In computer science, serializing tokens are a concept in concurrency control arising from the ongoing development of DragonFly BSD. According to Matthew Dillon, they are most akin to SPLs, except a token works across multiple CPUs while SPLs only work within a single CPU's domain.

Serializing tokens allow programmers to write multiprocessor-safe code without themselves or the lower level subsystems needing to be aware of every single entity that may also be holding the same token.

Tokens and mutual exclusion (mutex) mechanisms are locks. Unlike mutexes, tokens do not exclude other threads from accessing the resource while they are blocked or asleep. A thread sharing resources with other threads can be stopped and started for a variety of reasons:

The following table summarizes the properties of tokens and mutexes.

Issues such as deadlock and priority inversion can be very difficult to avoid, and require coordination at many levels of the kernel. Because locking with tokens does not deadlock and acquired tokens need not be atomic when later operations block, it allow much simpler code than mutexes.

... If you look at FreeBSD-5, you will notice that FreeBSD-5 passes held mutexes down the subroutine stack quite often, in order to allow some very deep procedural level to temporarily release a mutex in order to switch or block or deal with a deadlock. There is a great deal of code pollution in FreeBSD-5 because of this (where some procedures must be given knowledge of the mutexes held by other unrelated procedures in order to function properly).

— Matthew Dillon

The following pseudocode and explanations illustrate how serializing tokens work.