编辑 @Mike 指出我下面代码中的 try_lock 函数是不安全的,并且访问器创建也会产生竞争条件。 (来自每个人的)建议使我确信我走上了错误的道路。
原始问题
嵌入式微 Controller 上的锁定要求与多线程不同,我无法将多线程示例转换为我的嵌入式应用程序。通常我没有任何类型的操作系统或线程,只有 main
以及硬件定期调用的任何中断函数。
我需要从中断中填充缓冲区是很常见的,但在 main
中处理它是很常见的。 .我创建了 IrqMutex
下面的类来尝试安全地实现这一点。每个试图访问缓冲区的人都通过 IrqMutexAccessor
被分配了一个唯一的 ID。 ,那么他们每个人都可以 try_lock()
和 unlock()
.阻塞的想法 lock()
函数在中断时不起作用,因为除非您允许中断完成,否则没有其他代码可以执行,因此 unlock()
代码永远不会运行。然而,我确实使用了 main()
中的阻塞锁。偶尔打码。
但是,我知道没有 C++11 内存屏障(在许多嵌入式平台上不可用),双重检查锁不起作用。老实说,尽管阅读了很多关于它的信息,但我真的不明白内存访问重新排序如何/为什么会导致问题。我认为使用 volatile sig_atomic_t(可能结合使用唯一 ID)使其与双重检查锁不同。但我希望有人可以:确认以下代码正确 , 解释为什么它不安全 , 或 提供一种更好的方法来实现此目的 .
class IrqMutex {
friend class IrqMutexAccessor;
private:
std::sig_atomic_t accessorIdEnum;
volatile std::sig_atomic_t owner;
protected:
std::sig_atomic_t nextAccessor(void) { return ++accessorIdEnum; }
bool have_lock(std::sig_atomic_t accessorId) {
return (owner == accessorId);
}
bool try_lock(std::sig_atomic_t accessorId) {
// Only try to get a lock, while it isn't already owned.
while (owner == SIG_ATOMIC_MIN) {
// <-- If an interrupt occurs here, both attempts can get a lock at the same time.
// Try to take ownership of this Mutex.
owner = accessorId; // SET
// Double check that we are the owner.
if (owner == accessorId) return true;
// Someone else must have taken ownership between CHECK and SET.
// If they released it after CHECK, we'll loop back and try again.
// Otherwise someone else has a lock and we have failed.
}
// This shouldn't happen unless they called try_lock on something they already owned.
if (owner == accessorId) return true;
// If someone else owns it, we failed.
return false;
}
bool unlock(std::sig_atomic_t accessorId) {
// Double check that the owner called this function (not strictly required)
if (owner == accessorId) {
owner = SIG_ATOMIC_MIN;
return true;
}
// We still return true if the mutex was unlocked anyway.
return (owner == SIG_ATOMIC_MIN);
}
public:
IrqMutex(void) : accessorIdEnum(SIG_ATOMIC_MIN), owner(SIG_ATOMIC_MIN) {}
};
// This class is used to manage our unique accessorId.
class IrqMutexAccessor {
friend class IrqMutex;
private:
IrqMutex& mutex;
const std::sig_atomic_t accessorId;
public:
IrqMutexAccessor(IrqMutex& m) : mutex(m), accessorId(m.nextAccessor()) {}
bool have_lock(void) { return mutex.have_lock(accessorId); }
bool try_lock(void) { return mutex.try_lock(accessorId); }
bool unlock(void) { return mutex.unlock(accessorId); }
};
因为只有一个处理器,并且没有线程处理互斥锁,我认为它的用途与正常用途略有不同。我反复遇到两个主要用例。我意识到这两个示例被简化了一点,但是这些模式的某些版本出现在我工作的每个项目的许多外围设备中,我想要一段可重用的代码,可以在各种嵌入式平台上安全地处理这个问题。我包含了 C 标记,因为所有这些都可以直接转换为 C 代码,并且在某些嵌入式编译器上,这些都是可用的。所以我试图找到一种保证在 C 和 C++ 中都能工作的通用方法。
struct ExampleCounter {
volatile long long int value;
IrqMutex mutex;
} exampleCounter;
struct ExampleBuffer {
volatile char data[256];
volatile size_t index;
IrqMutex mutex; // One mutex per buffer.
