Android pthread mutex 实现分析
在Android ICS中，pthead库对应的路径为：
Android\bionic\libc\bionic\pthread.c
Android\bionic\libc\bionic\pthread-atfork.c
Android\bionic\libc\bionic\pthread-rwlocks.c
Android\bionic\libc\bionic\pthread-timers.c
Android\bionic\libc\bionic\pthread_internal.h
Android\bionic\libc\include\pthread.h
其中mutex在pthread.c中，相关的API有：
//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);
pthread_mutex_t对应一个结构体，包含一个int字段
typedef struct{ int volatile value;} pthread_mutex_t;
int字段分为5个部分，
/*
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)
*/
定义了一些macro来获得各个部分，其中mutex的type分为3类，有Normal, Recursive, Errorcheck，Recursize允许递归，Errorcheck会检测当前的状态。
#define MUTEX_OWNER(m) (((m)->value >> 16) & 0xffff)
#define MUTEX_COUNTER(m) (((m)->value >> 2) & 0xfff)
#define MUTEX_TYPE_MASK 0xc000
#define MUTEX_TYPE_NORMAL 0x0000
#define MUTEX_TYPE_RECURSIVE 0x4000
#define MUTEX_TYPE_ERRORCHECK 0x8000
#define MUTEX_COUNTER_SHIFT 2
#define MUTEX_COUNTER_MASK 0x1ffc
#define MUTEX_SHARED_MASK 0x2000
enum {
PTHREAD_MUTEX_NORMAL = 0,
PTHREAD_MUTEX_RECURSIVE = 1,
PTHREAD_MUTEX_ERRORCHECK = 2,
PTHREAD_MUTEX_ERRORCHECK_NP = PTHREAD_MUTEX_ERRORCHECK,
PTHREAD_MUTEX_RECURSIVE_NP = PTHREAD_MUTEX_RECURSIVE,
PTHREAD_MUTEX_DEFAULT = PTHREAD_MUTEX_NORMAL
};
1 Mutex属性
pthread_mutexattr_t用来描述mutex的属性，
typedef long pthread_mutexattr_t;
属性包含了mutex的type，以及sharedflag。对应于mutex_t
* 15-14 type mutex type
* 13 shared process-shared flag
/* a mutex attribute holds the following fields
*
* bits: name description
* 0-3 type type of mutex
* 4 shared process-shared flag
*/
#define MUTEXATTR_TYPE_MASK 0x000f
#define MUTEXATTR_SHARED_MASK 0x0010
#define PTHREAD_PROCESS_PRIVATE 0
#define PTHREAD_PROCESS_SHARED 1
1.1 init和destory
init将type设置为Default，也就是Normal（0）也就是，type=Normal， shared 为Private
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_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_NORMAL;
break;
case PTHREAD_MUTEX_RECURSIVE:
value |= MUTEX_TYPE_RECURSIVE;
break;
case PTHREAD_MUTEX_ERRORCHECK:
value |= MUTEX_TYPE_ERRORCHECK;
break;
default:
return EINVAL;
}
mutex->value = value;
return 0;
}
//destory将attr置为-1
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(mutex);
if (ret != 0)
return ret;
mutex->value = 0xdead10cc;
return 0;
}1.2 settype 和gettype
对pthread_mutexattr_t的type字段进行取值和赋值
int pthread_mutexattr_gettype(const pthread_mutexattr_t *attr, int *type)
{
if (attr) {
int atype = (*attr & MUTEXATTR_TYPE_MASK);
if (atype >= PTHREAD_MUTEX_NORMAL &&
atype <= PTHREAD_MUTEX_ERRORCHECK) {
*type = atype;
return 0;
}
}
return EINVAL;
}
int pthread_mutexattr_settype(pthread_mutexattr_t *attr, int type)
{
if (attr && type >= PTHREAD_MUTEX_NORMAL &&
type <= PTHREAD_MUTEX_ERRORCHECK ) {
*attr = (*attr & ~MUTEXATTR_TYPE_MASK) | type;
return 0;
}
return EINVAL;
}1.3 setpshared和getpshared
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;
}
int pthread_mutexattr_getpshared(pthread_mutexattr_t *attr, int *pshared)
{
if (!attr || !pshared)
