Debugging a Stall or a Freeze

[Valdis.Kletnieks at vt.edu]

On Thu, 25 Jul 2013 13:56:47 -0400, Salam Farhat said:

> When the guest OS freezes I get the following messages seen below. I would
> like to know what is a good approach for debugging this issue. I am not
> sure what a process stall is. Is that a deadlock?
>
>
> [  780.357876] BUG: soft lockup - CPU#0 stuck for 22s! [nautilus:1382]
> [  780.361658] Process nautilus (pid: 1382, ti=dca12000 task=dc837230 task.ti=d)
> [  780.361658]
> Stack:
> [  780.361658] Call Trace:
> [  780.361658] Code: 90 b8 43 64 03 c1 b9 40 64 03 c1 e9 49 ff ff ff 90 55 ba 0
> [  808.356372] BUG: soft lockup - CPU#0 stuck for 22s!

That's probably not a deadlock.  That's code stuck in an infinite loop,
probably while running in a non-interruptible state.

Too bad we didn't get a stack dump out of it, that would tell us what
code is hung in a loop.

For debugging deadlocks, turning on CONFIG_PROVE_LOCKING=y in the .config
is the best bet - that will fire an alert not only when the kernel *does*
lock up, but also if there's even a *possible* deadlock (for instance, if one
section takes 2 locks in the order A B, it will trigger if it ever spots
another chunk of code taking B and then A - even if that doesn't actually
trigger a deadlock because neither lock is held at the time).


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