多執行緒的安全隱患

一塊資源可能會被多個執行緒共享，也就是多個執行緒可能會訪問同一塊資源；當多個執行緒訪問同一塊資源時，很容易引發資料錯亂和資料安全問題### 問題案例
賣票和存錢取錢的兩個案例，具體見下面代碼

@interface BaseDemo: NSObject

- (void)moneyTest;
- (void)ticketTest;

#pragma mark - 暴露給子類去使用
- (void)__saveMoney;
- (void)__drawMoney;
- (void)__saleTicket;
@end

@interface BaseDemo()
@property (assign, nonatomic) int money;
@property (assign, nonatomic) int ticketsCount;
@end

@implementation BaseDemo

/**
 存錢、取錢演示
 */
- (void)moneyTest
{
    self.money = 100;
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self __saveMoney];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self __drawMoney];
        }
    });
}

/**
 存錢
 */
- (void)__saveMoney
{
    int oldMoney = self.money;
    sleep(.2);
    oldMoney += 50;
    self.money = oldMoney;
    
    NSLog(@"存50，還剩%d元 - %@", oldMoney, [NSThread currentThread]);
}

/**
 取錢
 */
- (void)__drawMoney
{
    int oldMoney = self.money;
    sleep(.2);
    oldMoney -= 20;
    self.money = oldMoney;
    
    NSLog(@"取20，還剩%d元 - %@", oldMoney, [NSThread currentThread]);
}

/**
 賣1張票
 */
- (void)__saleTicket
{
    int oldTicketsCount = self.ticketsCount;
    sleep(.2);
    oldTicketsCount--;
    self.ticketsCount = oldTicketsCount;
    NSLog(@"還剩%d張票 - %@", oldTicketsCount, [NSThread currentThread]);
}

/**
 賣票演示
 */
- (void)ticketTest
{
    self.ticketsCount = 15;
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self __saleTicket];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self __saleTicket];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self __saleTicket];
        }
    });
}

@end


@interface ViewController ()
@property (strong, nonatomic) BaseDemo *demo;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    BaseDemo *demo = [[BaseDemo alloc] init];
    [demo ticketTest];
    [demo moneyTest];
}
@end

解決方案是使用執行緒同步技術（同步，就是協同步調，按預定的先后次序進行，常見的方案就是加鎖

執行緒同步方案

iOS中的執行緒同步方案有以下這些

OSSpinLock- os_unfair_lock- pthread_mutex- dispatch_semaphore- dispatch_queue(DISPATCH_QUEUE_SERIAL)- NSLock- NSRecursiveLock- NSCondition- NSConditionLock- @synchronized#### OSSpinLock
OSSpinLock叫做”自旋鎖”，等待鎖的執行緒會處于忙等（busy-wait）狀態，一直占用著CPU資源用OSSpinLock來解決上述示例問題

@interface OSSpinLockDemo: BaseDemo

@end

#import "OSSpinLockDemo.h"
#import <libkern/OSAtomic.h>

@interface OSSpinLockDemo()
@property (assign, nonatomic) OSSpinLock moneyLock;
// @property (assign, nonatomic) OSSpinLock ticketLock;
@end

@implementation OSSpinLockDemo

- (instancetype)init
{
    if (self = [super init]) {
        self.moneyLock = OS_SPINLOCK_INIT;
        // self.ticketLock = OS_SPINLOCK_INIT;
    }
    return self;
}

- (void)__drawMoney
{
    OSSpinLockLock(&_moneyLock);
    
    [super __drawMoney];
    
    OSSpinLockUnlock(&_moneyLock);
}

- (void)__saveMoney
{
    OSSpinLockLock(&_moneyLock);
    
    [super __saveMoney];
    
    OSSpinLockUnlock(&_moneyLock);
}

- (void)__saleTicket
{
	// 不用屬性，用一個靜態變數也可以
	static OSSpinLock ticketLock = OS_SPINLOCK_INIT;
    OSSpinLockLock(&ticketLock);
    
    [super __saleTicket];
    
    OSSpinLockUnlock(&ticketLock);
}

@end

static的問題

上面的ticketLock也可以用static來修飾作為內部靜態變數來使用

#define	OS_SPINLOCK_INIT    0

由于OS_SPINLOCK_INIT就是0，所以才可以用static來修飾；static只能在編譯時賦值一個確定值，不能動態賦予一個函式值

// 這樣賦值一個函式回傳值是會報錯的
static OSSpinLock ticketLock = [NSString stringWithFormat:@"haha"];