return EINVAL;
*pshared = (*attr & MUTEXATTR_SHARED_MASK) ? PTHREAD_PROCESS_SHARED
: PTHREAD_PROCESS_PRIVATE;
return 0;
}2 Mutex实现
2.1 Mutex init和 destory
如果attr==NULL，则采用默认属性, type =normal, shared = private,
否则，感觉属性的type和shared进行设置
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_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_NORMAL;
break;
case PTHREAD_MUTEX_RECURSIVE:
value |= MUTEX_TYPE_RECURSIVE;
break;
case PTHREAD_MUTEX_ERRORCHECK:
value |= MUTEX_TYPE_ERRORCHECK;
break;
default:
return EINVAL;
}
mutex->value = value;
return 0;
}
如果mutex当前不被lock，则设置value为0xdead10cc
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(mutex);
if (ret != 0)
return ret;
mutex->value = 0xdead10cc;
return 0;
}
2.2 lock, unlock, trylock, timedlock
int pthread_mutex_lock(pthread_mutex_t *mutex)
{
int mtype, tid, new_lock_type, shared;
if (__unlikely(mutex == NULL))
return EINVAL;
mtype = (mutex->value & MUTEX_TYPE_MASK);
shared = (mutex->value & MUTEX_SHARED_MASK);
/* Handle normal case first */
if ( __likely(mtype == MUTEX_TYPE_NORMAL) ) {//type为normal，
_normal_lock(mutex);
return 0;
}
/* Do we already own this recursive or error-check mutex ? */
tid = __get_thread()->kernel_id;
if ( tid == MUTEX_OWNER(mutex) )//已经持有mutex，递归的情况
{
int oldv, counter;
if (mtype == MUTEX_TYPE_ERRORCHECK) {
/* trying to re-lock a mutex we already acquired */
return EDEADLK;
}
/*
* We own the mutex, but other threads are able to change
* the contents (e.g. promoting it to "contended"), so we
* need to hold the global lock.
*/
_recursive_lock();//更新counter
oldv = mutex->value;
counter = (oldv + (1 << MUTEX_COUNTER_SHIFT)) & MUTEX_COUNTER_MASK;
mutex->value = (oldv & ~MUTEX_COUNTER_MASK) | counter;
_recursive_unlock();
return 0;
}
/* We don't own the mutex, so try to get it.
*
* First, we try to change its state from 0 to 1, if this
* doesn't work, try to change it to state 2.
*/
new_lock_type = 1;
/* compute futex wait opcode and restore shared flag in mtype */
mtype |= shared;
for (;;) {
int oldv;
_recursive_lock();
oldv = mutex->value;
if (oldv == mtype) { /* uncontended released lock => 1 or 2 */
mutex->value = ((tid << 16) | mtype | new_lock_type);
} else if ((oldv & 3) == 1) { /* locked state 1 => state 2 */
oldv ^= 3;
mutex->value = oldv;
}
_recursive_unlock();
if (oldv == mtype)
break;
/*
* The lock was held, possibly contended by others. From
* now on, if we manage to acquire the lock, we have to
* assume that others are still contending for it so that
* we'll wake them when we unlock it.
*/
new_lock_type = 2;
__futex_wait_ex(&mutex->value, shared, oldv, NULL);
}
return 0;
}
normal 的情况
/*
* Lock a non-recursive mutex.
*
* As noted above, there are three states:
* 0 (unlocked, no contention)
* 1 (locked, no contention)
* 2 (locked, contention)
*
* Non-recursive mutexes don't use the thread-id or counter fields, and the
* "type" value is zero, so the only bits that will be set are the ones in
* the lock state field.
