mdraid10 regression in 2.6.27.4 (possibly earlier)

Hi,

Some weeks ago I upgraded from 2.6.23 to 2.6.27.4. After a failed hard
drive I realized that re-adding drives to a degraded raid10 no longer
works (it adds the drive as a spare and never starts a resync). Booting
back into the old .23 kernel allowed me to complete and resync the array
as usual. Attached find a test case reliably failing on vanilla 2.6.27.4
with no patches.

Thank you