Linux中最低的锁定原语是什么

Question

What are the lowest locking primitives in linux. I know about futex_wait and futex_wake . But futexes can be used for signalling besides locking. What is the lowest locking primitive and where can I find it in the glibc library? Secondly, are the condition variables also based on futexes?

Answer 1

Read various documentations on futex -es, including futex(2) and futex(7) man pages. You'll learn that pthread locking primitives are implemented with a mixture of futexes and of assembly code (doing things non-expressible in portable C99)

^{(as I said in a comment)}

Answer 2

There is no such thing as "lowest", but locks for different scenarios and purposes and "signalling" is not the right criterion to distinguish different sorts of locks.

• There are locking primitives that preform an active wait, spinlocks, that are based on atomic operations. With the venue of C11, these aren't even OS features anymore, atomic_flag can be used for that.
• The other family of lock primitives suspend the calling thread during wait. In linux these are all implemented with a combination of atomic operations and futex as work horse underneath for the waiting part.

Spinlocks are best used in contexts where you know that the critical phase only a handful of assembler instructions and where an interruption of the critical phase by the scheduler or signals is unlikely. This is the case for much more contexts than many people tend to belief. Therefore the atomic_flag is an important new feature in the C language.

The other locks are best used in context where active wait would eat up a substantial part of system resources.