为什么memory_order_seq_cst没有保护我的原子操作运行序列？

Question

#include <assert.h>
#include <atomic>
#include <iostream>
#include <thread>
std::atomic_bool b(false);
std::atomic_bool lock{false};
void producer() {
b.store(true, std::memory_order_seq_cst);
lock.store(true, std::memory_order_seq_cst);
}
void consume() {
while (!lock.load(std::memory_order_seq_cst))
;
assert(b.load(std::memory_order_seq_cst));
b.store(false, std::memory_order_seq_cst);
lock.store(false, std::memory_order_seq_cst);
}
int main() {
std::thread t1([&]() {
while (true)
consume();
});
std::thread t2([&]() {
while (true)
producer();
});
t1.join();
t2.join();
}

consume中的断言永远不会失败，memory_order_seq_cst保证atmoic操作按写入顺序运行；

但是断言失败发生了：(

Answer 1

对于一对produce()和consume()，您的分析是正确的，但对于它们的无休止循环，则不是正确的。

线程1		线程2
b.store(true, std::memory_order_seq_cst);
lock.store(true, std::memory_order_seq_cst);
	while (!lock.load(std::memory_order_seq_cst));
	assert(b.load(std::memory_order_seq_cst));
b.store(true, std::memory_order_seq_cst);
	b.store(false, std::memory_order_seq_cst);
	lock.store(false, std::memory_order_seq_cst);
lock.store(true, std::memory_order_seq_cst);
	while (!lock.load(std::memory_order_seq_cst));
	assert(b.load(std::memory_order_seq_cst));断言触发

void producer() {
b.store(true, std::memory_order_seq_cst);      // 1
lock.store(true, std::memory_order_seq_cst);   // 2
}
void consume() {
while (!lock.load(std::memory_order_seq_cst))
;                                            // 3
assert(b.load(std::memory_order_seq_cst));     // 4
b.store(false, std::memory_order_seq_cst);      // 5
lock.store(false, std::memory_order_seq_cst);   // 6
}

以下步骤将使断言失败：1->5->6->2->3->4