OSSpinLock的問題

OSSpinLock現在已經不再安全，可能會出現優先級反轉問題

由于多執行緒的本質是在不同執行緒之間進行來回的調度，每個執行緒可能對應分配的資源優先級不同；如果優先級低的執行緒先進行了加鎖并準備執行代碼，這時優先級高的執行緒就會在外面回圈等待加鎖；但因為其優先級高，所以CPU可能會大量的給其分配任務，那么就沒辦法處理優先級低的執行緒；優先級低的執行緒就無法繼續往下執行代碼，那么也就沒辦法解鎖，所以又會變成了互相等待的局面，造成死鎖，這也是蘋果現在廢棄了OSSpinLock的原因

解決辦法

用嘗試加鎖OSSpinLockTry來替換OSSpinLockLock，如果沒有加鎖才會進到判斷里執行代碼并加鎖，避免了因上鎖了一直在回圈等待的問題

// 用賣票的函式來舉例，其他幾個加鎖的方法也是同樣
- (void)__saleTicket
{
    if (OSSpinLockTry(&_ticketLock)) {
        [super __saleTicket];
        
        OSSpinLockUnlock(&_ticketLock);
    }
}

通過匯編來分析

我們通過斷點來分析加鎖之后做了什么

我們在賣票的加鎖代碼處打上斷點，并通過轉匯編的方式一步步呼叫分析

轉成匯編后呼叫OSSpinLockLock

內部會呼叫_OSSpinLockLockSlow

核心部分，在_OSSpinLockLockSlow會進行比較，然后執行到斷點處又會再次跳回0x7fff5e73326f再次執行代碼

所以通過匯編底層執行邏輯，我們能看出OSSpinLock是會不斷回圈去呼叫判斷的，只有解鎖之后才會往下執行代碼

鎖的等級

OSSpinLock自旋鎖是高等級的鎖（High-level lock），因為會一直回圈呼叫

os_unfair_lock

蘋果現在用os_unfair_lock用于取代不安全的OSSpinLock ，從iOS10開始才支持

從底層呼叫看，等待os_unfair_lock鎖的執行緒會處于休眠狀態，并非忙等

修改示例代碼如下

#import "BaseDemo.h"

@interface OSUnfairLockDemo: BaseDemo

@end

#import "OSUnfairLockDemo.h"
#import <os/lock.h>

@interface OSUnfairLockDemo()

@property (assign, nonatomic) os_unfair_lock moneyLock;
@property (assign, nonatomic) os_unfair_lock ticketLock;
@end

@implementation OSUnfairLockDemo

- (instancetype)init
{
    if (self = [super init]) {
        self.moneyLock = OS_UNFAIR_LOCK_INIT;
        self.ticketLock = OS_UNFAIR_LOCK_INIT;
    }
    return self;
}

- (void)__saleTicket
{
    os_unfair_lock_lock(&_ticketLock);
    
    [super __saleTicket];
    
    os_unfair_lock_unlock(&_ticketLock);
}

- (void)__saveMoney
{
    os_unfair_lock_lock(&_moneyLock);
    
    [super __saveMoney];
    
    os_unfair_lock_unlock(&_moneyLock);
}

- (void)__drawMoney
{
    os_unfair_lock_lock(&_moneyLock);
    
    [super __drawMoney];
    
    os_unfair_lock_unlock(&_moneyLock);
}

@end

如果不寫os_unfair_lock_unlock，那么所有的執行緒都會卡在os_unfair_lock_lock進入睡眠，不會再繼續執行代碼，這種情況叫做死鎖

通過匯編來分析

我們也通過斷點來分析加鎖之后做了什么

首先會呼叫os_unfair_lock_lock

然后會呼叫os_unfair_lock_lock_slow

然后在os_unfair_lock_lock_slow中會執行__ulock_wait

核心部分，代碼執行到syscall會直接跳出斷點，不再執行代碼，也就是進入了睡眠

所以通過匯編底層執行邏輯，我們能看出os_unfair_lock一旦進行了加鎖，就會直接進入休眠，等待解鎖后喚醒再繼續執行代碼，也由此可以認為os_unfair_lock是互斥鎖

syscall的呼叫可以理解為系統級別的呼叫進入睡眠，會直接卡住執行緒，不再執行代碼

鎖的等級

我們進到os_unfair_lock的頭檔案lock.h，可以看到注釋說明os_unfair_lock是一個低等級的鎖（Low-level lock），因為一旦發現加鎖后就會自動進入睡眠

pthread_mutex

互斥鎖

mutex叫做”互斥鎖”，等待鎖的執行緒會處于休眠狀態

使用代碼如下

@interface MutexDemo: BaseDemo

@end

#import "MutexDemo.h"
#import <pthread.h>

@interface MutexDemo()
@property (assign, nonatomic) pthread_mutex_t ticketMutex;
@property (assign, nonatomic) pthread_mutex_t moneyMutex;
@end