} exampleBuffers[2];
const volatile char * const REGISTER;
// This accessor shouldn't be created in an interrupt or a race condition can occur.
static IrqMutexAccessor myMutex(exampleCounter.mutex);
void __irqQuickFunction(void) {
// Obtain a lock, add the data then unlock all within one function call.
if (myMutex.try_lock()) {
exampleCounter.value++;
myMutex.unlock();
} else {
// If we failed to obtain a lock, we skipped this update this one time.
}
}
// These accessors shouldn't be created in an interrupt or a race condition can occur.
static IrqMutexAccessor myMutexes[2] = {
IrqMutexAccessor(exampleBuffers[0].mutex),
IrqMutexAccessor(exampleBuffers[1].mutex)
};
void __irqLongFunction(void) {
static size_t bufferIndex = 0;
// Check if we have a lock.
if (!myMutex[bufferIndex].have_lock() and !myMutex[bufferIndex].try_lock()) {
// If we can't get a lock try the other buffer
bufferIndex = (bufferIndex + 1) % 2;
// One buffer should always be available so the next line should always be successful.
if (!myMutex[bufferIndex].try_lock()) return;
}
// ... at this point we know we have a lock ...
// Get data from the hardware and modify the buffer here.
const char c = *REGISTER;
exampleBuffers[bufferIndex].data[exampleBuffers[bufferIndex].index++] = c;
// We may keep the lock for multiple function calls until the end of packet.
static const char END_PACKET_SIGNAL = '\0';
if (c == END_PACKET_SIGNAL) {
// Unlock this buffer so it can be read from main.
myMutex[bufferIndex].unlock();
// Switch to the other buffer for next time.
bufferIndex = (bufferIndex + 1) % 2;
}
}
int main(void) {
while (true) {
// Mutex for counter
static IrqMutexAccessor myCounterMutex(exampleCounter.mutex);
// Change counter value
if (EVERY_ONCE_IN_A_WHILE) {
// Skip any updates that occur while we are updating the counter.
while(!myCounterMutex.try_lock()) {
// Wait for the interrupt to release its lock.
}
// Set the counter to a new value.
exampleCounter.value = 500;
// Updates will start again as soon as we unlock it.
myCounterMutex.unlock();
}
// Mutexes for __irqLongFunction.
static IrqMutexAccessor myBufferMutexes[2] = {
IrqMutexAccessor(exampleBuffers[0].mutex),
IrqMutexAccessor(exampleBuffers[1].mutex)
};
// Process buffers from __irqLongFunction.
for (size_t i = 0; i < 2; i++) {
// Obtain a lock so we can read the data.
if (!myBufferMutexes[i].try_lock()) continue;
// Check that the buffer isn't empty.
if (exampleBuffers[i].index == 0) {
myBufferMutexes[i].unlock(); // Don't forget to unlock.
continue;
}
// ... read and do something with the data here ...
exampleBuffer.index = 0;
myBufferMutexes[i].unlock();
}
}
}
}
另请注意,我使用了 volatile
在任何由中断例程读取或写入的变量上(除非该变量仅从中断访问,如 static bufferIndex
中的 __irqLongFunction
值)。我读过互斥锁消除了对 volatile
的一些需要在多线程代码中,但我认为这不适用于这里。 我是否使用了正确数量的 volatile
? 我用它:ExampleBuffer[].data[256]
, ExampleBuffer[].index
, 和 ExampleCounter.value
.
