Lock-Free

Non-blocking

In computer science, an algorithm is called non-blocking if failure or suspension of any thread cannot cause failure or suspension of another thread;for some operations, these algorithms provide a useful alternative to traditional blocking implementations.

A non-blocking algorithm is lock-free if there is guaranteed system-wide progress.

and wait-free if there is also guaranteed per-thread progress.【4】

什么是lock-free

People often describe lock-free programming as programming without mutexes, which are also referred to as locks. That’s true, but it’s only part of the story. The generally accepted definition, based on academic literature, is a bit more broad. At its essence, lock-free is a property used to describe some code, without saying too much about how that code was actually written.

Basically, if some part of your program satisfies the following conditions, then that part can rightfully be considered lock-free. Conversely, if a given part of your code doesn’t satisfy these conditions, then that part is not lock-free.

In this sense, the lock in lock-free does not refer directly to mutexes, but rather to the possibility of “locking up” the entire application in some way, whether it’s deadlock, livelock – or even due to hypothetical thread scheduling decisions made by your worst enemy. That last point sounds funny, but it’s key. Shared mutexes are ruled out trivially, because as soon as one thread obtains the mutex, your worst enemy could simply never schedule that thread again. Of course, real operating systems don’t work that way – we’re merely defining terms.

A final precision: Operations that are designed to block do not disqualify the algorithm. For example, a queue’s pop operation may intentionally block when the queue is empty. The remaining codepaths can still be considered lock-free.【1】

lock-free相关技术

Atomic Read-Modify-Write Operations

基本定义：

In computer science, read-modify-write is a class of atomic operations (such as test-and-set, fetch-and-add, and compare-and-swap) that both read a memory location and write a new value into it simultaneously, either with a completely new value or some function of the previous value. These operations prevent race conditions in multi-threaded applications. Typically they are used to implement mutexes or semaphores. These atomic operations are also heavily used in non-blocking synchronization.【2】

Compare-and-swap

RMW的一种，维基百科的描述如下：

In computer science, compare-and-swap (CAS) is an atomic instruction used in multithreading to achieve synchronization. It compares the contents of a memory location to a given value and, only if they are the same, modifies the contents of that memory location to a given new value. This is done as a single atomic operation. The atomicity guarantees that the new value is calculated based on up-to-date information; if the value had been updated by another thread in the meantime, the write would fail. The result of the operation must indicate whether it performed the substitution; this can be done either with a simple boolean response (this variant is often called compare-and-set), or by returning the value read from the memory location (not the value written to it).【3】

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