[ebuild     U ] sys-fs/device-mapper-1.02.22-r5 [1.02.10-r1]
[ebuild     U ] sys-fs/udev-115-r1 [104-r12]
[blocks B     ] <sys-fs/udev-115-r1 (is blocking sys-fs/device-mapper-1.02.22-r5)
[blocks B     ] <sys-fs/device-mapper-1.02.19-r1 (is blocking sys-fs/udev-115-r1)

$sudo emerge -1p udev

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild     U ] sys-fs/udev-115-r1 [104-r12]
[blocks B     ] <sys-fs/device-mapper-1.02.19-r1 (is blocking sys-fs/udev-115-r1)