6.1.1. Pessimistic and optimistic locking

中文名

乐观锁

外文名

Optimistic locking

介    绍

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应    用

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乐观锁介绍

示例

优点

缺点

实现

编辑

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添加描述符

自旋锁是计算机科学用于多线程同步的一种锁，线程反复检查锁变量是否可用。由于线程在这一过程中保持执行，因此是一种忙等待。一旦获取了自旋锁，线程会一直保持该锁，直至显式释放自旋锁。

自旋锁避免了进程上下文的调度开销，因此对于线程只会阻塞很短时间的场合是有效的。因此操作系统的实现在很多地方往往用自旋锁。Windows操作系统提供的轻型读写锁（SRW Lock）内部就用了自旋锁。显然，单核CPU不适于使用自旋锁，这里的单核CPU指的是单核单线程的CPU，因为，在同一时间只有一个线程是处在运行状态，假设运行线程A发现无法获取锁，只能等待解锁，但因为A自身不挂起，所以那个持有锁的线程B没有办法进入运行状态，只能等到操作系统分给A的时间片用完，才能有机会被调度。这种情况下使用自旋锁的代价很高。

获取、释放自旋锁，实际上是读写自旋锁的存储内存或寄存器。因此这种读写操作必须是原子的。通常用test-and-set等原子操作来实现。^[1]

Windows内核API[编辑]

• KeInitializeSpinLock //初始化自旋锁
• KeAcquireSpinLock //获取自旋锁
• KeReleaseSpinLock //释放自旋锁

https://en.wikipedia.org/wiki/Spinlock

In software engineering, a spinlock is a lock which causes a thread trying to acquire it to simply wait in a loop ("spin") while repeatedly checking if the lock is available. Since the thread remains active but is not performing a useful task, the use of such a lock is a kind of busy waiting. Once acquired, spinlocks will usually be held until they are explicitly released, although in some implementations they may be automatically released if the thread being waited on (the one which holds the lock) blocks, or "goes to sleep".

Because they avoid overhead from operating system process rescheduling or context switching, spinlocks are efficient if threads are likely to be blocked for only short periods. For this reason, operating-system kernels often use spinlocks. However, spinlocks become wasteful if held for longer durations, as they may prevent other threads from running and require rescheduling. The longer a thread holds a lock, the greater the risk that the thread will be interrupted by the OS scheduler while holding the lock. If this happens, other threads will be left "spinning" (repeatedly trying to acquire the lock), while the thread holding the lock is not making progress towards releasing it. The result is an indefinite postponement until the thread holding the lock can finish and release it. This is especially true on a single-processor system, where each waiting thread of the same priority is likely to waste its quantum (allocated time where a thread can run) spinning until the thread that holds the lock is finally finished.

Implementing spin locks correctly offers challenges because programmers must take into account the possibility of simultaneous access to the lock, which could cause race conditions. Generally, such an implementation is possible only with special assembly-language instructions, such as atomic test-and-set operations and cannot be easily implemented in programming languages not supporting truly atomic operations.^[1] On architectures without such operations, or if high-level language implementation is required, a non-atomic locking algorithm may be used, e.g. Peterson's algorithm. However, such an implementation may require more memory than a spinlock, be slower to allow progress after unlocking, and may not be implementable in a high-level language if out-of-order execution is allowed.

Example implementation[edit]

The following example uses x86 assembly language to implement a spinlock. It will work on any Intel 80386 compatible processor.

; Intel syntax

locked:                      ; The lock variable. 1 = locked, 0 = unlocked.

     dd      0

spin_lock:

     mov     eax, 1          ; Set the EAX register to 1.

     xchg    eax, [locked]   ; Atomically swap the EAX register with

                             ;  the lock variable.

                             ; This will always store 1 to the lock, leaving

                             ;  the previous value in the EAX register.

     test    eax, eax        ; Test EAX with itself. Among other things, this will

                             ;  set the processor's Zero Flag if EAX is 0.

                             ; If EAX is 0, then the lock was unlocked and

                             ;  we just locked it.

                             ; Otherwise, EAX is 1 and we didn't acquire the lock.

     jnz     spin_lock       ; Jump back to the MOV instruction if the Zero Flag is

                             ;  not set; the lock was previously locked, and so

                             ; we need to spin until it becomes unlocked.

     ret                     ; The lock has been acquired, return to the calling

                             ;  function.

spin_unlock:

     xor     eax, eax        ; Set the EAX register to 0.

     xchg    eax, [locked]   ; Atomically swap the EAX register with

                             ;  the lock variable.

     ret                     ; The lock has been released.

Significant optimizations[edit]

The simple implementation above works on all CPUs using the x86 architecture. However, a number of performance optimizations are possible:

On later implementations of the x86 architecture, spin_unlock can safely use an unlocked MOV instead of the slower locked XCHG. This is due to subtle memory ordering rules which support this, even though MOV is not a full memory barrier. However, some processors (some Cyrix processors, some revisions of the Intel Pentium Pro (due to bugs), and earlier Pentium and i486 SMP systems) will do the wrong thing and data protected by the lock could be corrupted. On most non-x86 architectures, explicit memory barrier or atomic instructions (as in the example) must be used. On some systems, such as IA-64, there are special "unlock" instructions which provide the needed memory ordering.

