after upgrading, ASM did not find the labeled disks in the system:
root@xyz ~# oracleasm scandisks
However, they can be found when specifying the Device Mapper search path:
root@xyz ~# oracleasm scandisks /dev/mapper/*
This is due to the fact that DM does not create /dev/dm-* devices for LVM volumes on RHEL5, and ASM (since Version XXX?) only looks for /dev/dm-, not for the user-friendly /dev/mapper/.
As per Oracle Note 558596.1, this can be fixed by commenting out a line in /etc/udev/rules.d/90-dm.rules (/etc/udev/rules.d/50-udev.rules in older RHEL5):
#KERNEL. "dm-0-9*", ACTION"add", OPTIONS+="ignore_device"
After reboot, /dev/dm-* devices are created and ASM finds the disks when scanning.