本文详细解析了Mutex的构造、初始化、锁定、尝试锁定及销毁的过程。通过分析Mutex的不同类型及其内部实现原理，帮助读者深入理解Mutex的工作机制。
1. 先从mutex的构造开始
/*
* Simple mutex class. The implementation is system-dependent.
*
* The mutex must be unlocked by the thread that locked it. They are not
* recursive, i.e. the same thread can't lock it multiple times.
*/
class Mutex {
public:
enum {
PRIVATE = 0,
SHARED = 1
};
Mutex();
Mutex(const char* name);
Mutex(int type, const char* name = NULL);
#if defined(HAVE_PTHREADS)
virtual
#endif
~Mutex();
// lock or unlock the mutex
status_t lock();
// gaia add +
status_t lockTimeout(unsigned msecs);
// gaia add -
void unlock();
// lock if possible; returns 0 on success, error otherwise
status_t tryLock();
// Manages the mutex automatically. It'll be locked when Autolock is
// constructed and released when Autolock goes out of scope.
class Autolock {
public:
// gaia add +
#if 1
inline Autolock(Mutex& mutex) : mLock(mutex) { mStatus = mLock.lock(); }
inline Autolock(Mutex* mutex) : mLock(*mutex) { mStatus = mLock.lock(); }
inline Autolock(Mutex& mutex, unsigned msec) : mLock(mutex), mMsec(msec) {
msec ? (mStatus = mLock.lockTimeout(msec)) : (mStatus = mLock.lock()); }
inline ~Autolock() { mLock.unlock(); }
status_t getStatus() { return mStatus; }
#else
inline Autolock(Mutex& mutex) : mLock(mutex) { mLock.lock(); }
inline Autolock(Mutex* mutex) : mLock(*mutex) { mLock.lock(); }
inline ~Autolock() { mLock.unlock(); }
#endif
// gaia add -
private:
Mutex& mLock;
// gaia add +
unsigned mMsec;
status_t mStatus;
// gaia add -
};
private:
friend class Condition;
// GAIA ADD for RecursiveMutex
friend class RecursiveMutex;
// A mutex cannot be copied
Mutex(const Mutex&);
Mutex& operator = (const Mutex&);
#if defined(HAVE_PTHREADS)
pthread_mutex_t mMutex;
//GAIA ADD
//protected:
Mutex(bool recursive, int type = 0, const char* name = NULL);
#else
void _init();
void* mState;
#endif
};这是Mutex的头文件定义， 根据系统不同而不同。
#if defined(HAVE_PTHREADS)
inline Mutex::Mutex() {
pthread_mutex_init(&mMutex, NULL);
}
inline Mutex::Mutex(const char* name) {
pthread_mutex_init(&mMutex, NULL);
}
inline Mutex::Mutex(int type, const char* name) {
if (type == SHARED) {
pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
pthread_mutex_init(&mMutex, &attr);
pthread_mutexattr_destroy(&attr);
} else {
pthread_mutex_init(&mMutex, NULL);
}
}
//GAIA ADD
#if defined(HAVE_PTHREADS)
inline Mutex::Mutex(bool recursive, int type, const char* name) {
(void)name;
pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
if (type == SHARED) {
pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
}
if (recursive) {
pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
}
pthread_mutex_init(&mMutex, &attr);
pthread_mutexattr_destroy(&attr);}
#endif
inline Mutex::~Mutex() {
pthread_mutex_destroy(&mMutex);
}
inline status_t Mutex::lock() {
return -pthread_mutex_lock(&mMutex);
}
// gaia add +
inline status_t Mutex::lockTimeout(unsigned msec) {
#if HOST_BUILD
(void)msec;
return -pthread_mutex_lock(&mMutex);
#else
return -pthread_mutex_lock_timeout_np(&mMutex, msec);
#endif
}
// gaia add -
inline void Mutex::unlock() {
pthread_mutex_unlock(&mMutex);
}
inline status_t Mutex::tryLock() {
return -pthread_mutex_trylock(&mMutex);
}
#endif // HAVE_PTHREADS这段代码不复杂，粗看基本能看懂。
那么接下来详细介绍。
从构造函数入手：
Mutex();
Mutex(const char* name);
Mutex(int type, const char* name = NULL);inline Mutex::Mutex(int type, const char* name) {
if (type == SHARED) {
pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
pthread_mutex_init(&mMutex, &attr);
pthread_mutexattr_destroy(&attr);
} else {
pthread_mutex_init(&mMutex, NULL);
}
}
enum {
PRIVATE = 0,
SHARED = 1
};
Mutex调用的是Bionic中的pthread:bionic/libc/bionic/pthread.c.