*/
static __inline__ void
_normal_lock(pthread_mutex_t* mutex)
{
/* We need to preserve the shared flag during operations */
int shared = mutex->value & MUTEX_SHARED_MASK;
/*
* The common case is an unlocked mutex, so we begin by trying to
* change the lock's state from 0 to 1. __atomic_cmpxchg() returns 0
* if it made the swap successfully. If the result is nonzero, this
* lock is already held by another thread.
*/
if (__atomic_cmpxchg(shared|0, shared|1, &mutex->value ) != 0) {
/*
* We want to go to sleep until the mutex is available, which
* requires promoting it to state 2. We need to swap in the new
* state value and then wait until somebody wakes us up.
*
* __atomic_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.
*/
while (__atomic_swap(shared|2, &mutex->value ) != (shared|0))//等待muex
__futex_wait_ex(&mutex->value, shared, shared|2, 0);
}
ANDROID_MEMBAR_FULL();
}
在recursive的情况下，需要使用mutex来包含counter，
static pthread_mutex_t __recursive_lock = PTHREAD_MUTEX_INITIALIZER;
static void
_recursive_lock(void)
{
_normal_lock(&__recursive_lock);
}
static void
_recursive_unlock(void)
{
_normal_unlock(&__recursive_lock );
}
unlock的情况
int pthread_mutex_unlock(pthread_mutex_t *mutex)
{
int mtype, tid, oldv, shared;
if (__unlikely(mutex == NULL))
return EINVAL;
mtype = (mutex->value & MUTEX_TYPE_MASK);
shared = (mutex->value & MUTEX_SHARED_MASK);
/* Handle common case first */
if (__likely(mtype == MUTEX_TYPE_NORMAL)) {//normal
_normal_unlock(mutex);
return 0;
}
/* Do we already own this recursive or error-check mutex ? */
tid = __get_thread()->kernel_id;
if ( tid != MUTEX_OWNER(mutex) )//error check的情况，错误退出
return EPERM;
/* We do, decrement counter or release the mutex if it is 0 */
_recursive_lock();
oldv = mutex->value;
if (oldv & MUTEX_COUNTER_MASK) {
mutex->value = oldv - (1 << MUTEX_COUNTER_SHIFT);
oldv = 0;
} else {
mutex->value = shared | mtype;
}
_recursive_unlock();
/* Wake one waiting thread, if any */
if ((oldv & 3) == 2) {
__futex_wake_ex(&mutex->value, shared, 1);
}
return 0;
}
/*
* Release a non-recursive mutex. The caller is responsible for determining
* that we are in fact the owner of this lock.
*/
static __inline__ void
_normal_unlock(pthread_mutex_t* mutex)
{
ANDROID_MEMBAR_FULL();
/* We need to preserve the shared flag during operations */
int shared = mutex->value & MUTEX_SHARED_MASK;
/*
* The mutex state will be 1 or (rarely) 2. We use an atomic decrement
* to release the lock. __atomic_dec() returns the previous value;
* if it wasn't 1 we have to do some additional work.
