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我无法理解何时强制将变量写入内存，即使是在互斥锁 block 之外。对于下面令人费解的代码，我深表歉意，因为我已经剥离了处理 reader 是否决定某些数据是否过时的逻辑。需要注意的重要一点是，99.9% 的时间，读者会走快速路径，同步必须非常快，这就是为什么我使用原子 int32 进行通信的原因陈旧性以及现在是否需要慢速路径。
我有以下设置，我“相当”确定这是无竞争的:
#define NUM_READERS 10
BigObject mSharedObject;
std::atomic_int32_t mStamp = 1;
std::mutex mMutex;
std::condition_variable mCondition;
int32_t mWaitingReaders = 0;
void reader() {
for (;;) { // thread loop
for (;;) { // spin until stamp is acceptible
int32_t stamp = mStamp.load();
if (stamp > 0) { // fast path
if (stampIsAcceptible(stamp) &&
mStamp.compare_exchange_weak(stamp, stamp + 1)) {
break;
}
} else { // slow path
// tell the loader (writer) that we're halted
std::unique_lock<mutex> lk(mMutex);
mWaitingReaders++;
mCondition.notify_all();
while (mWaitingReaders != 0) {
mCondition.wait(lk);
} // ###
lk.unlock();
// *** THIS IS WHERE loader's CHANGES TO mSharedObject
// *** MUST BE VISIBLE TO THIS THREAD!
}
}
// stamp acceptible; mSharedObject guaranteed not written to
mSharedObject.accessAndDoFunStuff();
mStamp.fetch_sub(1); // part of hidden staleness logic
}
}
void loader() {
for (;;) { // thread loop
// spin until we somehow decide we want to change mSharedObject!
while (meIsHappySleeping()) {}
// we want to modify mSharedObject, so set mStamp to 0 and wait
// for readers to see this and report that they are now waiting
int32_t oldStamp = mStamp.exchange(0);
unique_lock<mutex> lk(mMutex);
while (mWaitingReaders != NUM_READERS) {
mCondition.wait(lk);
}
// all readers are waiting. start writing to mSharedObject
mSharedObject.loadFromFile("example.foo");
mStamp.store(oldStamp);
mWaitingReaders = 0; // report completion
lk.unlock();
mCondition.notify_all();
// *** NOW loader's CHANGES TO mSharedObject
// *** MUST BE VISIBLE TO THE READER THREADS!
}
}
void setup() {
for (int i = 0; i < NUM_READERS; i++) {
std::thread t(reader); t.detach();
}
std::thead t(loader); t.detach();
}
*** 星号标记的部分是我关心的。这是因为虽然我的代码排除了竞争(据我所知)，mSharedObject 在被 loader() 写入时仅受互斥体保护。因为 reader() 需要非常快(如上所述)，我不希望它对 mSharedObject 的只读访问必须由互斥锁保护。
一个“保证”的解决方案是在 ### 行引入一个线程局部变量 const BigObject *latestObject，它被设置为 &mSharedObject 然后使用它进行访问。但这是不好的做法吗？而且真的有必要吗？原子操作/互斥锁释放操作能保证读者看到变化吗？
谢谢!