那么接下来深入到pthread中。
pthread针对不同Mutex设置了不同的type
/* Mutex types. */
enum
{
PTHREAD_MUTEX_TIMED_NP,
PTHREAD_MUTEX_RECURSIVE_NP,
PTHREAD_MUTEX_ERRORCHECK_NP,
PTHREAD_MUTEX_ADAPTIVE_NP
#if defined __USE_UNIX98 || defined __USE_XOPEN2K8
,
PTHREAD_MUTEX_NORMAL = PTHREAD_MUTEX_TIMED_NP,
PTHREAD_MUTEX_RECURSIVE = PTHREAD_MUTEX_RECURSIVE_NP,
PTHREAD_MUTEX_ERRORCHECK = PTHREAD_MUTEX_ERRORCHECK_NP,
PTHREAD_MUTEX_DEFAULT = PTHREAD_MUTEX_NORMAL
#endif
#ifdef __USE_GNU
/* For compatibility. */
, PTHREAD_MUTEX_FAST_NP = PTHREAD_MUTEX_TIMED_NP
#endif
};相关的API罗列如下：
//pthread mutexattr 操作
int pthread_mutexattr_init(pthread_mutexattr_t *attr);
int pthread_mutexattr_destroy(pthread_mutexattr_t *attr);
int pthread_mutexattr_gettype(const pthread_mutexattr_t *attr, int *type);
int pthread_mutexattr_settype(pthread_mutexattr_t *attr, int type);
int pthread_mutexattr_setpshared(pthread_mutexattr_t *attr, int pshared);
int pthread_mutexattr_getpshared(pthread_mutexattr_t *attr, int *pshared);
//pthread mutex 操作
int pthread_mutex_init(pthread_mutex_t *mutex,
const pthread_mutexattr_t *attr);
int pthread_mutex_destroy(pthread_mutex_t *mutex);
int pthread_mutex_lock(pthread_mutex_t *mutex);
int pthread_mutex_unlock(pthread_mutex_t *mutex);
int pthread_mutex_trylock(pthread_mutex_t *mutex);
int pthread_mutex_timedlock(pthread_mutex_t *mutex, struct timespec* ts);
基础结构
pthread_mutexattr_t，
pthread_mutex_t
typedef long pthread_mutexattr_t;
a mutex attribute holds the following fields
bits: name description
0-3 type type of mutex
4 shared process-shared flag
typedef struct
{
int volatile value;
} pthread_mutex_t;
a mutex is implemented as a 32-bit integer holding the following fields
bits: name description
31-16 tid owner thread's kernel id (recursive and errorcheck only)
15-14 type mutex type
13 shared process-shared flag
12-2 counter counter of recursive mutexes
1-0 state lock state (0, 1 or 2)
先把这个解释下，在后面会用到的：/
//tid, 表示拥有这个mutex的线程kernel id
//type 表示mutex的类型 /normal : recursive: error
//process-shared flag, 是否共享锁，/private : shared
//counter主要用于递归mutex；
//state 表示锁的状态；
先解析如下这段代码：
pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
pthread_mutex_init(&mMutex, &attr);
pthread_mutexattr_destroy(&attr);锁的初始化分两步：
a. 初始化mutexattr;
pthread_mutexattr_t用来描述mutex的属性:
a mutex attribute holds the following fields
*
* bits: name description
* 0-3 type type of mutex
* 4 shared process-shared flag
对应的掩码：
#define MUTEXATTR_TYPE_MASK 0x000f
#define MUTEXATTR_SHARED_MASK 0x0010
shared： Flage为
#define PTHREAD_PROCESS_PRIVATE 0
#define PTHREAD_PROCESS_SHARED 1
pthread_mutexattr_t对应于pthread_mutex_t
* 15-14 type mutex type
* 13 shared process-shared flag
a. 初始化attr源码
int pthread_mutexattr_init(pthread_mutexattr_t *attr)
{
if (attr) {
*attr = PTHREAD_MUTEX_DEFAULT;
return 0;
} else {
return EINVAL;
}
}
PTHREAD_MUTEX_DEFAULT其实就是前面的
PTHREAD_MUTEX_DEFAULT = PTHREAD_MUTEX_NORMAL
接下来设置flag:
int pthread_mutexattr_setpshared(pthread_mutexattr_t *attr, int pshared)
{
if (!attr)
return EINVAL;
switch (pshared) {
case PTHREAD_PROCESS_PRIVATE:
*attr &= ~MUTEXATTR_SHARED_MASK;
return 0;
case PTHREAD_PROCESS_SHARED:
/* our current implementation of pthread actually supports shared
* mutexes but won't cleanup if a process dies with the mutex held.
