gpart resize vs. cache?

I'm tinkering with a disk image that automatically
fills whatever media you put it onto.  But I'm having
trouble with gpart resize failing.

Disk layout:
   MBR with two slices  mmcsd0s1 and mmcsd0s2
   bsdlabel with one partition mmcsd0s2a

Before I can use growfs, I have two gpart resize operations:

1)   gpart resize -i 2 mmcsd0

2)  gpart resize -i 1 mmcsd0s2

Step 1 resizes mmcsd0s2 and always succeeds.

Step 2 resizes mmcsd0s2a and always fails
with "No space on device."

BUT if I reboot between these steps, step #2
always succeeds.

I suspect that step #1 is updating the partition
information on disk but that step #2 is somehow
reading the old size of mmcsd0s2 and thus finding
that there is no available space to grow the partition.

gpart(1) doesn't say anything about caching of
disk partiition info and "gpart list" does show the
updated information after step #1.

Is there some trick that will force the partition
information in memory to be updated (short of
a reboot or unmount/remount the root filesystem)?

Tim