The solution (4) - cookies

• A device-mapper cookie is a 32-bit integer generated by a userspace program that is going to need to wait for udev.

• We pass the cookie into the kernel using the event number field which happens not to be used for input by the ioctls that need a cookie.

• The cookie is passed to udev when uevents are generated and appears as the environment variable DM_COOKIE.

• When LVM2 has several related events to wait for, it uses the same cookie for them all.

• Our current implementation uses a semaphore for each cookie. To avoid races, we increment it prior to submitting the ioctls and wait for it to drop to 0. The last udev rule decrements it.