$ go test -race github.com/cznic/exp/dbm
crashed inside __tsan_read_range on shadow memory for address 0x7ECC00 which is:

00000000007c3da0 0000000000003ec0 b semtable
00000000007c7c60 0000000000005240 b bufferList
00000000007ccea0 0000000000010000 B github.com/cznic/exp/dbm.onePage
00000000007dcea0 0000000000010000 B github.com/cznic/exp/dbm.zeroPage
00000000007ecea0 B end
00000000007ecea0 B enoptrbss

I suspect it has something do to with the way we map shadow for globals, both address
and size are not rounded to page size:
runtime∕race·MapShadow(noptrdata, enoptrbss - noptrdata);
so unaligned ending ends up not mapped.