前篇文章整理了iOS中常见的几种锁,我最常用的是@synchronized,接下来咱们来一同学习下其底层原理
@synchronized是怎数据结构严蔚敏么完结递归互斥的？是怎么源码编辑器完结可重入的呢？源码编辑器带着这两个问题去剖析源码。
咱们先用clang命令检查@synchronize源码中的图片d的.cpp中的完结
#import <Cocoa/Cocoa.h>
int main(int argc, const char * argv[]) {
@autoreleasepool {
NSObject *syObject = [NSObject alloc];
@synchronized (syObject) {
}
}
return NSApplicationMain(argc, argv);
}
NSObject *syObject = ((NSObject *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSObject"), sel_registerName("alloc"));
{
id _rethrow = 0;
id _sync_obj = (id)syObject;
objc_sync_enter(_sync_obj);
try {
struct _SYNC_EXIT {
_SYNC_EXIT(id arg) : sync_exit(arg) {}
~_SYNC_EXIT() {objc_sync_exit(sync_exit);}
id sync_exit;
} _sync_exit(_sync_obj);
} catch (id e) {_rethrow = e;}
{
struct _FIN { _FIN(id reth) : rethrow(reth) {}
~_FIN() { if (rethrow) objc_exception_throw(rethrow); }
id rethrow;
} _fin_force_rethow(_rethrow);}
}
编译后的代码剖析出，先去调用了objc_sync_enter然后是一个_SYNC_E链表排序XIT数据结构知识点总结的结构函数和析构函数，结构函数什么都没做，析构函数出了效果域就会调用的objc_approachsync_exit。
@synchronizios15ed会被编译成objc_sync_enter和objc_sync_exit。
接下来咱们经过源码检查他们都做了什么
objc_sync_enter
int objc_sync_enter(id obj) {
int result = OBJC_SYNC_SUCCESS;
if (obj) {
SyncData* data = 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;
}
objc_sync_exit
// End synchronizing on 'obj'.
// Returns OBJC_SYNC_SUCCESS or OBJC_SYNC_NOT_OWNING_THREAD_ERROR
int objc_sync_exit(id obj) {
int result = OBJC_SYNC_SUCCESS;
if (obj) {
SyncData* data = id2data(obj, RELEASE);
if (!data) {
result = OBJC_SYNC_NOT_OWNING_THREAD_ERROR;
} else {
bool okay = data->mutex.tryUnlock();
if (!okay) {
result = OBJC_SYNC_NOT_OWNING_THREAD_ERROR;
}
}
} else {
// @synchronized(nil) does nothing
}
return result;
}
objc_sync_enter和objc_sync链表的创建_exit都是先判定参数obj为nil就啥也不做，obj不为nil经过id2data获得一个SyncData的数据结构，而且经过id2data的第二个参数来区分是enter调用的id2data还是exit调用的id2data。SyncData里拿出递归锁加锁解锁操作。
先了解SyncData数据结构的数据结构：
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;
能够看出SyncData是单向链表结构,为每一个@synchronized 的参数object分配了一把递归锁和记载线程数量。这两个分配记载就是多线程下递归调用的底子。（@synchronized(objc1)相当于是SyncData->object=objc1）
using recursive_mutex_t = recursive_mutex_tt<LOCKDEBUG>;
class recursive_mutex_tt : nocopy_t {
os_unfair_recursive_lock mLock;
......
}
OS_UNFAIR_RECURSIVE_LOCK_AVAILABILITY
typedef struct os_unfair_recursive_lock_s {
os_unfair_lock ourl_lock;
uint32_t ourl_count;
} os_unfair_recursive_lock, *os_unfair_recursive_lock_t;
能够看到，其链表结构底子是根据os_unfair_lock的封装。在之前的版本，这个是根据pthread数据结构教程第5版李春葆答案_mute源码网站x_t的封装。
关于appstore这个锁能够持续看其定义：
咱们现在要关注的是SyncData里的成员是怎么在多线程下完结递归调用的源码。要害逻辑还得看id2data里边做了什么。
static SyncData* id2data(id object, enum usage why) {
// 从大局hash表中, 经过object获取锁
spinlock_t *lockp = &LOCK_FOR_OBJ(object);
// 从大局hash表中, 经过object获取指向SyncData单向链表的头指针
SyncData **listp = &LIST_FOR_OBJ(object);
// 查询后需求回来的结果
SyncData* result = NULL;
#if SUPPORT_DIRECT_THREAD_KEYS
/* 快缓存。 ：
2个固定的线程键 贮存一个单独的 SyncCacheItem
Fast cache: two fixed pthread keys store a single SyncCacheItem.