@implementation MutexDemo

- (void)__initMutex:(pthread_mutex_t *)mutex
{
    // 靜態初始化
    // 需要在定義這個變數時給予值才可以這么寫
    //        pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
    
//    // 初始化屬性
//    pthread_mutexattr_t attr;
//    pthread_mutexattr_init(&attr);
//    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_DEFAULT);
//    // 初始化鎖
//    pthread_mutex_init(mutex, &attr);
    
    // &attr傳NULL默認就是PTHREAD_MUTEX_DEFAULT
    pthread_mutex_init(mutex, NULL);
}

- (instancetype)init
{
    if (self = [super init]) {
        [self __initMutex:&_ticketMutex];
        [self __initMutex:&_moneyMutex];
    }
    return self;
}

- (void)__saleTicket
{
    pthread_mutex_lock(&_ticketMutex);
    
    [super __saleTicket];
    
    pthread_mutex_unlock(&_ticketMutex);
}

- (void)__saveMoney
{
    pthread_mutex_lock(&_moneyMutex);
    
    [super __saveMoney];
    
    pthread_mutex_unlock(&_moneyMutex);
}

- (void)__drawMoney
{
    pthread_mutex_lock(&_moneyMutex);
    
    [super __drawMoney];
    
    pthread_mutex_unlock(&_moneyMutex);
}

- (void)dealloc
{
  
  // 物件銷毀時呼叫
  	pthread_mutex_destroy(&_moneyMutex);
    pthread_mutex_destroy(&_ticketMutex);
}

pthread_mutex_t實際就是pthread_mutex *型別

遞回鎖

當屬性設定為PTHREAD_MUTEX_RECURSIVE時，就可以作為遞回鎖來使用

遞回鎖允許同一個執行緒對一把鎖進行重復加鎖；多個執行緒不可以用遞回鎖

- (void)__initMutex:(pthread_mutex_t *)mutex
{
    // 初始化屬性
    pthread_mutexattr_t attr;
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
    // 初始化鎖
    pthread_mutex_init(mutex, &attr);
}

- (void)otherTest
{
    pthread_mutex_lock(&_mutex);
    
    NSLog(@"%s", __func__);
    
    static int count = 0;
    if (count < 10) {
        count++;
        [self otherTest];
    }
    
    pthread_mutex_unlock(&_mutex);
}

根據條件來進行加鎖

我們可以設定一定的條件來選擇執行緒之間的呼叫進行加鎖解鎖，示例如下

@interface MutexDemo()
@property (assign, nonatomic) pthread_mutex_t mutex;
@property (assign, nonatomic) pthread_cond_t cond;
@property (strong, nonatomic) NSMutableArray *data;
@end

@implementation MutexDemo

- (instancetype)init
{
    if (self = [super init]) {
        // 初始化鎖
        pthread_mutex_init(&_mutex, NULL);        
        // 初始化條件
        pthread_cond_init(&_cond, NULL);
        
        self.data = https://www.cnblogs.com/funkyRay/archive/2021/04/08/[NSMutableArray array];
    }
    return self;
}

- (void)otherTest
{
    [[[NSThread alloc] initWithTarget:self selector:@selector(__remove) object:nil] start];
    
    [[[NSThread alloc] initWithTarget:self selector:@selector(__add) object:nil] start];
}

// 執行緒1
// 洗掉陣列中的元素
- (void)__remove
{
    pthread_mutex_lock(&_mutex);
    NSLog(@"__remove - begin");
    
    // 如果資料為空，那么設定條件等待喚醒
    // 等待期間會先解鎖，讓其他執行緒執行代碼
    if (self.data.count == 0) {
        pthread_cond_wait(&_cond, &_mutex);
    }
    
    [self.data removeLastObject];
    NSLog(@"洗掉了元素");
    
    pthread_mutex_unlock(&_mutex);
}

// 執行緒2
// 往陣列中添加元素
- (void)__add
{
    pthread_mutex_lock(&_mutex);
    
    sleep(1);
    
    [self.data addObject:@"Test"];
    NSLog(@"添加了元素");
    
