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Copied entirely from http://en.wikipedia.org/wiki/Serializing_tokens
In computer science, serializing tokens are a concept in concurrency control arising from the ongoing development of DragonFly BSD. According to Matt 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 vs. Mutexes
Tokens are similar to mutexes in that they can, if used correctly, prevent multiple threads from accessing a shared resource at the same time. Unlike mutexes, however, they do NOT exclude other threads from accessing the resource while they are blocked or asleep. In general terms, they're both locks: your thread gets a lock (which prevents other threads from having it), does some work, and then releases it for another thread to use.
It's important here to recall how threads interact with each other when sharing resources. There are a number of ways that a thread can be stopped and another thread to be started:
1. Timeslicing: the scheduler tries to ensure that all threads get a fair chance to run, so it runs each thread for a brief period of time (a timeslice) and then switches to another thread.
2. Concurrent Execution: In multiprocessor computers, your thread may also be run at exactly the same time as another thread on a different CPU.
3. Preemption: A thread may be preempted by a higher priority thread, such as a hardware interrupt or LWKT.
4. Voluntary Blocking: A thread may voluntarily block (aka "go to sleep" ) if it has to wait for something, has no work to do, or calls a function that blocks-- note that even the call to acquire a lock can block.
Remember: the purpose of a lock is to keep other threads out while your thread is working on something. This table summarizes the situations in which tokens and mutexes work correctly to keep other threads "out".
So what's the big deal? It seems like mutexes are the clear winner-- and in some cases it's important to be able to block and keep a lock. However, they also cause problems such as Deadlocks and Priority inversions. Dealing with these issues is very difficult and requires coordination at many different levels of the kernel:
"In fact, the fact that tokens do not deadlock coupled with the fact
that there is no expectation of atomicity for earlier acquired tokens
when later operations block leads to a great deal of code simplification.
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)." -- Matt Dillon
Obviously Matt has reason to promote his own solution to deadlocking, but he has a point: serializing tokens do a fine job of locking out other threads as long as you don't block while holding them. If you do, another thread will steal the lock and possibly change the data you were working on. You will reacquire the token when you are awakened, but you will have to make sure that your data is still consistent.
Serializing Tokens In Action
To show how serializing tokens actually work, let's see some pseudocode and what's going on behind the scenes.
References
A mailing list thread where Matt Dillon explains tokens in great detail
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