这就避免了对于一次只同步单个目标的线程运用SyncCache的malloc
This avoids malloc of the SyncCache for threads that only synchronize a single object at a time.
SYNC_DATA_DIRECT_KEY == SyncCacheItem.data
SYNC_COUNT_DIRECT_KEY == SyncCacheItem.lockCount
*/
// Check per-thread single-entry fast cache for matching object
bool fastCacheOccupied = NO;
//拿出快速缓存里边的SyncData
SyncData *data = (SyncData *)tls_get_direct(SYNC_DATA_DIRECT_KEY);
if (data) {
fastCacheOccupied = YES;
// 假如是同一个
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");
}
//加锁的时分（ENTER）
switch(why) {
case ACQUIRE: {
lockCount++;
//lockCount放入快速缓存
tls_set_direct(SYNC_COUNT_DIRECT_KEY, (void*)lockCount);
break;
}
//解锁的时分（EXIT）
case RELEASE:
lockCount--;
//取出加锁的时分的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
// SyncData中记载线程数量的-1
OSAtomicDecrement32Barrier(&result->threadCount);
}
break;
case CHECK:
// do nothing
break;
}
//回来
return result;
}
}
#endif
/*
SyncCache 查找
在线程的TLS中找objc目标，然后再保护2个count lockCount 和 threadCount
每个线程都只要一份
*/
// Check per-thread cache of already-owned locks for matching object
SyncCache *cache = fetch_cache(NO);
if (cache) {
unsigned int i;
//遍历查找
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--;
//假如==0，该线程已经运用完了
if (item->lockCount == 0) {
// remove from per-thread cache
cache->list[i] = cache->list[--cache->used];
// atomic because may collide with concurrent ACQUIRE
// threadCount -1。避免和加锁的时分通途
OSAtomicDecrement32Barrier(&result->threadCount);
}
break;
case CHECK:
// do nothing
break;
}
return result;
}
}
/*
sDataLists 查找
这儿加锁内容包括sDataLists查找，和创立SyncData，目的是为了避免创立重复的SyncData
*/
// 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;
//遍历链表
for (p = *listp; p != NULL; p = p->nextData) {
//找到SyncData
if ( p->object == object ) {
result = p;
// atomic because may collide with concurrent RELEASE
// threadCount + 1
OSAtomicIncrement32Barrier(&result->threadCount);
// 跳转：done
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
//利用链表里边的无用节点
if ( firstUnused != NULL ) {
result = firstUnused;
result->object = (objc_object *)object;
result->threadCount = 1;
goto done;
}
}
/*
创立SyncData
*/
// 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);
//放到头节点
result->nextData = *listp;
*listp = result;
done:
/* 缓存 */
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
// 支撑线程快速缓存，而且快速缓存没有东西
if (!fastCacheOccupied) {
//存储到快速缓存中
// Save in fast thread cache
tls_set_direct(SYNC_DATA_DIRECT_KEY, result);
tls_set_direct(SYNC_COUNT_DIRECT_KEY, (void*)1);
} else
#endif
{
//在线程缓存中存储
// Save in thread cache
if (!cache) cache = fetch_cache(YES);
cache->list[cache->used].data = result;
cache->list[cache->used].lockCount = 1;
cache->used++;
}
}
return result;
}
TLS快速缓存 查找
行数许多,能够分为几链表个部分,第一部分快速缓存查找:appreciate
1.TLS快速缓存中只存储了一个SyncDa源码中的图片ta数据，从这链表逆置儿取出的SyncData的object和@synchronized 的参数object做比照（相同则阐明是咱们要找到的SyncData）
2.假如找链表c语言到了SyncData，对lockCount和threadCount做记载更新，直接把SyncData回来出去；
3.假如没有找到SyncData，则ios下载进入下一部分。
注意⚠️TL源码交易平台S（thread Local Store）为线程本地存储。也就是说每条线程都会有一个这样的FastCa链表数据结构che。并不是整个过程只要一个FastCache。假如在FastCache找到就直接回来。
syncCache 查找
1.遍历带锁的每个线程的缓存，取出每一个SyncCacheItem，取出Sy数据结构严蔚敏ncCacheItem里边的SyncData，SyncData的object和@synchronized 的参数objios应用商店ect做比源码交易平台照（相同链表和数组的区别则阐明是咱们要找到的SyncData）
2.假如找到了SyncData，对lockCount和threadCount做记载更新，直接APP把SyncData回来出去；
3.假如没有找到SyncData，则进入第三部分。
typedef struct {
SyncData *data;
unsigned int lockCount; // number of times THIS THREAD locked this block
} SyncCacheItem;
typedef struct SyncCache {
unsigned int allocated;
unsigned int used;
SyncCacheItem list[0];
} SyncCache;
同样是在缓存找，因为源码编辑器Syios系统ncCache里边是数组，这儿遍历查找。能够看其链表的创建间fetch_cache(NO)中的代码：
static SyncCache *fetch_cache(bool create) {
_objc_pthread_data *data;
data = _objc_fetch_pthread_data(create);
if (!data) return NULL;
if (!data->syncCache) {
if (!create) { return NULL; }
else {
int count = 4;
data->syncCache = (SyncCache *) calloc(1, sizeof(SyncCache) + count*sizeof(SyncCacheItem));
data->syncCache->allocated = count;
}
} // Make sure there's at least one open slot in the list.
