On Mon, 12 Jan 2009, Pete French wrote:
I'm not sure if you've done this already, but the normal suggestions apply:
have you compiled with INVARIANTS/WITNESS/DDB/KDB/BREAK_TO_DEBUGGER, and do
any results / panics / etc result? Sometimes these debugging tools are
able to convert hangs into panics, which gives us much more ability to
debug them.
OK, I have now had a machine hand again, with the correct debug options in
the kernel. The screen looked like this when I went to restart it:
http://toybox.twisted.org.uk/~pete/71_lor2.png
It had not, however, dropped into any kind of debugger. Also there appear to
me console messages after the lock order reversal - is that normal ?
Lock order reversals are warnings of potential deadlock due to a lock cycle,
but deadlocks may not actually result, either because it's a false positive
(some locking construct that is deadlock free but involves lock cycles), or
because a cycle didn't actually form. The message is suggestive, but if you
have significant system activity after the message, then it may be unrelated.
The machine did stay up for a signifanct amount of time before doing this. I
notice that it is more or less identical to the one I posted whenI had
WITNESS_KDB in the kernel too, so maybe those results arent entirely
suprious after all ?
Given it hasnt dropped to a debugger, is there anything else I can try ?
Features like WITNESS and INVARIANTS may change the timing of the kernel
making certain race conditions less likely; I'd run with them for a bit and
see if you can reproduce the hang with them present, as they will make
debugging the problem a lot easier, if it's possible.
Robert N M Watson
Computer Laboratory
University of Cambridge
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