There is a "dd if=/dev/sda3 of=991754715136-1000204140543/old_uuid" running which has the device open (pid 21620), but it finishes well before mkswap starts. I don't see anything else accessing /dev/sda3 in the straces in between, so something from outside interferes here (triggered by udev or similar).
Another observation from the strace: when mkswap runs, /dev/sda3 is already enabled again:
21626 open("/proc/swaps", O_RDONLY|O_CLOEXEC) = 4 t\t\t\tType\ t\tSize\ tUsed\tPriority \n/dev/ sda3 partition\ t8251388\ t0\t-1\ n", 1024) O_EXCL| O_CLOEXEC) = -1 EBUSY (Device or resource busy)
21626 read(4, "Filename\
= 100
[...]
21626 open("/dev/sda3", O_RDONLY|
There is a "dd if=/dev/sda3 of=991754715136 -1000204140543/ old_uuid" running which has the device open (pid 21620), but it finishes well before mkswap starts. I don't see anything else accessing /dev/sda3 in the straces in between, so something from outside interferes here (triggered by udev or similar).