if (data->syncCache->allocated == data->syncCache->used) {
data->syncCache->allocated *= 2;
data->syncCache = (SyncCache *) realloc(data->syncCache, sizeof(SyncCache) + data->syncCache->allocated * sizeof(SyncCacheItem));
}
return data->syncCache;
}
其间_o链表的创建bjc_pthread_approachdata结构如下:
typedef struct {
struct _objc_initializing_classes *initializingClasses; // for +initialize
struct SyncCache *syncCache; // for @synchronize
struct alt_handler_list *handlerList; // for exception alt handlers
char *printableNames[4]; // temporary demangled names for logging
const char **classNameLookups; // for objc_getClass() hooks
unsigned classNameLookupsAllocated;
unsigned classNameLookupsUsed;
// If you add new fields here, don't forget to update
// _objc_pthread_destroyspecific()
} _objc_pthread_data;
这儿也进一步阐明晰TLS，syncCache也是每个线程中都存在一份的。
很明显线程缓存保存了很多的SyncData+lockCount。
sDataLists 查找 或 创立SyncData
假如在快速缓存和缓存里边都没有找到，这时分是这个线程第一次走到@synchronized的当地，系统会去sDataLists里边去找对应的SyncData目标.sDataLists是大局Hash表,在id2data函数一开头就获取了大局Hash表的元素
// Use multiple parallel lists to decrease contention among unrelated objects.
#define LOCK_FOR_OBJ(obj) sDataLists[obj].lock
#define LIST_FOR_OBJ(obj) sDataLists[obj].data
static StripedMap<SyncList> sDataLists;
template<typename T>
class StripedMap {
#if TARGET_OS_IPHONE && !TARGET_OS_SIMULATOR
enum { StripeCount = 8 };
#else
enum { StripeCount = 64 };
#endif
struct PaddedT {
T value alignas(CacheLineSize);
};
PaddedT array[StripeCount];
......
}
struct SyncList {
SyncData *data;
spinlock_t lock;
constexpr SyncList() : data(nil), lock(fork_unsafe_lock) { }
};
sDataLists是StripedMap类型,Strios16ipedMap存储的是 真机下8张表/模拟器下64张表，每张表里存储的是许多的SyncList = SyncData单向链表 + lock。这儿也阐明晰源码一个问题,为什么@siOSynchronized在模拟器中的性能会很差,因为在模拟链表数据结构器中会从64张表中去查找锁,而真机是从8张表中查找锁.
1.遍历大局Hash表StripedMap，取出的SyncData单向链表的object和@synchronized 的参数object做比照（相同则阐明ios16是咱们要找到的SyncappointmentData源码精灵永久兑换码），假如比照不是同一个，会找链表下一个元素比照。
2.当前没有与object相关的SyncData，则直接回来nil
3.找到一个没用过的SyncData，就对其缓存到TLS快速缓存和线程缓存，并回来这个SyncDa数据结构知识点总结ta
4.假如是TLS快速缓存和线程缓ios是苹果还是安卓存和大局Hash表StripedMap都没有找到，阐明object被第一次加源码网站锁，去创立一个SyncData回来它。
缓存到线程中
在sDataLists 查找 及 创立SyncData后会调用done,这儿只会在Enter的时分履行，假如支撑快速缓存而且快速缓存里边没有值，那么在快速缓存里边去添加，便利下次递归的时分来加锁。不然ios系统就在线程iOS缓存里边添加。
id2da源码编辑器下载ta在找锁的过程中运用了类似三级缓存的流程，这样的目的是为了在多线程中办理锁，而且让线程以最快的速度拿到锁，来完结加锁解锁的操作，然后提升功率。appetite
总结
- 在快速缓存中没有找到
- 在线程缓存中也没有找到
- 在大局的sDataLists中也没有找到
- 那就新建一个SyncData