    // 一旦添加了元素，變發送條件信號，讓等待洗掉的條件繼續執行代碼，并再次加鎖
    // 信號（通知一個條件）
    pthread_cond_signal(&_cond);
    // 廣播（通知所有條件）
//    pthread_cond_broadcast(&_cond);
    
    pthread_mutex_unlock(&_mutex);
}

- (void)dealloc
{
    pthread_mutex_destroy(&_mutex);
    pthread_cond_destroy(&_cond);
}

@end

通過匯編來分析

我們通過斷點來分析加鎖之后做了什么

首先會執行pthread_mutex_lock

然后會執行pthread_mutex_firstfit_lock_slow

然后會執行pthread_mutex_firstfit_lock_wait

然后會執行__psynch_mutexwait

核心部分，在__psynch_mutexwait里，代碼執行到syscall會直接跳出斷點，不再執行代碼，也就是進入了睡眠

所以pthread_mutex和os_unfair_lock一樣，都是在加鎖之后會進入到睡眠

鎖的等級

pthread_mutex和os_unfair_lock一樣，都是低等級的鎖（Low-level lock）

NSLock

NSLock是對mutex普通鎖的封裝

NSLock遵守了<NSLocking>協議，支持以下兩個方法

@protocol NSLocking

- (void)lock;
- (void)unlock;

@end

其他常用方法

// 嘗試解鎖
- (BOOL)tryLock;

// 設定一個時間等待加鎖，時間到了如果還不能成功加鎖就回傳NO
- (BOOL)lockBeforeDate:(NSDate *)limit;

具體使用看下面代碼

@interface NSLockDemo: BaseDemo

@end

@interface NSLockDemo()
@property (strong, nonatomic) NSLock *ticketLock;
@property (strong, nonatomic) NSLock *moneyLock;
@end

@implementation NSLockDemo


- (instancetype)init
{
    if (self = [super init]) {
        self.ticketLock = [[NSLock alloc] init];
        self.moneyLock = [[NSLock alloc] init];
    }
    return self;
}

- (void)__saleTicket
{
    [self.ticketLock lock];
    
    [super __saleTicket];
    
    [self.ticketLock unlock];
}

- (void)__saveMoney
{
    [self.moneyLock lock];
    
    [super __saveMoney];
    
    [self.moneyLock unlock];
}

- (void)__drawMoney
{
    [self.moneyLock lock];
    
    [super __drawMoney];
    
    [self.moneyLock unlock];
}

@end

分析底層實作

由于NSLock是不開源的，我們可以通過GNUstep Base來分析具體實作

找到NSLock.m可以看到initialize初始化方法里是創建的pthread_mutex_t物件，所以可以確定NSLock是對pthread_mutex的面向物件的封裝

@implementation NSLock

+ (id) allocWithZone: (NSZone*)z
{
  if (self == baseLockClass && YES == traceLocks)
    {
      return class_createInstance(tracedLockClass, 0);
    }
  return class_createInstance(self, 0);
}

+ (void) initialize
{
  static BOOL	beenHere = NO;

  if (beenHere == NO)
    {
      beenHere = YES;

      /* Initialise attributes for the different types of mutex.
       * We do it once, since attributes can be shared between multiple
       * mutexes.
       * If we had a pthread_mutexattr_t instance for each mutex, we would
       * either have to store it as an ivar of our NSLock (or similar), or
       * we would potentially leak instances as we couldn't destroy them
       * when destroying the NSLock.  I don't know if any implementation
       * of pthreads actually allocates memory when you call the
       * pthread_mutexattr_init function, but they are allowed to do so
       * (and deallocate the memory in pthread_mutexattr_destroy).
       */
      pthread_mutexattr_init(&attr_normal);
      pthread_mutexattr_settype(&attr_normal, PTHREAD_MUTEX_NORMAL);
      pthread_mutexattr_init(&attr_reporting);
      pthread_mutexattr_settype(&attr_reporting, PTHREAD_MUTEX_ERRORCHECK);
      pthread_mutexattr_init(&attr_recursive);
      pthread_mutexattr_settype(&attr_recursive, PTHREAD_MUTEX_RECURSIVE);

