tstile lockups - test case

[Donald Lee <MacPPC2%c.icompute.com@localhost>]

At 11:55 PM +0000 3/30/13, Valery Ushakov wrote:
>Donald Lee <MacPPC2%c.icompute.com@localhost> wrote:
>> At 5:22 PM +0000 3/30/13, Valery Ushakov wrote:
>>
>>>Next time it hangs you can break into DDB from console and check with
>>>ps (ddb command, not ordinary command; see ddb(4)).  I've seen such
>>>lockup once on my newly installed mini g4 when I was copying over
>>>several cvs trees from the old machine.
>> 
>> Console does not respond (aside from the first CR, sometimes).
>> I don't think I can use ddb.  I'll read the man page......  Thanks
>
>Press Ctrl-Alt-Esc on the console keyboard to break into ddb.  It
>should work on tstiled machine even when normal console input isn't.
>IIRC, NetBSD 6 should have the necessary fixes for USB keyboard to be
>usable with ddb.
>
>-uwe

I can reproduce the tstile hang fairly easily and am looking for
a path to fix it.  I don't really know how to use ddb, and am not very
familiar with the internals of the kernel.  What I have done so
far is put together a test case that reliably causes the problem
in a few hours or less.

The test case is just a shell script.  I have apache running on the
machine, and have a shell script doing
wget operations on http://127.0.0.1/...  The network interface
is down (gem0) when the failure occurs, so it doesn't look like a driver
problem.  When the hang occurs, I cannot get a command launched, so no
user-level debugging is possible, but I can break into ddb with the
ctrl-alt-esc sequence.  When I break into ddb I can do "ps", and see
many, many processes waiting on "tstile".

I have run this test maybe 10 times, and the failures all look about
the same.

This smells to me like a race condition - a small timing window that gets
hit somewhere.  Those are always fun to find. ;->

I have run the same test case on a VM instance of an i386 NetBSD install,
and it does not fail (so far), so I'm pretty sure it's a macppc-specific
problem.

Anyone have suggestions on how to track this down?  What's the shortest path
to my getting enough ddb expertise to help track this down, or getting my
test case in the hands of someone with the requisite skill?

Thanks,

-dgl-