[m-rev.] For review: Fix a crash that can occur in parallel, stack-segment grades.

[Paul Bone]

On Wed, May 26, 2010 at 02:14:22PM +1000, Paul Bone wrote:
> 
> For review by anyone.
> 
> I'll commit this once the bootchecks finish in a number of grades.

I've committed this.  I'll address any review comments post-commit.

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