Comment 24 for bug 1552539

Another observation from the strace: when mkswap runs, /dev/sda3 is already enabled again:

21626 open("/proc/swaps", O_RDONLY|O_CLOEXEC) = 4
21626 read(4, "Filename\t\t\t\tType\t\tSize\tUsed\tPriority\n/dev/sda3 partition\t8251388\t0\t-1\n", 1024)
 = 100
[...]
21626 open("/dev/sda3", O_RDONLY|O_EXCL|O_CLOEXEC) = -1 EBUSY (Device or resource busy)

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).