最佳答案
我为长答案道歉,但也许它适合一个长问题。
要回答你的第一个问题,我会说你的 IrqMutex 实现是不安全的。让我试着解释我在哪里看到问题。
功能 nextAccessor
std::sig_atomic_t nextAccessor(void) { return ++accessorIdEnum; }
该函数存在竞争条件,因为增量运算符不是原子的,尽管它位于标记为
volatile
的原子值上。 .涉及3个操作:读取accessorIdEnum
的当前值,增加它,然后写回结果。如果两个 IrqMutexAccessor
s 是同时创建的,它们可能获得相同的 ID。功能
try_lock
try_lock
函数也有竞争条件。一个线程(例如主线程)可以进入 while
循环,然后在获得所有权之前,另一个线程(例如中断)也可以进入 while
循环并取得锁的所有权(返回 true
)。然后第一个线程可以继续,移动到 owner = accessorId
,因此“也”取得了锁。所以两个线程(或者你的 main
线程和一个中断)可以 try_lock
同时在一个无主互斥锁上并且都返回 true
.通过 RAII 禁用中断
我们可以通过使用 RAII 禁用中断来实现某种程度的简单性和封装,例如以下类:
class InterruptLock {
public:
InterruptLock() {
prevInterruptState = currentInterruptState();
disableInterrupts();
}
~InterruptLock() {
restoreInterrupts(prevInterruptState);
}
private:
int prevInterruptState; // Whatever type this should be for the platform
InterruptLock(const InterruptLock&); // Not copy-constructable
};
我建议禁用中断以获得互斥体实现本身所需的原子性。例如类似的东西:
bool try_lock(std::sig_atomic_t accessorId) {
InterruptLock lock;
if (owner == SIG_ATOMIC_MIN) {
owner = accessorId;
return true;
}
return false;
}
bool unlock(std::sig_atomic_t accessorId) {
InterruptLock lock;
if (owner == accessorId) {
owner = SIG_ATOMIC_MIN;
return true;
}
return false;
}
根据您的平台,这可能看起来不同,但您明白了。
正如您所说,这提供了一个平台,可以将通用代码中的禁用和启用中断抽象出来,并将其封装到这一类中。
互斥和中断
说了我将如何考虑实现互斥类之后,我实际上不会为您的用例使用互斥类。正如您所指出的,互斥锁与中断并不能很好地配合使用,因为中断无法“阻止”尝试获取互斥锁。出于这个原因,对于直接与中断交换数据的代码,我会强烈考虑直接禁用中断(在主“线程”接触数据的很短的时间内)。
所以你的计数器可能看起来像这样:
volatile long long int exampleCounter;
void __irqQuickFunction(void) {
exampleCounter++;
}
...
// Change counter value
if (EVERY_ONCE_IN_A_WHILE) {
InterruptLock lock;
exampleCounter = 500;
}
在我看来,这更容易阅读,更容易推理,并且在出现争用(即错过计时器节拍)时不会“滑倒”。
关于缓冲区用例,我强烈建议不要为多个中断周期持有锁。锁/互斥锁应该只保留“接触”一块内存所需的最轻微的时刻——刚好足以读取或写入它。进来,出去。
所以这就是缓冲示例的样子:
struct ExampleBuffer {
char data[256];
} exampleBuffers[2];
ExampleBuffer* volatile bufferAwaitingConsumption = nullptr;
ExampleBuffer* volatile freeBuffer = &exampleBuffers[1];
const volatile char * const REGISTER;
void __irqLongFunction(void) {
static const char END_PACKET_SIGNAL = '\0';
static size_t index = 0;
static ExampleBuffer* receiveBuffer = &exampleBuffers[0];
// Get data from the hardware and modify the buffer here.
const char c = *REGISTER;
receiveBuffer->data[index++] = c;
// End of packet?
if (c == END_PACKET_SIGNAL) {
// Make the packet available to the consumer
bufferAwaitingConsumption = receiveBuffer;
// Move on to the next buffer
receiveBuffer = freeBuffer;
freeBuffer = nullptr;
index = 0;
}
}
int main(void) {
while (true) {
// Fetch packet from shared variable
ExampleBuffer* packet;
{
InterruptLock lock;
packet = bufferAwaitingConsumption;
bufferAwaitingConsumption = nullptr;
}
if (packet) {
// ... read and do something with the data here ...