To reduce inter-CPU bus traffic, code trying to acquire a lock should loop reading without trying to write anything until it reads a changed value. Because of MESI caching protocols, this causes the cache line for the lock to become "Shared"; then there is remarkably no bus traffic while a CPU waits for the lock. This optimization is effective on all CPU architectures that have a cache per CPU, because MESI is so widespread. On Hyper-Threading CPUs, pausing with rep nop gives additional performance by hinting the core that it can work on the other thread while the lock spins waiting.^[2]

Transactional Synchronization Extensions and other hardware transactional memory instruction sets serve to replace locks in most cases. Although locks are still required as a fallback, they have the potential to greatly improve performance by having the processor handle entire blocks of atomic operations. This feature is built into some mutex implementations, for example in glibc. The Hardware Lock Elision (HLE) in x86 is a weakened but backwards-compatible version of TSE, and we can use it here for locking without losing any compatibility. In this particular case, the processor can choose to not lock until two threads actually conflict with each other.^[3]

A simpler version of the test can use the cmpxchg instruction on x86, or the __sync_bool_compare_and_swap built into many Unix compilers.

With the optimizations applied, a sample would look like:

; In C: while(!__sync_bool_compare_and_swap(&locked, 0, 1)) while(locked) __builtin_ia32_pause();

spin_lock:

    mov     ecx, 1             ; Set the ECX register to 1.

retry:

    xor     eax, eax           ; Zero out EAX, because cmpxchg compares against EAX.

    XACQUIRE lock cmpxchg ecx, [locked]

                               ; atomically decide: if locked is zero, write ECX to it.

                               ;  XACQUIRE hints to the processor that we are acquiring a lock.

    je      out                ; If we locked it (old value equal to EAX: 0), return.

pause:

    mov     eax, [locked]      ; Read locked into EAX.

    test    eax, eax           ; Perform the zero-test as before.

    jz      retry              ; If it's zero, we can retry.

    rep nop                    ; Tell the CPU that we are waiting in a spinloop, so it can

                               ;  work on the other thread now. Also written as the "pause".

    jmp     pause              ; Keep check-pausing.

out:

    ret                        ; All done.

spin_unlock:

    XRELEASE mov [locked], 0   ; Assuming the memory ordering rules apply, release the

                               ;  lock variable with a "lock release" hint.

    ret                        ; The lock has been released.

Alternatives[edit]

The primary disadvantage of a spinlock is that, while waiting to acquire a lock, it wastes time that might be productively spent elsewhere. There are two ways to avoid this:

1. Do not acquire the lock. In many situations it is possible to design data structures that do not require locking, e.g. by using per-thread or per-CPU data and disabling interrupts.
2. Switch to a different thread while waiting. This typically involves attaching the current thread to a queue of threads waiting for the lock, followed by switching to another thread that is ready to do some useful work. This scheme also has the advantage that it guarantees that resource starvation does not occur as long as all threads eventually relinquish locks they acquire and scheduling decisions can be made about which thread should progress first. Spinlocks that never entail switching, usable by real-time operating systems, are sometimes called raw spinlocks.^[4]

Most operating systems (including Solaris, Mac OS X and FreeBSD) use a hybrid approach called "adaptive mutex". The idea is to use a spinlock when trying to access a resource locked by a currently-running thread, but to sleep if the thread is not currently running. (The latter is always the case on single-processor systems.)^[5]

OpenBSD attempted to replace spinlocks with ticket locks which enforced first-in-first-out behaviour, however this resulted in more CPU usage in the kernel and larger applications, such as Firefox, becoming much slower.^[6][7]

See also[edit]

• Synchronization
• Busy spin
• Deadlock
• Seqlock
• Ticket lock

References[edit]

External links[edit]

• pthread_spin_lock documentation from The Open Group Base Specifications Issue 6, IEEE Std 1003.1, 2004 Edition
• Variety of spinlock Implementations from Concurrency Kit
• Article "User-Level Spin Locks - Threads, Processes & IPC" by Gert Boddaert
• Article Spin Lock Example in Java
• Paper "The Performance of Spin Lock Alternatives for Shared-Memory Multiprocessors" by Thomas E. Anderson
• Paper "Algorithms for Scalable Synchronization on Shared-Memory Multiprocessors" by John M. Mellor-Crummey and Michael L. Scott. This paper received the 2006 Dijkstra Prize in Distributed Computing.
• Spin-Wait Lock by Jeffrey Richter
• Austria C++ SpinLock Class Reference
• Interlocked Variable Access(Windows)
• Operating Systems: Three Easy Pieces (Chapter: Locks)

https://docs.microsoft.com/en-us/dotnet/standard/threading/spinlock

The SpinLock structure is a low-level, mutual-exclusion synchronization primitive that spins while it waits to acquire a lock. On multicore computers, when wait times are expected to be short and when contention is minimal, SpinLock can perform better than other kinds of locks. However, we recommend that you use SpinLock only when you determine by profiling that the System.Threading.Monitor method or the Interlocked methods are significantly slowing the performance of your program.

SpinLock may yield the time slice of the thread even if it has not yet acquired the lock. It does this to avoid thread-priority inversion, and to enable the garbage collector to make progress. When you use a SpinLock, ensure that no thread can hold the lock for more than a very brief time span, and that no thread can block while it holds the lock.

Because SpinLock is a value type, you must explicitly pass it by reference if you intend the two copies to refer to the same lock.

For more information about how to use this type, see System.Threading.SpinLock. For an example, see How to: Use SpinLock for Low-Level Synchronization.

SpinLock supports a thread-tracking mode that you can use during the development phase to help track the thread that is holding the lock at a specific time. Thread-tracking mode is very useful for debugging, but we recommend that you turn it off in the release version of your program because it may slow performance. For more information, see How to: Enable Thread-Tracking Mode in SpinLock.