如果可能的话,我想使用临界区来实现它。该锁不必是进程安全的,只需要是线程安全的。你有什么关于如何实现的想法吗?
虽然这是一个老问题,但以下内容应该有效。它不会在争用时自旋。如果读者几乎没有争用,则产生的额外成本有限,因为SetEvent
被懒惰地调用(查看编辑历史以获取没有此优化的更重量级版本)。
#include <windows.h>
typedef struct _RW_LOCK {
CRITICAL_SECTION countsLock;
CRITICAL_SECTION writerLock;
HANDLE noReaders;
int readerCount;
BOOL waitingWriter;
} RW_LOCK, *PRW_LOCK;
void rwlock_init(PRW_LOCK rwlock)
{
InitializeCriticalSection(&rwlock->writerLock);
InitializeCriticalSection(&rwlock->countsLock);
/*
* Could use a semaphore as well. There can only be one waiter ever,
* so I'm showing an auto-reset event here.
*/
rwlock->noReaders = CreateEvent (NULL, FALSE, FALSE, NULL);
}
void rwlock_rdlock(PRW_LOCK rwlock)
{
/*
* We need to lock the writerLock too, otherwise a writer could
* do the whole of rwlock_wrlock after the readerCount changed
* from 0 to 1, but before the event was reset.
*/
EnterCriticalSection(&rwlock->writerLock);
EnterCriticalSection(&rwlock->countsLock);
++rwlock->readerCount;
LeaveCriticalSection(&rwlock->countsLock);
LeaveCriticalSection(&rwlock->writerLock);
}
int rwlock_wrlock(PRW_LOCK rwlock)
{
EnterCriticalSection(&rwlock->writerLock);
/*
* readerCount cannot become non-zero within the writerLock CS,
* but it can become zero...
*/
if (rwlock->readerCount > 0) {
EnterCriticalSection(&rwlock->countsLock);
/* ... so test it again. */
if (rwlock->readerCount > 0) {
rwlock->waitingWriter = TRUE;
LeaveCriticalSection(&rwlock->countsLock);
WaitForSingleObject(rwlock->noReaders, INFINITE);
} else {
/* How lucky, no need to wait. */
LeaveCriticalSection(&rwlock->countsLock);
}
}
/* writerLock remains locked. */
}
void rwlock_rdunlock(PRW_LOCK rwlock)
{
EnterCriticalSection(&rwlock->countsLock);
assert (rwlock->readerCount > 0);
if (--rwlock->readerCount == 0) {
if (rwlock->waitingWriter) {
/*
* Clear waitingWriter here to avoid taking countsLock
* again in wrlock.
*/
rwlock->waitingWriter = FALSE;
SetEvent(rwlock->noReaders);
}
}
LeaveCriticalSection(&rwlock->countsLock);
}
void rwlock_wrunlock(PRW_LOCK rwlock)
{
LeaveCriticalSection(&rwlock->writerLock);
}
CRITICAL_SECTION
来降低读者的成本:
countsLock
is replaced with writerLock
in rdlock and rdunlock
rwlock->waitingWriter = FALSE
is removed in wrunlock
wrlock's body is changed to
EnterCriticalSection(&rwlock->writerLock);
rwlock->waitingWriter = TRUE;
while (rwlock->readerCount > 0) {
LeaveCriticalSection(&rwlock->writerLock);
WaitForSingleObject(rwlock->noReaders, INFINITE);
EnterCriticalSection(&rwlock->writerLock);
}
rwlock->waitingWriter = FALSE;
/* writerLock remains locked. */
请查看来自英特尔线程构建模块的spin_rw_mutex...
