How can I find out what other tasks are threads of (or with) a given
task?

--Guy

There's no direct command to do it.  But, for example,
all threads of a task should have the same name and the
same task->tgid right?

So for example, I just executed a task named "mkthreads", pid 27610,
which in turn creates 10 threads of itself, all of which will have
the same task->tgid:

crash> foreach mkthreads task | grep -e COMMAND -e tgid
PID: 27610  TASK: ddb2c000  CPU: 0   COMMAND: "mkthreads"
  tgid = 27610,
PID: 27611  TASK: c82de000  CPU: 0   COMMAND: "mkthreads"
  tgid = 27610,
PID: 27612  TASK: d016a000  CPU: 0   COMMAND: "mkthreads"
  tgid = 27610,
PID: 27613  TASK: c3f2e000  CPU: 0   COMMAND: "mkthreads"
  tgid = 27610,
PID: 27614  TASK: db95e000  CPU: 0   COMMAND: "mkthreads"
  tgid = 27610,
PID: 27615  TASK: d6dfa000  CPU: 0   COMMAND: "mkthreads"
  tgid = 27610,
PID: 27616  TASK: ddad6000  CPU: 0   COMMAND: "mkthreads"
  tgid = 27610,
PID: 27617  TASK: cd1a4000  CPU: 0   COMMAND: "mkthreads"
  tgid = 27610,
PID: 27618  TASK: d45de000  CPU: 0   COMMAND: "mkthreads"
  tgid = 27610,
PID: 27619  TASK: cef58000  CPU: 0   COMMAND: "mkthreads"
  tgid = 27610,
PID: 27620  TASK: cdc38000  CPU: 0   COMMAND: "mkthreads"
  tgid = 27610,
crash>

Dave