cam_semaphore

 

1. 什么是忙碌等待？

　　在软件工程中，忙碌等待（也称自旋；英语：Busy waiting、busy-looping、spinning）是一种以进程反复检查一个条件是否为真为根本的技术，条件可能为键盘输入或某个锁是否可用。忙碌等待也可以用来产生一个任意的时间延迟，若系统没有提供生成特定时间长度的方法，则需要用到忙碌等待。不同的计算机处理器速度差异很大，特别是一些处理器设计为可能根据外部因素（例如操作系统上的负载）动态调整速率。因此，忙碌等待这种时间延迟技术容易产生不可预知、甚至不一致的结果，除非实现代码来确定处理器执行“什么都不做”循环的速度，或者循环代码明确检查实时时钟。

　　在某些情况下，忙碌等待是有效的策略，特别是实现自旋锁设计的操作系统上运行对称多处理。不过一般来说，忙碌等待是应该避免的反模式[1]，处理器时间应该用来执行其他任务，而不是浪费在无用的活动上。

对于多核CPU，忙碌等待的优点是不切换线程，避免了由此付出的代价。因此一些多线程同步机制不使用切换到内核态的同步对象，而是以用户态的自旋锁或其衍生机制（如轻型读写锁）来做同步，付出的时间复杂度相差3个数量级。忙碌等待可使用一些机制来降低CPU功耗，如Windows系统中调用YieldProcessor，实际上是调用了SIMD指令_mm_pause。

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>

volatile int i = 0; /* i is global, so it is visible to all functions.
                       It's also marked volatile, because it
                       may change in a way which is not predictable by the compiler,
                       here from a different thread. */

/* f1 uses a spinlock to wait for i to change from 0. */
static void *f1(void *p) {
    while (i == 0) {
        /* do nothing - just keep checking over and over */
    }
    printf("i's value has changed to %d.\n", i);
    return NULL;
}

static void *f2(void *p) {
    sleep(60);   /* sleep for 60 seconds */
    i = 99;
    printf("t2 has changed the value of i to %d.\n", i);
    return NULL;
}

int main() {
    int rc;
    pthread_t t1, t2;
    rc = pthread_create(&t1, NULL, f1, NULL);
    if (rc != 0) {
        fprintf(stderr, "pthread f1 failed\n");
        return EXIT_FAILURE;
    }
    rc = pthread_create(&t2, NULL, f2, NULL);
    if (rc != 0) {
        fprintf(stderr, "pthread f2 failed\n");
        return EXIT_FAILURE;
    }
    pthread_join(t1, NULL);
    pthread_join(t2, NULL);
    puts("All pthreads finished.");
    return 0;
}

 

 

　　忙碌等待，顾名思义，是忙着等待，而不是闲着等待。忙和闲针对的是CPU；而等待针对的是共享资源。 

　　从上述示例中，可以看到，线程 f1 在等待条件（i==0）的过程中，需要CPU不断的查询而导致CPU无法空闲。所以，在大部分的程序设计中，应该避免这种情况的发生。

 

2. 什么是信号量？为什么需要信号量？

　　信号标（英语：Semaphore）又称为号志、旗语，是一个同步对象，用于保持在0至指定最大值之间的一个计数值。当线程完成一次对该semaphore对象的等待（wait）时，该计数值减一；当线程完成一次对semaphore对象的释放（release）时，计数值加一。当计数值为0，则线程等待该semaphore对象不再能成功直至该semaphore对象变成signaled状态。semaphore对象的计数值大于0，为signaled状态；计数值等于0，为nonsignaled状态.

　　semaphore对象适用于控制一个仅支持有限个用户的共享资源，是一种不需要使用忙碌等待（busy waiting）的方法。 

typedef struct {
    int val;
    pthread_mutex_t mutex;
    pthread_cond_t cond;
} cam_semaphore_t;

static inline void cam_sem_init(cam_semaphore_t *s, int n)
{
    pthread_mutex_init(&(s->mutex), NULL);
    pthread_cond_init(&(s->cond), NULL);
    s->val = n;
}

static inline void cam_sem_post(cam_semaphore_t *s)
{
    pthread_mutex_lock(&(s->mutex));
    s->val++;
    pthread_cond_signal(&(s->cond));
    pthread_mutex_unlock(&(s->mutex));
}

static inline int cam_sem_wait(cam_semaphore_t *s)
{
    int rc = 0;
    pthread_mutex_lock(&(s->mutex));
    while (s->val == 0)
        rc = pthread_cond_wait(&(s->cond), &(s->mutex));
    s->val--;
    pthread_mutex_unlock(&(s->mutex));
    return rc;
}

static inline void cam_sem_destroy(cam_semaphore_t *s)
{
    pthread_mutex_destroy(&(s->mutex));
    pthread_cond_destroy(&(s->cond));
    s->val = 0;
}

 

 　　对着代码，一句一句的细细品味。

　　当 计数值 为 0 时，相当于一个互斥锁。

 

3. Android Mutex、Condition

/*
 * Copyright (C) 2007 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#ifndef _LIBS_UTILS_MUTEX_H
#define _LIBS_UTILS_MUTEX_H

#include <stdint.h>
#include <sys/types.h>
#include <time.h>

#if !defined(_WIN32)
# include <pthread.h>
#endif

#include <utils/Errors.h>
#include <utils/Timers.h>

// ---------------------------------------------------------------------------
namespace android {
// ---------------------------------------------------------------------------

class Condition;