// Once we're done with the buffer, we need to release it back to the producer
{
InterruptLock lock;
freeBuffer = packet;
}
}
}
}
这段代码可以说更容易推理,因为中断和主循环之间只有两个共享内存位置:一个将数据包从中断传递到主循环,另一个将空缓冲区传递回中断。我们也只接触“锁定”下的那些变量,并且只在“移动”值所需的最短时间。 (为简单起见,当主循环需要很长时间才能释放缓冲区时,我跳过了缓冲区溢出逻辑)。
确实,在这种情况下甚至可能不需要锁,因为我们只是读取和写入简单的值,但是禁用中断的成本并不高,否则犯错误的风险在我看来是不值得的观点。
编辑
正如评论中所指出的,上述解决方案旨在仅解决多线程问题,并省略了溢出检查。这是更完整的解决方案,在溢出条件下应该是健壮的:
const size_t BUFFER_COUNT = 2;
struct ExampleBuffer {
char data[256];
ExampleBuffer* next;
} exampleBuffers[BUFFER_COUNT];
volatile size_t overflowCount = 0;
class BufferList {
public:
BufferList() : first(nullptr), last(nullptr) { }
// Atomic enqueue
void enqueue(ExampleBuffer* buffer) {
InterruptLock lock;
if (last)
last->next = buffer;
else {
first = buffer;
last = buffer;
}
}
// Atomic dequeue (or returns null)
ExampleBuffer* dequeueOrNull() {
InterruptLock lock;
ExampleBuffer* result = first;
if (first) {
first = first->next;
if (!first)
last = nullptr;
}
return result;
}
private:
ExampleBuffer* first;
ExampleBuffer* last;
} freeBuffers, buffersAwaitingConsumption;
const volatile char * const REGISTER;
void __irqLongFunction(void) {
static const char END_PACKET_SIGNAL = '\0';
static size_t index = 0;
static ExampleBuffer* receiveBuffer = &exampleBuffers[0];
// Recovery from overflow?
if (!receiveBuffer) {
// Try get another free buffer
receiveBuffer = freeBuffers.dequeueOrNull();
// Still no buffer?
if (!receiveBuffer) {
overflowCount++;
return;
}
}
// Get data from the hardware and modify the buffer here.
const char c = *REGISTER;
if (index < sizeof(receiveBuffer->data))
receiveBuffer->data[index++] = c;
// End of packet, or out of space?
if (c == END_PACKET_SIGNAL) {
// Make the packet available to the consumer
buffersAwaitingConsumption.enqueue(receiveBuffer);
// Move on to the next free buffer
receiveBuffer = freeBuffers.dequeueOrNull();
index = 0;
}
}
size_t getAndResetOverflowCount() {
InterruptLock lock;
size_t result = overflowCount;
overflowCount = 0;
return result;
}
int main(void) {
// All buffers are free at the start
for (int i = 0; i < BUFFER_COUNT; i++)
freeBuffers.enqueue(&exampleBuffers[i]);
while (true) {
// Fetch packet from shared variable
ExampleBuffer* packet = dequeueOrNull();
if (packet) {
// ... read and do something with the data here ...
// Once we're done with the buffer, we need to release it back to the producer
freeBuffers.enqueue(packet);
}
size_t overflowBytes = getAndResetOverflowCount();
if (overflowBytes) {
// ...
}
}
}
关键变化:
getAndResetOverflowCount
将其传达给主线程。 BufferList
),它支持原子出队和入队。前面的示例也使用了队列,但长度为 0-1(一个项目已入队或未入队),因此队列的实现只是一个变量。在空闲缓冲区用完的情况下,接收队列可能有 2 个项目,因此我将其升级为适当的队列,而不是添加更多共享变量。 关于c++ - 带中断的互斥安全(嵌入式固件),我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/27409024/