Re: Big problems with 7.1 locking up :-(


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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