      /* To emulate OSX behavior, we need to be able both to detect deadlocks
       * (so we can log them), and also hang the thread when one occurs.
       * the simple way to do that is to set up a locked mutex we can
       * force a deadlock on.
       */
      pthread_mutex_init(&deadlock, &attr_normal);
      pthread_mutex_lock(&deadlock);

      baseConditionClass = [NSCondition class];
      baseConditionLockClass = [NSConditionLock class];
      baseLockClass = [NSLock class];
      baseRecursiveLockClass = [NSRecursiveLock class];

      tracedConditionClass = [GSTracedCondition class];
      tracedConditionLockClass = [GSTracedConditionLock class];
      tracedLockClass = [GSTracedLock class];
      tracedRecursiveLockClass = [GSTracedRecursiveLock class];

      untracedConditionClass = [GSUntracedCondition class];
      untracedConditionLockClass = [GSUntracedConditionLock class];
      untracedLockClass = [GSUntracedLock class];
      untracedRecursiveLockClass = [GSUntracedRecursiveLock class];
    }
}

NSRecursiveLock

NSRecursiveLock也是對mutex遞回鎖的封裝，API跟NSLock基本一致

NSCondition

NSCondition是對mutex和cond的封裝

具體使用代碼如下

@interface NSConditionDemo()
@property (strong, nonatomic) NSCondition *condition;
@property (strong, nonatomic) NSMutableArray *data;
@end

@implementation NSConditionDemo

- (instancetype)init
{
    if (self = [super init]) {
        self.condition = [[NSCondition alloc] init];
        self.data = https://www.cnblogs.com/funkyRay/archive/2021/04/08/[NSMutableArray array];
    }
    return self;
}

- (void)otherTest
{
    [[[NSThread alloc] initWithTarget:self selector:@selector(__remove) object:nil] start];
    
    [[[NSThread alloc] initWithTarget:self selector:@selector(__add) object:nil] start];
}

// 執行緒1
// 洗掉陣列中的元素
- (void)__remove
{
    [self.condition lock];
    NSLog(@"__remove - begin");
    
    if (self.data.count == 0) {
        // 等待
        [self.condition wait];
    }
    
    [self.data removeLastObject];
    NSLog(@"洗掉了元素");
    
    [self.condition unlock];
}

// 執行緒2
// 往陣列中添加元素
- (void)__add
{
    [self.condition lock];
    
    sleep(1);
    
    [self.data addObject:@"Test"];
    NSLog(@"添加了元素");
    // 信號
    [self.condition signal];
    
    // 廣播
//    [self.condition broadcast];
    [self.condition unlock];
    