* Nevertheless, it's better than nothing. Shared mutexes are used
* by surfaceflinger and audioflinger.
*/
*attr |= MUTEXATTR_SHARED_MASK;
return 0;
}
return EINVAL;
}b. 初始化mutex;
#define MUTEX_TYPE_SHIFT 14
#define MUTEX_TYPE_LEN 2
#define MUTEX_TYPE_MASK FIELD_MASK(MUTEX_TYPE_SHIFT,MUTEX_TYPE_LEN)
#define MUTEX_TYPE_NORMAL 0 /* Must be 0 to match __PTHREAD_MUTEX_INIT_VALUE */
#define MUTEX_TYPE_RECURSIVE 1
#define MUTEX_TYPE_ERRORCHECK 2
#define MUTEX_TYPE_TO_BITS(t) FIELD_TO_BITS(t, MUTEX_TYPE_SHIFT, MUTEX_TYPE_LEN)
#define MUTEX_TYPE_BITS_NORMAL MUTEX_TYPE_TO_BITS(MUTEX_TYPE_NORMAL)
#define MUTEX_TYPE_BITS_RECURSIVE MUTEX_TYPE_TO_BITS(MUTEX_TYPE_RECURSIVE)
#define MUTEX_TYPE_BITS_ERRORCHECK MUTEX_TYPE_TO_BITS(MUTEX_TYPE_ERRORCHECK)
int pthread_mutex_init(pthread_mutex_t *mutex,
const pthread_mutexattr_t *attr)
{
int value = 0;
if (mutex == NULL)
return EINVAL;
if (__likely(attr == NULL)) {
mutex->value = MUTEX_TYPE_BITS_NORMAL;
return 0;
}
if ((*attr & MUTEXATTR_SHARED_MASK) != 0)
value |= MUTEX_SHARED_MASK;
switch (*attr & MUTEXATTR_TYPE_MASK) {
case PTHREAD_MUTEX_NORMAL:
value |= MUTEX_TYPE_BITS_NORMAL;//设置相应位置
break;
case PTHREAD_MUTEX_RECURSIVE:
value |= MUTEX_TYPE_BITS_RECURSIVE;
break;
case PTHREAD_MUTEX_ERRORCHECK:
value |= MUTEX_TYPE_BITS_ERRORCHECK;
break;
default:
return EINVAL;
}
mutex->value = value;
return 0;
}
那么Mutex就创建好了。inline status_t Mutex::lock() {
return -pthread_mutex_lock(&mMutex);
}
进入pthread
int pthread_mutex_lock(pthread_mutex_t *mutex) {
if (!__g_enable_deadlock_detection) {
int err = pthread_mutex_lock_impl(mutex);
return err;
}
int64_t startTime = uptime_millis();
int err = pthread_mutex_trylock_impl(mutex);
int wait_tid = gettid();
if (err) {
// two cases:
// 1. mutex hold by other thread
// 2. normal mutex acquired again
int mvalue = mutex->value;
int mtype = mvalue & MUTEX_TYPE_MASK;//设置type
int tid = MUTEX_OWNER_FROM_BITS(mvalue);
#if NONRECURSIVE_REENTRANT_DETECT
if (mtype == MUTEX_TYPE_BITS_NORMAL) {
if (tid != 0 && wait_tid == tid) { // 2
__dump_me("nonrecursive-mutex-lock-twice", tid, tid);
}
}
#endif
#if MUTEX_WARNING_ON
int i = 0;
for (i = 0; i < DEADLOCK_TIMEOUT/DEADLOCK_WARNING_PERIOD; ++i) {
err = pthread_mutex_lock_timeout_np_impl(mutex, DEADLOCK_WARNING_PERIOD);
if (!err)
break;
mvalue = mutex->value;
tid = MUTEX_OWNER_FROM_BITS(mvalue);
__show_mutex_info(mutex, uptime_millis() - startTime, tid);
}
#else
err = pthread_mutex_lock_timeout_np_impl(mutex, DEADLOCK_TIMEOUT);
