per process, a per
  the original design?
<braunr> it's clean
<braunr> you don't have to define async-signal-safe functions
<braunr> it's like using sigwait() yourself in a separate thread, or
  multiplexing them through signalfd()
<svante_> Regardless of the advantages, isn't two threads per process a
  waste of resources?
<braunr> sure it is
<braunr> but does it really matter ?
<braunr> mach and the hurd were intended to be "hyperthreaded"


IRC, freenode, #hurd, 2013-09-17

<teythoon> I just realized that I know next to nothing about signal
  handling on the Hurd...
<teythoon> especially /hurd/inits role in it
<teythoon> reading glibcs kill.c it does not involve /hurd/init at all, but
  /hurd/init is full of proxying code for the msg protocol
<teythoon> ah, /hurd/init mitms the signal handling logic in the libc for
  its own signals
<teythoon> for msg_sig_post it sends a reply immediately, and then
  processes the signal, I wonder why that is done
<teythoon> also it "forwards" any signals it receives to the child it
  spawned (like /etc/hurd/runsystem), I wonder why...
<teythoon> good thing the comments tell what is done, not why...
<teythoon> so in theory kill -HUP 1 should have been forwarded to the
  "runsystem" process, I wonder why that does not work if that one execs
<braunr> teythoon: can't help you there :/
<teythoon> braunr: I think I sorted it out on my own, we'll see how that
  works out in practice ;)
<braunr> good

IRC, freenode, #hurd, 2013-09-18

<teythoon> braunr: I figured out why /hurd/init does this strange thing
  with the msg protocol
<teythoon> braunr: it has no signal thread
<teythoon> I wonder how /hurd/exec and the initial filesystem handle
<teythoon> err, afaics the signal thread is created in fork(), so any
  process not created using it (ie manually using task_create) should lack
  the signal thread, no?
<teythoon> that'd be the root fs, /hurd/{exec,init,auth,proc} and
  /etc/hurd/runsystem (the child started by /hurd/init)
<teythoon> but I see only /hurd/init doing something about it, namely
  setting a msgport and handling the msg protocol, relaying any messages to
  the signal handling logic in the glibc