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#pragma once
#include <util/system/spinlock.h>
// State can be one of:
// 0) [NOT_USED] State = 0:
// * any reader can acquire lock (State += 2: -> READING)
// * one writer can acquire lock (State = -1: -> WRITING)
// 1) [READING] even number:
// * State/2 = number of active readers
// * no writers are waiting
// * any reader can aqcuire lock (State += 2: -> READING)
// * active readers can release lock (State -= 2)
// * one writer can acquire lock (State += 1: -> WAITING)
// 2) [WAITING] odd number > 0:
// * (State-1)/2 = number of active readers
// * no more readers/writers can acquire lock
// * active readers can release lock (State -= 2: -> WAITING)
// * exactly one writer is waiting for active readers to release lock
// * if no more active readers left (State == 1) waiting writer acquires lock (State = -1: -> WRITING)
// 3) [WRITING] State = -1
// * exactly one active writer
// * no active readers
// * no more readers/writers can acquire lock
// * writer can release lock (State = 0: -> READING)
struct TRWSpinLock {
TAtomic State; // must be initialized by 'TRWSpinLock myLock = {0};' construction
void Init() noexcept {
State = 0;
}
void AcquireRead() noexcept {
while (true) {
TAtomic a = AtomicGet(State);
if ((a & 1) == 0 && AtomicCas(&State, a + 2, a)) {
break;
}
SpinLockPause();
}
}
void ReleaseRead() noexcept {
AtomicAdd(State, -2);
}
void AcquireWrite() noexcept {
while (true) {
TAtomic a = AtomicGet(State);
if ((a & 1) == 0 && AtomicCas(&State, a + 1, a)) {
break;
}
SpinLockPause();
}
while (!AtomicCas(&State, TAtomicBase(-1), 1)) {
SpinLockPause();
}
}
void ReleaseWrite() noexcept {
AtomicSet(State, 0);
}
};
struct TRWSpinLockReadOps {
static inline void Acquire(TRWSpinLock* t) noexcept {
t->AcquireRead();
}
static inline void Release(TRWSpinLock* t) noexcept {
t->ReleaseRead();
}
};
struct TRWSpinLockWriteOps {
static inline void Acquire(TRWSpinLock* t) noexcept {
t->AcquireWrite();
}
static inline void Release(TRWSpinLock* t) noexcept {
t->ReleaseWrite();
}
};
using TReadSpinLockGuard = TGuard<TRWSpinLock, TRWSpinLockReadOps>;
using TWriteSpinLockGuard = TGuard<TRWSpinLock, TRWSpinLockWriteOps>;
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