#endif
if (err) { // 1
__dump_me("mutex-acquire-lock-timeout", tid, wait_tid);
} else {
while (1) {
int newval = mvalue;
newval |= (mvalue & FIELD_MASK(0, MUTEX_OWNER_SHIFT)) | MUTEX_OWNER_TO_BITS(wait_tid);
if (__unlikely(__bionic_cmpxchg(mvalue, newval, &mutex->value) != 0)) {
mvalue = mutex->value;
continue;
}
ANDROID_MEMBAR_FULL();
return 0;
}
}
} else {
int mvalue = mutex->value;
int mtype = mutex->value & MUTEX_TYPE_MASK;
if (mtype == MUTEX_TYPE_BITS_NORMAL) {
while (1) {
int newval = mvalue;
newval |= (mvalue & FIELD_MASK(0, MUTEX_OWNER_SHIFT)) | MUTEX_OWNER_TO_BITS(wait_tid);
//表示本线程已经持有了Mutex,那么更新其tid
if (__unlikely(__bionic_cmpxchg(mvalue, newval, &mutex->value) != 0)) {
mvalue = mutex->value;
continue;
}
ANDROID_MEMBAR_FULL();
return 0;
}
}
}
return err;
}
__LIBC_HIDDEN__
int pthread_mutex_lock_impl(pthread_mutex_t *mutex)
{
int mvalue, mtype, tid, new_lock_type, shared;
if (__unlikely(mutex == NULL))
return EINVAL;
mvalue = mutex->value;
mtype = (mvalue & MUTEX_TYPE_MASK);
shared = (mvalue & MUTEX_SHARED_MASK);
/* Handle normal case first */
//情况1
if ( __likely(mtype == MUTEX_TYPE_BITS_NORMAL) ) {//normal type调用
_normal_lock(mutex, shared);
return 0;
}
//情况2
/* Do we already own this recursive or error-check mutex ? */
tid = __get_thread()->kernel_id;
if ( tid == MUTEX_OWNER_FROM_BITS(mvalue) )
return _recursive_increment(mutex, mvalue, mtype);
/* Add in shared state to avoid extra 'or' operations below */
mtype |= shared;
//情况3
/* First, if the mutex is unlocked, try to quickly acquire it.
* In the optimistic case where this works, set the state to 1 to
* indicate locked with no contention */
if (mvalue == mtype) {
int newval = MUTEX_OWNER_TO_BITS(tid) | mtype | MUTEX_STATE_BITS_LOCKED_UNCONTENDED;
if (__bionic_cmpxchg(mvalue, newval, &mutex->value) == 0) {
ANDROID_MEMBAR_FULL();
return 0;
}
/* argh, the value changed, reload before entering the loop */
mvalue = mutex->value;
}
//情况4
for (;;) {
int newval;
/* if the mutex is unlocked, its value should be 'mtype' and
* we try to acquire it by setting its owner and state atomically.
* NOTE: We put the state to 2 since we _know_ there is contention
* when we are in this loop. This ensures all waiters will be
* unlocked.
*/
if (mvalue == mtype) {
newval = MUTEX_OWNER_TO_BITS(tid) | mtype | MUTEX_STATE_BITS_LOCKED_CONTENDED;
/* TODO: Change this to __bionic_cmpxchg_acquire when we
* implement it to get rid of the explicit memory
* barrier below.