}
@end
```##### 分析底層實作`NSCondition`也遵守了`NSLocking`協議，說明其內部已經封裝了鎖的相關代碼

@interface NSCondition : NSObject {
@private
void *_priv;
}

(void)wait;
(BOOL)waitUntilDate:(NSDate *)limit;
(void)signal;
(void)broadcast;

@property (nullable, copy) NSString *name API_AVAILABLE(macos(10.5), ios(2.0), watchos(2.0), tvos(9.0));

@end

我們通過`GNUstep Base`也可以看到其初始化方法里對`pthread_mutex_t`進行了封裝

@implementation NSCondition

(id) allocWithZone: (NSZone*)z
{
if (self == baseConditionClass && YES == traceLocks)
{
return class_createInstance(tracedConditionClass, 0);
}
return class_createInstance(self, 0);
}
(void) initialize
{
[NSLock class]; // Ensure mutex attributes are set up.
}

(id) init
{
if (nil != (self = [super init]))
{
if (0 != pthread_cond_init(&_condition, NULL))
{
DESTROY(self);
}
else if (0 != pthread_mutex_init(&_mutex, &attr_reporting))
{
pthread_cond_destroy(&_condition);
DESTROY(self);
}
}
return self;
}


#### NSConditionLock
`NSConditionLock`是對`NSCondition`的進一步封裝，可以設定具體的條件值

通過設定條件值可以對執行緒做依賴控制執行順序，具體使用見示例代碼

@interface NSConditionLockDemo()
@property (strong, nonatomic) NSConditionLock *conditionLock;
@end

@implementation NSConditionLockDemo

(instancetype)init
{
// 創建的時候可以設定一個條件
// 如果不設定，默認就是0
if (self = [super init]) {
self.conditionLock = [[NSConditionLock alloc] initWithCondition:1];
}
return self;
}
(void)otherTest
{
[[[NSThread alloc] initWithTarget:self selector:@selector(__one) object:nil] start];

[[[NSThread alloc] initWithTarget:self selector:@selector(__two) object:nil] start];

[[[NSThread alloc] initWithTarget:self selector:@selector(__three) object:nil] start];
}
(void)__one
{
// 不需要任何條件，只有沒有鎖就加鎖
[self.conditionLock lock];

NSLog(@"__one");
sleep(1);

[self.conditionLock unlockWithCondition:2];
}
(void)__two
{
// 根據對應條件來加鎖
[self.conditionLock lockWhenCondition:2];

NSLog(@"__two");
sleep(1);

[self.conditionLock unlockWithCondition:3];
}
(void)__three
{
[self.conditionLock lockWhenCondition:3];

NSLog(@"__three");

[self.conditionLock unlock];
}

@end

// 列印的先后順序為：1、2、3


#### dispatch\_queue\_t 
我們可以直接使用`GCD`的串行佇列，也是可以實作執行緒同步的，具體代碼可以參考`GCD`部分的示例代碼

#### dispatch_semaphore
`semaphore`叫做”信號量”

信號量的初始值，可以用來控制執行緒并發訪問的最大數量

示例代碼如下

@interface SemaphoreDemo()
@property (strong, nonatomic) dispatch_semaphore_t semaphore;
@property (strong, nonatomic) dispatch_semaphore_t ticketSemaphore;
@property (strong, nonatomic) dispatch_semaphore_t moneySemaphore;
@end

@implementation SemaphoreDemo

(instancetype)init
{
if (self = [super init]) {
// 初始化信號量
// 最多只開5條執行緒，也就是可以5條執行緒同時訪問同一塊空間，然后加鎖，其他執行緒再進來就只能等待了
self.semaphore = dispatch_semaphore_create(5);
// 最多只開1條執行緒
self.ticketSemaphore = dispatch_semaphore_create(1);
self.moneySemaphore = dispatch_semaphore_create(1);
}
return self;
}
(void)__drawMoney
{
dispatch_semaphore_wait(self.moneySemaphore, DISPATCH_TIME_FOREVER);

[super __drawMoney];

dispatch_semaphore_signal(self.moneySemaphore);
}
(void)__saveMoney
{
dispatch_semaphore_wait(self.moneySemaphore, DISPATCH_TIME_FOREVER);

[super __saveMoney];

dispatch_semaphore_signal(self.moneySemaphore);
}
(void)__saleTicket
{
// 如果信號量的值>0就減1，然后往下執行代碼
// 當信號量的值<=0時，當前執行緒就會進入休眠等待（直到信號量的值>0）
dispatch_semaphore_wait(self.ticketSemaphore, DISPATCH_TIME_FOREVER);

[super __saleTicket];

// 讓信號量的值加1
dispatch_semaphore_signal(self.ticketSemaphore);
}

@end


#### @synchronized

`@synchronized`是對`mutex`遞回鎖的封裝

示例代碼如下

@interface SynchronizedDemo: BaseDemo

@end

@implementation SynchronizedDemo

(void)__drawMoney
{
// @synchronized需要加鎖的是同一個物件才行
@synchronized([self class]) {
[super __drawMoney];
}
}
(void)__saveMoney
{
@synchronized([self class]) { // objc_sync_enter
[super __saveMoney];
} // objc_sync_exit
}
(void)__saleTicket
{
static NSObject *lock;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
lock = [[NSObject alloc] init];
});

@synchronized(lock) {
[super __saleTicket];
}
}
@end


##### 原始碼分析
我們可以通程序式運行中轉匯編看到，最終都會呼叫`objc_sync_enter`

![](https://img2020.cnblogs.com/blog/2320802/202104/2320802-20210408144446882-975064667.jpg)

我們可以通過`objc4`中`objc-sync.mm`來分析對應的原始碼實作

int objc_sync_enter(id obj)
{
int result = OBJC_SYNC_SUCCESS;

if (obj) {