spin_rw_mutex
严格位于用户空间,使用自旋等待进行阻塞。
这是一个老问题,但也许有人会发现这很有用。我们开发了一个高性能的开源RWLock
for Windows,它自动使用Vista+ SRWLock
Michael提到,如果可用,否则回退到用户空间实现。
作为额外的奖励,它有四种不同的“口味”(虽然你可以坚持基本的,这也是最快的),每个都提供更多的同步选项。它从基本的RWLock()
开始,它是非可重入的,仅限于单进程同步,并且不能交换读/写锁定,到完全跨进程IPC RWLock,支持可重入和读/写降级。
如上所述,它们在可能时动态切换到Vista+轻量级读写锁,以获得最佳性能,但您根本不必担心,因为它将回退到Windows XP及其类似系统上的完全兼容实现。
我使用了临界区来编写以下代码。
class ReadWriteLock {
volatile LONG writelockcount;
volatile LONG readlockcount;
CRITICAL_SECTION cs;
public:
ReadWriteLock() {
InitializeCriticalSection(&cs);
writelockcount = 0;
readlockcount = 0;
}
~ReadWriteLock() {
DeleteCriticalSection(&cs);
}
void AcquireReaderLock() {
retry:
while (writelockcount) {
Sleep(0);
}
EnterCriticalSection(&cs);
if (!writelockcount) {
readlockcount++;
}
else {
LeaveCriticalSection(&cs);
goto retry;
}
LeaveCriticalSection(&cs);
}
void ReleaseReaderLock() {
EnterCriticalSection(&cs);
readlockcount--;
LeaveCriticalSection(&cs);
}
void AcquireWriterLock() {
retry:
while (writelockcount||readlockcount) {
Sleep(0);
}
EnterCriticalSection(&cs);
if (!writelockcount&&!readlockcount) {
writelockcount++;
}
else {
LeaveCriticalSection(&cs);
goto retry;
}
LeaveCriticalSection(&cs);
}
void ReleaseWriterLock() {
EnterCriticalSection(&cs);
writelockcount--;
LeaveCriticalSection(&cs);
}
};
要执行自旋等待,请注释掉 Sleep(0) 的行。
这是我能想到的最小解决方案:
http://www.baboonz.org/rwlock.php
并原样粘贴:
/** A simple Reader/Writer Lock.
This RWL has no events - we rely solely on spinlocks and sleep() to yield control to other threads.
I don't know what the exact penalty is for using sleep vs events, but at least when there is no contention, we are basically
as fast as a critical section. This code is written for Windows, but it should be trivial to find the appropriate
equivalents on another OS.
**/
class TinyReaderWriterLock
{
public:
volatile uint32 Main;
static const uint32 WriteDesireBit = 0x80000000;
void Noop( uint32 tick )
{
if ( ((tick + 1) & 0xfff) == 0 ) // Sleep after 4k cycles. Crude, but usually better than spinning indefinitely.
Sleep(0);
}
TinyReaderWriterLock() { Main = 0; }
~TinyReaderWriterLock() { ASSERT( Main == 0 ); }
void EnterRead()
{
for ( uint32 tick = 0 ;; tick++ )
{
uint32 oldVal = Main;
if ( (oldVal & WriteDesireBit) == 0 )
{
if ( InterlockedCompareExchange( (LONG*) &Main, oldVal + 1, oldVal ) == oldVal )
break;
}
Noop(tick);
}
}
void EnterWrite()
{
for ( uint32 tick = 0 ;; tick++ )
{
if ( (tick & 0xfff) == 0 ) // Set the write-desire bit every 4k cycles (including cycle 0).
_InterlockedOr( (LONG*) &Main, WriteDesireBit );
uint32 oldVal = Main;
if ( oldVal == WriteDesireBit )
{
if ( InterlockedCompareExchange( (LONG*) &Main, -1, WriteDesireBit ) == WriteDesireBit )
break;
}
Noop(tick);
}
}
void LeaveRead()
{
ASSERT( Main != -1 );
InterlockedDecrement( (LONG*) &Main );
}
void LeaveWrite()
{
ASSERT( Main == -1 );
InterlockedIncrement( (LONG*) &Main );
}
};
如果您已经知道一种仅使用互斥量的解决方案,那么您应该能够修改它以使用临界区。
我们使用了两个临界区和一些计数器来自己实现。它适合我们的需求 - 我们有非常少的写入者数量,写入者优先于读取者等等。我不能公开我们的代码,但可以说不使用互斥量和信号量也是可能的。
请查看我的实现:
https://github.com/coolsoftware/LockLib
VRWLock是一个实现单写多读逻辑的C++类。
还可以查看测试项目TestLock.sln。
更新。下面是读者和写者的简单代码:
LONG gCounter = 0;
// reader
for (;;) //loop
{
LONG n = InterlockedIncrement(&gCounter);
// n = value of gCounter after increment
if (n <= MAX_READERS) break; // writer does not write anything - we can read
InterlockedDecrement(&gCounter);
}
// read data here
InterlockedDecrement(&gCounter); // release reader
// writer
for (;;) //loop
{
LONG n = InterlockedCompareExchange(&gCounter, (MAX_READERS+1), 0);
// n = value of gCounter before attempt to replace it by MAX_READERS+1 in InterlockedCompareExchange
// if gCounter was 0 - no readers/writers and in gCounter will be MAX_READERS+1
// if gCounter was not 0 - gCounter stays unchanged
if (n == 0) break;
}
// write data here
InterlockedExchangeAdd(&gCounter, -(MAX_READERS+1)); // release writer
VRWLock类支持自旋次数和线程特定引用计数,允许释放已终止线程的锁。