*/
if (__unlikely(__bionic_cmpxchg(mvalue, newval, &mutex->value) != 0)) {
mvalue = mutex->value;
continue;
}
ANDROID_MEMBAR_FULL();
return 0;
}
//情况5
/* the mutex is already locked by another thread, if its state is 1
* we will change it to 2 to indicate contention. */
if (MUTEX_STATE_BITS_IS_LOCKED_UNCONTENDED(mvalue)) {
newval = MUTEX_STATE_BITS_FLIP_CONTENTION(mvalue); /* locked state 1 => state 2 */
if (__unlikely(__bionic_cmpxchg(mvalue, newval, &mutex->value) != 0)) {
mvalue = mutex->value;
continue;
}
mvalue = newval;
}
/* wait until the mutex is unlocked */
__futex_wait_ex(&mutex->value, shared, mvalue, NULL);
mvalue = mutex->value;
}
/* NOTREACHED */
}情况1：Normal。调用_normal_lock
static __inline__ void
_normal_lock(pthread_mutex_t* mutex, int shared)
{
/* convenience shortcuts */
const int unlocked = shared | MUTEX_STATE_BITS_UNLOCKED;
const int locked_uncontended = shared | MUTEX_STATE_BITS_LOCKED_UNCONTENDED;
取得locked状态，
//锁状态宏如下：
下面是锁的三种状态：
无锁
非竞争锁
竞争锁
后面两中情况应该是这样理解，如果这个锁无持有这，无锁状态，那么当第一个持有这，持有它的时候，将它设为uncontended状态
如果非本线程持有，不管是否获取到了该锁，都将它设置为contended状态，表示这个锁是几个线程竞争的（待验证）
//#define MUTEX_STATE_BITS_UNLOCKED MUTEX_STATE_TO_BITS(MUTEX_STATE_UNLOCKED)
//#define MUTEX_STATE_BITS_LOCKED_UNCONTENDED MUTEX_STATE_TO_BITS(MUTEX_STATE_LOCKED_UNCONTENDED)
//#define MUTEX_STATE_BITS_LOCKED_CONTENDED MUTEX_STATE_TO_BITS(MUTEX_STATE_LOCKED_CONTENDED)
/* * The common case is an unlocked mutex, so we begin by trying to * change the lock's state from 0 (UNLOCKED) to 1 (LOCKED). * __bionic_cmpxchg() returns 0 if it made the swap successfully. * If the result is nonzero, this lock is already held by another thread.
翻译为：
尝试去将lock状态从unlocked转换为locked，如果转换成功则返回0
其实际就是将Mutex->value与unlocked比较，如果两个值相同，那么将value设置为locked_uncontended
*/ if (__bionic_cmpxchg(unlocked, locked_uncontended, &mutex->value) != 0) { const int locked_contended = shared | MUTEX_STATE_BITS_LOCKED_CONTENDED;
/*返回非0，说明mutex->value不是unlocked状态，那么应该被其他线程占用着
* We want to go to sleep until the mutex is available, which * requires promoting it to state 2 (CONTENDED). We need to * swap in the new state value and then wait until somebody wakes us up. * * __bionic_swap() returns the previous value. We swap 2 in and * see if we got zero back; if so, we have acquired the lock. If * not, another thread still holds the lock and we wait again. * * The second argument to the __futex_wait() call is compared * against the current value. If it doesn't match, __futex_wait() * returns immediately (otherwise, it sleeps for a time specified * by the third argument; 0 means sleep forever). This ensures * that the mutex is in state 2 when we go to sleep on it, which * guarantees a wake-up call. */
//循环等待其他线程释放这个mutex
while (__bionic_swap(locked_contended, &mutex->value) != unlocked) __futex_wait_ex(&mutex->value, shared, locked_contended, 0); } ANDROID_MEMBAR_FULL();}
__ATOMIC_INLINE__ int
__bionic_cmpxchg(int32_t old_value, int32_t new_value, volatile int32_t* ptr)
{
/* We must return 0 on success */
return __sync_bool_compare_and_swap(ptr, old_value, new_value) == 0;
//也就是如果*ptr==old_value，那么将*ptr设置为new_value，返回true;否则返回false;
}
__ATOMIC_INLINE__ int32_t
__bionic_swap(int32_t new_value, volatile int32_t* ptr)
{
int32_t prev;
do {
prev = *ptr;
status = __sync_val_compare_and_swap(ptr, prev, new_value);
} while (status == 0);//如果prev == ptr那么将new_value设置到ptr, 返回prev;
return prev;
}static __inline__ __attribute__((always_inline))
int _recursive_increment(pthread_mutex_t* mutex, int mvalue, int mtype)
{
if (mtype == MUTEX_TYPE_BITS_ERRORCHECK) {
/* trying to re-lock a mutex we already acquired */
return EDEADLK;
}
/* Detect recursive lock overflow and return EAGAIN.
