A week ago, I posted this:
http://groups.google.com/groups?selm=linux.kernel.3CA20C9B.20309%40us.ibm.com

Nobody had anything to sayabout it, so here's a patch.  It moves the 
disassociate_ctty(1) up, and releases the BKl after it gets done.  Is 
this a sane thing to do, or do some of those exit_*() functions still 
need the tty?

The patch reduces hold times of the BKL in do_exit() by a factor of 100. 
   They were on the order of 200us, now they're about 1.5us.  However, 
those numbers were on Martin Bligh's NUMA-Q box, so they represent a 
serious worst-case scenario.

The patch is stable, but I don't properly understand the consequences of 
moving the disassociate_ctty(1) up.  I guess we could do this instead:

  	lock_kernel();
  	sem_exit();
	unlock_kernel();
	
  	__exit_files(tsk);
  	__exit_fs(tsk);
  	exit_namespace(tsk);
  	exit_sighand(tsk);
  	exit_thread();
	
	lock_kernel();
	if (current->leader)
		disassociate_ctty(1);
	unlock_kernel();

But, I hesitated to do that because of the lock bouncing consequences on 
the NUMA-Q.

-- 
Dave Hansen
haveblue@us.ibm.com