Big problems with 7.1 locking up :-(

[Robert Watson]

On Tue, 13 Jan 2009, Pete French wrote:

>> I can't (fortunately) make it lock up. I have a DL360 G5 which is unused 
>> atm. and can test on it if needed.
>
> Would it be possible to install that under amd64 and hammer it with DNS 
> requests ? I have been trying to think what the difference might be between 
> my webservers and the machines which are freezing, and the opnly one I an 
> come up with is UDP traffic as the locking machines are serving DNS and also 
> NFS.

There are significant changes in UDP locking between 7.0 and 7.1, so it could 
be that we're looking at a regression there.  If you're able to reproduce this 
reliably, it might well be worth doing a little search-and-replace in 
udp_usrreq.c along the following lines:

   INP_RLOCK_ASSERT -> INP_WLOCK_ASSERT
   INP_RLOCK -> INP_WLOCK
   INP_RUNLOCK -> INP_WUNLOCK

However, before making these changes for debugging purposes, make sure it's 
100% reproduceable without them in the configuration so that we don't find 
ourselves barking up the wrong tree.  Normally deadlocks along these lines 
*do* allow breaking into the debugger from a serial console, but since there 
are significant changes here in 7.1 it is worth trying to see if this might be 
related.

Robert N M Watson
Computer Laboratory
University of Cambridge