* This is safe because only the owner thread can modify the
* counter bits in the mutex value. */
if (MUTEX_COUNTER_BITS_WILL_OVERFLOW(mvalue)) {
return EAGAIN;
}
/* We own the mutex, but other threads are able to change
* the lower bits (e.g. promoting it to "contended"), so we
* need to use an atomic cmpxchg loop to update the counter.
*/
for (;;) {
/* increment counter, overflow was already checked */
int newval = mvalue + MUTEX_COUNTER_BITS_ONE;
if (__likely(__bionic_cmpxchg(mvalue, newval, &mutex->value) == 0)) {
/* mutex is still locked, not need for a memory barrier */
return 0;
}
/* the value was changed, this happens when another thread changes
* the lower state bits from 1 to 2 to indicate contention. This
* cannot change the counter, so simply reload and try again.
*/
mvalue = mutex->value;
}
}
情况3： 没怎么弄懂，应该是一种优化的情况。
mtype |= shared;
/* First, if the mutex is unlocked, try to quickly acquire it.
* In the optimistic case where this works, set the state to 1 to
* indicate locked with no contention
*///下面的值：tid拥有的mutex value|mtype|locke_uncontended竞争锁，其实也就是表示如果
mvalue除了type和shared字段，其他字段都是空的，即value == mtype|shared,那么表示这个mutex没有任何线程获得，
那么立即将它设置为1，表示其为非竞争锁
if (mvalue == mtype) { int newval = MUTEX_OWNER_TO_BITS(tid) | mtype | MUTEX_STATE_BITS_LOCKED_UNCONTENDED;
if (__bionic_cmpxchg(mvalue, newval, &mutex->value) == 0) { ANDROID_MEMBAR_FULL(); return 0; } /* argh, the value changed, reload before entering the loop */
mvalue = mutex->value; }
* bits: name description
* 31-16 tid owner thread's kernel id (recursive and errorcheck only)
* 15-14 type mutex type
* 13 shared process-shared flag
* 12-2 counter counter of recursive mutexes
* 1-0 state lock state (0, 1 or 2)
则可能如下情况： a. tid!=0 b. counter!=0; c. state!=0; 还是回到源码：
for (;;) {
int newval;
/* if the mutex is unlocked, its value should be 'mtype' and
* we try to acquire it by setting its owner and state atomically.
* NOTE: We put the state to 2 since we _know_ there is contention
* when we are in this loop. This ensures all waiters will be
* unlocked.
*/
if (mvalue == mtype) {//之前的情况，其他都是0，mvalue == mtype|shared ，但是之前之所以失败，肯定其不是uncontended状态，因此现在将它设置为contended 状态
newval = MUTEX_OWNER_TO_BITS(tid) | mtype | MUTEX_STATE_BITS_LOCKED_CONTENDED;
/* TODO: Change this to __bionic_cmpxchg_acquire when we
* implement it to get rid of the explicit memory
* barrier below.
*/
if (__unlikely(__bionic_cmpxchg(mvalue, newval, &mutex->value) != 0)) {
mvalue = mutex->value;
continue;
}
ANDROID_MEMBAR_FULL();
return 0;
}
/* the mutex is already locked by another thread, if its state is 1
* we will change it to 2 to indicate contention. */表示被其他线程持有，那么将其设置为contended，表示这个锁存在竞争了。
if (MUTEX_STATE_BITS_IS_LOCKED_UNCONTENDED(mvalue)) {
newval = MUTEX_STATE_BITS_FLIP_CONTENTION(mvalue); /* locked state 1 => state 2 */
if (__unlikely(__bionic_cmpxchg(mvalue, newval, &mutex->value) != 0)) {
mvalue = mutex->value;
continue;
}
mvalue = newval;
}
/* wait until the mutex is unlocked */
__futex_wait_ex(&mutex->value, shared, mvalue, NULL);
mvalue = mutex->value;
}
/* NOTREACHED */
__futex_wait_ex相关的东西 需要以后好好研究。应该持续等待其他线程唤醒，对应的是
__futex_wake_ex__LIBC_HIDDEN__
int pthread_mutex_unlock_impl(pthread_mutex_t *mutex)
{
int mvalue, mtype, tid, oldv, shared;
if (__unlikely(mutex == NULL))
return EINVAL;
mvalue = mutex->value;
mtype = (mvalue & MUTEX_TYPE_MASK);
shared = (mvalue & MUTEX_SHARED_MASK);
//情况1.