    SyncData* data = https://www.cnblogs.com/funkyRay/archive/2021/04/08/id2data(obj, ACQUIRE);
    ASSERT(data);
    data->mutex.lock();
} else {
    // @synchronized(nil) does nothing
    if (DebugNilSync) {
        _objc_inform("NIL SYNC DEBUG: @synchronized(nil); set a breakpoint on objc_sync_nil to debug");
    }
    objc_sync_nil();
}

return result;

}


可以看到會根據傳進來的`obj`找到對應的`SyncData`

typedef struct alignas(CacheLineSize) SyncData {
struct SyncData* nextData;
DisguisedPtr<objc_object> object;
int32_t threadCount; // number of THREADS using this block
recursive_mutex_t mutex;
} SyncData;


在找到`SyncData`里面的成員變數`recursive_mutex_t`的真實型別，里面有一個遞回鎖

using recursive_mutex_t = recursive_mutex_tt;

class recursive_mutex_tt : nocopy_t {
os_unfair_recursive_lock mLock; // 遞回鎖

public:
constexpr recursive_mutex_tt() : mLock(OS_UNFAIR_RECURSIVE_LOCK_INIT) {
lockdebug_remember_recursive_mutex(this);
}

constexpr recursive_mutex_tt(__unused const fork_unsafe_lock_t unsafe)
    : mLock(OS_UNFAIR_RECURSIVE_LOCK_INIT)
{ }

void lock()
{
    lockdebug_recursive_mutex_lock(this);
    os_unfair_recursive_lock_lock(&mLock);
}

void unlock()
{
    lockdebug_recursive_mutex_unlock(this);

    os_unfair_recursive_lock_unlock(&mLock);
}

void forceReset()
{
    lockdebug_recursive_mutex_unlock(this);

    bzero(&mLock, sizeof(mLock));
    mLock = os_unfair_recursive_lock OS_UNFAIR_RECURSIVE_LOCK_INIT;
}

bool tryLock()
{
    if (os_unfair_recursive_lock_trylock(&mLock)) {
        lockdebug_recursive_mutex_lock(this);
        return true;
    }
    return false;
}

bool tryUnlock()
{
    if (os_unfair_recursive_lock_tryunlock4objc(&mLock)) {
        lockdebug_recursive_mutex_unlock(this);
        return true;
    }
    return false;
}

void assertLocked() {
    lockdebug_recursive_mutex_assert_locked(this);
}

void assertUnlocked() {
    lockdebug_recursive_mutex_assert_unlocked(this);
}

};


然后我們分析獲取`SyncData`的實作方法`id2data`，通過`obj`從`LIST_FOR_OBJ`真正取出資料

static SyncData* id2data(id object, enum usage why)
{
spinlock_t *lockp = &LOCK_FOR_OBJ(object);
SyncData *listp = &LIST_FOR_OBJ(object);
SyncData result = NULL;

if SUPPORT_DIRECT_THREAD_KEYS

// Check per-thread single-entry fast cache for matching object
bool fastCacheOccupied = NO;
SyncData *data = https://www.cnblogs.com/funkyRay/archive/2021/04/08/(SyncData *)tls_get_direct(SYNC_DATA_DIRECT_KEY);
if (data) {
    fastCacheOccupied = YES;

    if (data->object == object) {
        // Found a match in fast cache.
        uintptr_t lockCount;

        result = data;
        lockCount = (uintptr_t)tls_get_direct(SYNC_COUNT_DIRECT_KEY);
        if (result->threadCount <= 0  ||  lockCount <= 0) {
            _objc_fatal("id2data fastcache is buggy");
        }

        switch(why) {
        case ACQUIRE: {
            lockCount++;
            tls_set_direct(SYNC_COUNT_DIRECT_KEY, (void*)lockCount);
            break;
        }
        case RELEASE:
            lockCount--;
            tls_set_direct(SYNC_COUNT_DIRECT_KEY, (void*)lockCount);
            if (lockCount == 0) {
                // remove from fast cache
                tls_set_direct(SYNC_DATA_DIRECT_KEY, NULL);
                // atomic because may collide with concurrent ACQUIRE
                OSAtomicDecrement32Barrier(&result->threadCount);
            }
            break;
        case CHECK:
            // do nothing
            break;
        }

        return result;
    }
}

endif

// Check per-thread cache of already-owned locks for matching object
SyncCache *cache = fetch_cache(NO);
if (cache) {
    unsigned int i;
    for (i = 0; i < cache->used; i++) {
        SyncCacheItem *item = &cache->list[i];
        if (item->data->object != object) continue;

        // Found a match.
        result = item->data;
        if (result->threadCount <= 0  ||  item->lockCount <= 0) {
            _objc_fatal("id2data cache is buggy");
        }
            
        switch(why) {
        case ACQUIRE:
            item->lockCount++;
            break;
        case RELEASE:
            item->lockCount--;
            if (item->lockCount == 0) {
