On Mon, 31 Aug 98 21:08:49 CDT 
 Chuck Cranor <chuck@maria.wustl.edu> wrote:

 > i believe the pool allocator has uncovered a design problem in the
 > process exit code path.

I agree.

 > looking at kern_exit.c, the final thing it does is set curproc
 > to NULL and call cpu_exit.   once curproc is NULL, we are no longer
 > allowed to sleep.    that means that we really have no business
 > calling vm_map_lock() in cpu_exit()  or any function it calls [because 
 > vm_map_lock() can sleep if the map is busy... not likely to be the 
 > case for the kernel map because we are single threaded right now, 
 > but it is best not to depend on that in the future...]
 > 
 > looking at the cpu_exit() functions on various ports, they all
 > seem to call uvmspace_free() and various pmap functions.  i believe 
 > there are several calls to vm_map_lock in that general area.   
 > this seems bad.   [maybe we should add a DIAGNOSTIC to vm_map_lock 
 > to check for a NULL curproc to catch these?]
 > 
 > 
 > it isn't clear to me what the best way to clean this up is.
 > i guess one temptation might be to push the clearing of curproc
 > back later into the cpu_exit function...?

Setting curproc == NULL later seems OK at first, and I started down that
path...

However...

Note that the process is already SZOMB at this point.  There are two
issues here:

	(1) Clock interrupt might deref already freed structure (can
	    deal with this by also testing for SZOMB in addition for
	    valid curproc).

	(2) Can't wakeup a SZOMB process.

Charles Hannum mentioned to me some "zombifier daemon" diffs he'd worked
out once, that make the exit code a lot less evil.  He no longer has them,
but we have discussed how it worked, and I think I can implement it tomorrow.
It actually make more code MI!

Stay tuned.

Jason R. Thorpe                                       thorpej@nas.nasa.gov
NASA Ames Research Center                            Home: +1 408 866 1912
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