/* Handle common case first */
if (__likely(mtype == MUTEX_TYPE_BITS_NORMAL)) {
_normal_unlock(mutex, shared);
return 0;
}
//情况2.
/* Do we already own this recursive or error-check mutex ? */ tid = __get_thread()->kernel_id; if ( tid != MUTEX_OWNER_FROM_BITS(mvalue) ) return EPERM;
//情况3.
/* If the counter is > 0, we can simply decrement it atomically. * Since other threads can mutate the lower state bits (and only the * lower state bits), use a cmpxchg to do it. */ if (!MUTEX_COUNTER_BITS_IS_ZERO(mvalue)) { for (;;) { int newval = mvalue - MUTEX_COUNTER_BITS_ONE; if (__likely(__bionic_cmpxchg(mvalue, newval, &mutex->value) == 0)) { /* success: we still own the mutex, so no memory barrier */ return 0; } /* the value changed, so reload and loop */ mvalue = mutex->value; } } /* the counter is 0, so we're going to unlock the mutex by resetting * its value to 'unlocked'. We need to perform a swap in order * to read the current state, which will be 2 if there are waiters * to awake. * * TODO: Change this to __bionic_swap_release when we implement it * to get rid of the explicit memory barrier below. */
//情况4.
ANDROID_MEMBAR_FULL(); /* RELEASE BARRIER */ mvalue = __bionic_swap(mtype | shared | MUTEX_STATE_BITS_UNLOCKED, &mutex->value); /* Wake one waiting thread, if any */ if (MUTEX_STATE_BITS_IS_LOCKED_CONTENDED(mvalue)) { __futex_wake_ex(&mutex->value, shared, 1); } return 0;}
情况1：表示normal type进入到_normal_unlock
_normal_unlock(pthread_mutex_t* mutex, int shared)
{
ANDROID_MEMBAR_FULL();
/*
* The mutex state will be 1 or (rarely) 2. We use an atomic decrement
* to release the lock. __bionic_atomic_dec() returns the previous value;
* if it wasn't 1 we have to do some additional work.
*///理解为下：
bionic_atomic_dec调用的__sync_fetch_and_add，是先获取state，然后-1,那么如果当前状态不是uncontended，则是contended，即需要唤醒其他等待的线程；
如果状态时uncontende,-1之后变为了0，表示为unlock状态。
if (__bionic_atomic_dec(&mutex->value) != (shared|MUTEX_STATE_BITS_LOCKED_UNCONTENDED)) { /* * Start by releasing the lock. The decrement changed it from * "contended lock" to "uncontended lock", which means we still * hold it, and anybody who tries to sneak in will push it back * to state 2. * * Once we set it to zero the lock is up for grabs. We follow * this with a __futex_wake() to ensure that one of the waiting * threads has a chance to grab it. * * This doesn't cause a race with the swap/wait pair in * _normal_lock(), because the __futex_wait() call there will * return immediately if the mutex value isn't 2. */ mutex->value = shared; /* * Wake up one waiting thread. We don't know which thread will be * woken or when it'll start executing -- futexes make no guarantees * here. There may not even be a thread waiting. * * The newly-woken thread will replace the 0 we just set above * with 2, which means that when it eventually releases the mutex * it will also call FUTEX_WAKE. This results in one extra wake * call whenever a lock is contended, but lets us avoid forgetting * anyone without requiring us to track the number of sleepers. * * It's possible for another thread to sneak in and grab the lock * between the zero assignment above and the wake call below. If * the new thread is "slow" and holds the lock for a while, we'll * wake up a sleeper, which will swap in a 2 and then go back to * sleep since the lock is still held. If the new thread is "fast", * running to completion before we call wake, the thread we * eventually wake will find an unlocked mutex and will execute. * Either way we have correct behavior and nobody is orphaned on * the wait queue. */ __futex_wake_ex(&mutex->value, shared, 1); }}
情况2：既然不满足1情况，同时mutex非本线程持有，因此将其作为error处理；情况3：递归mutex
if (!MUTEX_COUNTER_BITS_IS_ZERO(mvalue)) {//递归锁，表示计数非0，那么只是减去1
for (;;) {
int newval = mvalue - MUTEX_COUNTER_BITS_ONE;//将其对应位减去1
if (__likely(__bionic_cmpxchg(mvalue, newval, &mutex->value) == 0)) {
/* success: we still own the mutex, so no memory barrier */
return 0;
}
/* the value changed, so reload and loop */
mvalue = mutex->value;
}
} /* the counter is 0, so we're going to unlock the mutex by resetting
* its value to 'unlocked'. We need to perform a swap in order
* to read the current state, which will be 2 if there are waiters
* to awake.
