PV operation primitive

" Atomic manipulation (atomic operation) You don't need to synchronized", This is a cliche of multithreaded programming . Atomic operation means that it will not be Thread scheduling The mechanism interrupts the operation ; Once this operation begins , It runs until the end , There won't be any context switch [1]  （ Switch to another thread ）.

Atomic operations are mostly implemented in assembly language , because c Language can't do this .

Atomicity cannot be guaranteed by software alone -- Must be supported by hardware , So it's architecture related . stay x86 On the platform ,CPU Provides the ability to Means of bus locking .CPU There's a lead on the chip #HLOCK pin, If you prefix an instruction in an assembly language program "LOCK", The assembled machine code makes CPU In executing this command, put #HLOCK pin The potential of the current is lowered , Continue until the end of this command release , To lock the bus , So other things on the same bus CPU You can't access the memory through the bus for the time being , This ensures the atomicity of this instruction in a multiprocessor environment .

                                                                                                                                                                                  —— From baidu baike

And the reason why the operating system passes PV Operation to access semaphores can be used to achieve process mutually exclusive synchronization , It is also because PV Primitives are non interruptible , It's atomic manipulation . According to the introduction to atomic operation in Baidu Encyclopedia above , In single processor mode , When multiple processes access critical resources concurrently ,P Operation primitives can put the process that first executes to the primitive code in a non interruptible state , At this time, other concurrent processes cannot pass CPU Interrupt the process and preempt CPU resources , To enter block state , Wait until the process executes v After operation primitive , Wake up one of the blocked processes and resume ready state .