*/
if (__atomic_dec(&mutex->value) != (shared|1)) {
/*
* 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);//唤醒等待的
}
}
trylock
int pthread_mutex_trylock(pthread_mutex_t *mutex)
{
int mtype, tid, oldv, shared;
if (__unlikely(mutex == NULL))
return EINVAL;
mtype = (mutex->value & MUTEX_TYPE_MASK);
shared = (mutex->value & MUTEX_SHARED_MASK);
/* Handle common case first */
if ( __likely(mtype == MUTEX_TYPE_NORMAL) )
{
if (__atomic_cmpxchg(shared|0, shared|1, &mutex->value) == 0) {
ANDROID_MEMBAR_FULL();
return 0;
}
return EBUSY;
}
/* Do we already own this recursive or error-check mutex ? */
tid = __get_thread()->kernel_id;
if ( tid == MUTEX_OWNER(mutex) )//当前thread已经持有mutex
{
int counter;
if (mtype == MUTEX_TYPE_ERRORCHECK) {
/* already locked by ourselves */
return EDEADLK;
}
//更新counter计数器
_recursive_lock();
oldv = mutex->value;
counter = (oldv + (1 << MUTEX_COUNTER_SHIFT)) & MUTEX_COUNTER_MASK;
mutex->value = (oldv & ~MUTEX_COUNTER_MASK) | counter;
_recursive_unlock();
return 0;
}
/* Restore sharing bit in mtype */
mtype |= shared;
/* Try to lock it, just once. */
_recursive_lock();
oldv = mutex->value;
if (oldv == mtype) /* uncontended released lock => state 1 */
mutex->value = ((tid << 16) | mtype | 1);
_recursive_unlock();
if (oldv != mtype)
return EBUSY;
return 0;
}timeout
int pthread_mutex_lock_timeout_np(pthread_mutex_t *mutex, unsigned msecs)
{
clockid_t clock = CLOCK_MONOTONIC;
struct timespec abstime;
struct timespec ts;
int mtype, tid, oldv, new_lock_type, shared;
/* compute absolute expiration time */
__timespec_to_relative_msec(&abstime, msecs, clock);
if (__unlikely(mutex == NULL))
return EINVAL;
mtype = (mutex->value & MUTEX_TYPE_MASK);
shared = (mutex->value & MUTEX_SHARED_MASK);
/* Handle common case first */
if ( __likely(mtype == MUTEX_TYPE_NORMAL) )//normal的情况
{
/* fast path for uncontended lock */
if (__atomic_cmpxchg(shared|0, shared|1, &mutex->value) == 0) {
ANDROID_MEMBAR_FULL();
return 0;
}
//在while循环中等待，直到超时或者获得mutex
/* loop while needed */
while (__atomic_swap(shared|2, &mutex->value) != (shared|0)) {
if (__timespec_to_absolute(&ts, &abstime, clock) < 0)
return EBUSY;
__futex_wait_ex(&mutex->value, shared, shared|2, &ts);
}
ANDROID_MEMBAR_FULL();
return 0;
}
/* Do we already own this recursive or error-check mutex ? */
tid = __get_thread()->kernel_id;
if ( tid == MUTEX_OWNER(mutex) )
{
int oldv, counter;
if (mtype == MUTEX_TYPE_ERRORCHECK) {
/* already locked by ourselves */
return EDEADLK;
}
_recursive_lock();
oldv = mutex->value;
counter = (oldv + (1 << MUTEX_COUNTER_SHIFT)) & MUTEX_COUNTER_MASK;
mutex->value = (oldv & ~MUTEX_COUNTER_MASK) | counter;
_recursive_unlock();
return 0;
}
/* We don't own the mutex, so try to get it.
*
* First, we try to change its state from 0 to 1, if this
* doesn't work, try to change it to state 2.
*/
new_lock_type = 1;
/* Compute wait op and restore sharing bit in mtype */
mtype |= shared;
//循环等待，直到超时
for (;;) {
int oldv;
struct timespec ts;
_recursive_lock();
oldv = mutex->value;
if (oldv == mtype) { /* uncontended released lock => 1 or 2 */
mutex->value = ((tid << 16) | mtype | new_lock_type);
} else if ((oldv & 3) == 1) { /* locked state 1 => state 2 */
oldv ^= 3;
mutex->value = oldv;
}
_recursive_unlock();
if (oldv == mtype)
break;
/*
* The lock was held, possibly contended by others. From
* now on, if we manage to acquire the lock, we have to
* assume that others are still contending for it so that
* we'll wake them when we unlock it.
*/
new_lock_type = 2;
//超时退出
if (__timespec_to_absolute(&ts, &abstime, clock) < 0)
return EBUSY;
__futex_wait_ex(&mutex->value, shared, oldv, &ts);
}
return 0;
}