after upgrading, ASM did not find the labeled disks in the system:

root@xyz ~# oracleasm scandisks
Reloading disk partitions: done
Cleaning any stale ASM disks...
Scanning system for ASM disks...
root@xyz ~# oracleasm listdisks
root@xyz ~#

However, they can be found when specifying the Device Mapper search path:

root@xyz ~# oracleasm scandisks /dev/mapper/*
Reloading disk partitions: done
Cleaning any stale ASM disks...
Scanning system for ASM disks...
Instantiating disk "A"
Instantiating disk "B"
Instantiating disk "C"
Instantiating disk "D"

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.