*
* TODO: Change this to __bionic_swap_release when we implement it
* to get rid of the explicit memory barrier below.
*/
ANDROID_MEMBAR_FULL(); /* RELEASE BARRIER */
mvalue = __bionic_swap(mtype | shared | MUTEX_STATE_BITS_UNLOCKED, &mutex->value);
/* Wake one waiting thread, if any */
if (MUTEX_STATE_BITS_IS_LOCKED_CONTENDED(mvalue)) {
__futex_wake_ex(&mutex->value, shared, 1);
}
//将state设置为unlocked，然后判定mvalue是什么状态，如果是contended状态，那么唤醒其他的线程。
return 0;
__ATOMIC_INLINE__ int32_t
__bionic_swap(int32_t new_value, volatile int32_t* ptr)
{
int32_t prev;
do {
prev = *ptr;
status = __sync_val_compare_and_swap(ptr, prev, new_value);
} while (status == 0);
return prev; 就是交换之后，返回先前的值
}
int pthread_mutex_trylock_impl(pthread_mutex_t *mutex)
{
int mvalue, mtype, tid, oldv, shared;
if (__unlikely(mutex == NULL))
return EINVAL;
mvalue = mutex->value;
mtype = (mvalue & MUTEX_TYPE_MASK);
shared = (mvalue & MUTEX_SHARED_MASK);
//情况1：
/* Handle common case first */
if ( __likely(mtype == MUTEX_TYPE_BITS_NORMAL) )
{
if (__bionic_cmpxchg(shared|MUTEX_STATE_BITS_UNLOCKED,
shared|MUTEX_STATE_BITS_LOCKED_UNCONTENDED,
&mutex->value) == 0) {
ANDROID_MEMBAR_FULL();
return 0;
}
return EBUSY;
}
//情况2：
/* Do we already own this recursive or error-check mutex ? */ tid = __get_thread()->kernel_id; if ( tid == MUTEX_OWNER_FROM_BITS(mvalue) ) return _recursive_increment(mutex, mvalue, mtype);
//情况3：
/* Same as pthread_mutex_lock, except that we don't want to wait, and * the only operation that can succeed is a single cmpxchg to acquire the * lock if it is released / not owned by anyone. No need for a complex loop. */
mtype |= shared | MUTEX_STATE_BITS_UNLOCKED;
//假设其处于unlocked状态，那么设置为tid|mtype|UNcontended。
mvalue = MUTEX_OWNER_TO_BITS(tid) | mtype | MUTEX_STATE_BITS_LOCKED_UNCONTENDED;
if (__likely(__bionic_cmpxchg(mtype, mvalue, &mutex->value) == 0)) {
ANDROID_MEMBAR_FULL();
return 0;
}
return EBUSY;
}
情况1：NormalType 要么获取到了，要么返回ebusy
情况2：表示本线程已经持有，那么+1就ok
情况3：如果处于unlocked状态，那么果断的将其设置为uncontende，如果失败返回ebusy.
5. destory
int pthread_mutex_destroy(pthread_mutex_t *mutex)
{
int ret;
/* use trylock to ensure that the mutex value is
* valid and is not already locked. */
ret = pthread_mutex_trylock_impl(mutex);
if (ret != 0)
return ret;
mutex->value = 0xdead10cc;
return 0;
}
进行trylock会发现没有被锁，那么就可以释放。设置为
0xdead10cc
这有什么特殊含义？貌似就是告诉你 dead lock
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