                // remove from per-thread cache
                cache->list[i] = cache->list[--cache->used];
                // atomic because may collide with concurrent ACQUIRE
                OSAtomicDecrement32Barrier(&result->threadCount);
            }
            break;
        case CHECK:
            // do nothing
            break;
        }

        return result;
    }
}

// Thread cache didn't find anything.
// Walk in-use list looking for matching object
// Spinlock prevents multiple threads from creating multiple 
// locks for the same new object.
// We could keep the nodes in some hash table if we find that there are
// more than 20 or so distinct locks active, but we don't do that now.

lockp->lock();

{
    SyncData* p;
    SyncData* firstUnused = NULL;
    for (p = *listp; p != NULL; p = p->nextData) {
        if ( p->object == object ) {
            result = p;
            // atomic because may collide with concurrent RELEASE
            OSAtomicIncrement32Barrier(&result->threadCount);
            goto done;
        }
        if ( (firstUnused == NULL) && (p->threadCount == 0) )
            firstUnused = p;
    }

    // no SyncData currently associated with object
    if ( (why == RELEASE) || (why == CHECK) )
        goto done;

    // an unused one was found, use it
    if ( firstUnused != NULL ) {
        result = firstUnused;
        result->object = (objc_object *)object;
        result->threadCount = 1;
        goto done;
    }
}

// Allocate a new SyncData and add to list.
// XXX allocating memory with a global lock held is bad practice,
// might be worth releasing the lock, allocating, and searching again.
// But since we never free these guys we won't be stuck in allocation very often.
posix_memalign((void **)&result, alignof(SyncData), sizeof(SyncData));
result->object = (objc_object *)object;
result->threadCount = 1;
new (&result->mutex) recursive_mutex_t(fork_unsafe_lock);
result->nextData = https://www.cnblogs.com/funkyRay/archive/2021/04/08/*listp;
*listp = result;

done:
lockp->unlock();
if (result) {
// Only new ACQUIRE should get here.
// All RELEASE and CHECK and recursive ACQUIRE are
// handled by the per-thread caches above.
if (why == RELEASE) {
// Probably some thread is incorrectly exiting
// while the object is held by another thread.
return nil;
}
if (why != ACQUIRE) _objc_fatal("id2data is buggy");
if (result->object != object) _objc_fatal("id2data is buggy");

if SUPPORT_DIRECT_THREAD_KEYS

    if (!fastCacheOccupied) {
        // Save in fast thread cache
        tls_set_direct(SYNC_DATA_DIRECT_KEY, result);
        tls_set_direct(SYNC_COUNT_DIRECT_KEY, (void*)1);
    } else

endif

    {
        // Save in thread cache
        if (!cache) cache = fetch_cache(YES);
        cache->list[cache->used].data = https://www.cnblogs.com/funkyRay/archive/2021/04/08/result;
        cache->list[cache->used].lockCount = 1;
        cache->used++;
    }
}

return result;

}


`LIST_FOR_OBJ`是一個哈希表，哈希表的實作就是將傳進來的`obj`作為`key`，然后對應的鎖為`value`

define LIST_FOR_OBJ(obj) sDataLists[obj].data // 哈希表

static StripedMap sDataLists;
// 哈希表的實作就是將傳進來的物件作為key，然后對應的鎖為value


通過原始碼分析我們也可以看出，`@synchronized`內部的鎖是遞回鎖

### 鎖的比較
#### 性能比較排序
下面是每個鎖按性能從高到低排序

- os\_unfair\_lock- OSSpinLock- dispatch\_semaphore- pthread\_mutex- dispatch\_queue(DISPATCH\_QUEUE\_SERIAL)- NSLock- NSCondition- pthread_mutex(recursive)- NSRecursiveLock- NSConditionLock- @synchronized

選擇性最高的鎖

- dispatch\_semaphore- pthread\_mutex

#### 互斥鎖、自旋鎖的比較
##### 什么情況使用自旋鎖
- 預計執行緒等待鎖的時間很短- 加鎖的代碼（臨界區）經常被呼叫，但競爭情況很少發生- CPU資源不緊張- 多核處理器

##### 什么情況使用互斥鎖
- 預計執行緒等待鎖的時間較長- 單核處理器（盡量減少CPU的消耗）- 臨界區有IO操作（IO操作比較占用CPU資源）- 臨界區代碼復雜或者回圈量大- 臨界區競爭非常激烈

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標籤：其他

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iOS底層原理（七）多執行緒（中）

多執行緒的安全隱患

執行緒同步方案

static的問題

OSSpinLock的問題

解決辦法

通過匯編來分析

鎖的等級

os_unfair_lock

通過匯編來分析

鎖的等級

pthread_mutex

互斥鎖

遞回鎖

根據條件來進行加鎖

通過匯編來分析

鎖的等級

NSLock

分析底層實作

NSRecursiveLock

NSCondition

if SUPPORT_DIRECT_THREAD_KEYS

endif

if SUPPORT_DIRECT_THREAD_KEYS

endif

define LIST_FOR_OBJ(obj) sDataLists[obj].data // 哈希表