Windows用户模式下的线程同步

Interlocked系列函数

• 原子访问:线程在访问某个资源的时候能保证没有其他线程会在同一时刻访问同一资源

函数名	功能
InterlockedExchangeAdd InterlockedExchangeAdd64	增/减
InterlockedExchange InterlockedExchange64 InterlockedExchangePointer	交换
InterlockedCompareExchange16 InterlockedCompareExchange InterlockedCompareExchange64 InterlockedCompareExchangePointer	比较赋值
InterlockedAnd16 InterlockedAnd InterlockedAnd64	与
InterlockedOr16 InterlockedOr InterlockedOr64	或
InterlockedXor16 InterlockedXor InterlockedXor64	异或

注意点:

• InterlockedXxx is atomic only with respect to other InterlockedXxx calls.
• The parameters for this function must be aligned on a 32-bit boundary; otherwise, the function will behave unpredictably on multiprocessor x86 systems and any non-x86 systems. See _aligned_malloc.(上述部分函数在MSDN解释里有这句话)
• 使用_aligned_malloc分配一块对齐过的内存。对应释放函数:_aligned_free

关键段

函数名	功能
InitializeCriticalSection	初始化
EnterCriticalSection	等待直至获取到关键段的所有权
LeaveCriticalSection	释放关键段的所有权
DeleteCriticalSection	资源释放
TryEnterCriticalSection	尝试获取关键段的所有权，此函数不会等待
InitializeCriticalSectionAndSpinCount	初始化具有螺旋锁的关键段 备注:线程被切换到等待前会先进行旋转，从而可能会提高性能
SetCriticalSectionSpinCount	设置关键段的螺旋锁的旋转次数 备注:用来保护进程堆的关键段所使用的旋转次数大约是4000

EnterCriticalSection注意事项

• 此函数超时会引发异常，超时时间在注册表中可以设置(默认30天左右)
• 线程可以在EnterCriticalSection成功后继续EnterCriticalSection或TryEnterCriticalSection，每一次成功获取访问权都需要对应使用LeaveCriticalSection
• 当线程调用EnterCriticalSection时，关键段的访问权已被占用，那么关键段会在内部使用一个事件内核对象，只有在第一次要用到事件内核对象时，系统才会去创建他(当进程能使用的内存过低时，这个创建可能会失败，从而会抛出异常)，此时系统会切换到内核模式，此过程开销很大。为此，使用附带螺旋锁的关键段的性能可能会比不使用螺旋锁的关键段的性能高

InitializeCriticalSection注意事项

• 在进程能使用的内存过低时，此函数可能会失败并会抛出异常

InitializeCriticalSectionAndSpinCount注意事项

• 此函数的旋转次数的最高位设为1，则会在初始化时创建一个与关键段相关联的事件内核对象，从而可能防止EnterCriticalSection调用时因为事件内核对象分配问题而抛出异常

Slim 读/写锁

函数名	功能
InitializeSRWLock	Initialize a slim reader/writer (SRW) lock.
AcquireSRWLockExclusive	Acquires a slim reader/writer (SRW) lock in exclusive mode.
AcquireSRWLockShared	Acquires a slim reader/writer (SRW) lock in shared mode.
ReleaseSRWLockExclusive	Releases a slim reader/writer (SRW) lock that was acquired in exclusive mode.
ReleaseSRWLockShared	Releases a slim reader/writer (SRW) lock that was acquired in shared mode.
TryAcquireSRWLockExclusive	Attempts to acquire a slim reader/writer (SRW) lock in exclusive mode. If the call is successful, the calling thread takes ownership of the lock. Win7以上才支持
TryAcquireSRWLockShared	Attempts to acquire a slim reader/writer (SRW) lock in shared mode. If the call is successful, the calling thread takes ownership of the lock. Win7以上才支持

注意点

• SRW locks do not need to be explicitly destroyed.
• An SRW lock is the size of a pointer. The advantage is that it is fast to update the lock state. The disadvantage is that very little state information can be stored, so SRW locks cannot be acquired recursively(递归地). In addition, a thread that owns an SRW lock in shared mode cannot upgrade(提升) its ownership of the lock to exclusive mode.
• SRWLock系列函数不支持Windows Xp，支持的最低版本是Windows Vista，关键段支持Windows Xp
• 线程使用Slim读/写锁相较于关键段更具有可伸缩性

• 参考资料 https://docs.microsoft.com/en-us/windows/desktop/Sync/slim-reader-writer--srw--locks