Reduce umtx-related work on exec and exit

[mjg]

Summary

Vast majority of the time there is no need to do anything. The patch below avoids relocking proc and locking umtx + thread lock in the common case.

[kib]

sys/kern/kern_umtx.c

4422

So your problem is this PROC_UNLOCK() ?

I think it can be done much simpler. Since the process is single-threaded, its thread list must be stable there and you can avoid acquiring the PROC_LOCK() in umtx_exec_hook at all.

Of course the proper comment is due.

[mjg]

• remove proc locking in the first place

[mjg]

sys/kern/kern_umtx.c

4422

I'm after avoidable locks in general. umtx shows up as a minor global lock in the grand scheme of things, I don't see what's the problem plugging it.

[kib]

sys/kern/sched_ule.c

1871

Why not write this part as

  if (td->td_lend_user_pri == prio ||
   td->td_user_pri == min(prio, td->td_base_user_pri) ||
   td->td_priority < td->td_user_pri)
	return;
  thread_lock(td);
  ...

[mjg]

sys/kern/sched_ule.c

1871

I wanted to keep this easier to modify should extra code be needed in the future.