/*
 * NOTE: This class is for code that builds on Win32.  Its usage is
 * deprecated for code which doesn't build for Win32.  New code which
 * doesn't build for Win32 should use std::mutex and std::lock_guard instead.
 *
 * 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();
    explicit    Mutex(const char* name);
    explicit    Mutex(int type, const char* name = NULL);
                ~Mutex();

    // lock or unlock the mutex
    status_t    lock();
    void        unlock();

    // lock if possible; returns 0 on success, error otherwise
    status_t    tryLock();

#if defined(__ANDROID__)
    // Lock the mutex, but don't wait longer than timeoutNs (relative time).
    // Returns 0 on success, TIMED_OUT for failure due to timeout expiration.
    //
    // OSX doesn't have pthread_mutex_timedlock() or equivalent. To keep
    // capabilities consistent across host OSes, this method is only available
    // when building Android binaries.
    //
    // FIXME?: pthread_mutex_timedlock is based on CLOCK_REALTIME,
    // which is subject to NTP adjustments, and includes time during suspend,
    // so a timeout may occur even though no processes could run.
    // Not holding a partial wakelock may lead to a system suspend.
    status_t    timedLock(nsecs_t timeoutNs);
#endif

    // Manages the mutex automatically. It'll be locked when Autolock is
    // constructed and released when Autolock goes out of scope.
    class Autolock {      // 非常常用的 Autolock，其作用范围是一个 大括号 
    public:
        inline explicit Autolock(Mutex& mutex) : mLock(mutex)  { mLock.lock(); }
        inline explicit Autolock(Mutex* mutex) : mLock(*mutex) { mLock.lock(); }
        inline ~Autolock() { mLock.unlock(); }
    private:
        Mutex& mLock;
    };

private:
    friend class Condition;

    // A mutex cannot be copied
                Mutex(const Mutex&);
    Mutex&      operator = (const Mutex&);

#if !defined(_WIN32)
    pthread_mutex_t mMutex;
#else
    void    _init();
    void*   mState;
#endif
};

// ---------------------------------------------------------------------------

#if !defined(_WIN32)

inline Mutex::Mutex() {
    pthread_mutex_init(&mMutex, NULL);
}
inline Mutex::Mutex(__attribute__((unused)) const char* name) {
    pthread_mutex_init(&mMutex, NULL);
}
inline Mutex::Mutex(int type, __attribute__((unused)) 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);
    }
}
inline Mutex::~Mutex() {
    pthread_mutex_destroy(&mMutex);
}
inline status_t Mutex::lock() {
    return -pthread_mutex_lock(&mMutex);
}
inline void Mutex::unlock() {
    pthread_mutex_unlock(&mMutex);
}
inline status_t Mutex::tryLock() {
    return -pthread_mutex_trylock(&mMutex);
}
#if defined(__ANDROID__)
inline status_t Mutex::timedLock(nsecs_t timeoutNs) {
    timeoutNs += systemTime(SYSTEM_TIME_REALTIME);
    const struct timespec ts = {
        /* .tv_sec = */ static_cast<time_t>(timeoutNs / 1000000000),
        /* .tv_nsec = */ static_cast<long>(timeoutNs % 1000000000),
    };
    return -pthread_mutex_timedlock(&mMutex, &ts);
}
#endif

#endif // !defined(_WIN32)

// ---------------------------------------------------------------------------

/*
 * Automatic mutex.  Declare one of these at the top of a function.
 * When the function returns, it will go out of scope, and release the
 * mutex.
 */

typedef Mutex::Autolock AutoMutex;

// ---------------------------------------------------------------------------
}; // namespace android
// ---------------------------------------------------------------------------

#endif // _LIBS_UTILS_MUTEX_H

 

 　　可以看到，Android对于Mutex的实现，其本质也是封装了 pthread_mutex。

/*
 * Copyright (C) 2007 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#ifndef _LIBS_UTILS_CONDITION_H
#define _LIBS_UTILS_CONDITION_H

#include <limits.h>
#include <stdint.h>
#include <sys/types.h>
#include <time.h>

#if !defined(_WIN32)
# include <pthread.h>
#endif

#include <utils/Errors.h>
#include <utils/Mutex.h>
#include <utils/Timers.h>

// ---------------------------------------------------------------------------
namespace android {
// ---------------------------------------------------------------------------

/*
 * Condition variable class.  The implementation is system-dependent.
 *
 * Condition variables are paired up with mutexes.  Lock the mutex,
 * call wait(), then either re-wait() if things aren't quite what you want,
 * or unlock the mutex and continue.  All threads calling wait() must
 * use the same mutex for a given Condition.
 *
 * On Android and Apple platforms, these are implemented as a simple wrapper
 * around pthread condition variables.  Care must be taken to abide by
 * the pthreads semantics, in particular, a boolean predicate must
 * be re-evaluated after a wake-up, as spurious wake-ups may happen.
 */
class Condition {
public:
    enum {
        PRIVATE = 0,
        SHARED = 1
    };

    enum WakeUpType {
        WAKE_UP_ONE = 0,
        WAKE_UP_ALL = 1
    };

    Condition();
    explicit Condition(int type);
    ~Condition();
    // Wait on the condition variable.  Lock the mutex before calling.
    // Note that spurious wake-ups may happen.
    status_t wait(Mutex& mutex);
    // same with relative timeout
    status_t waitRelative(Mutex& mutex, nsecs_t reltime);
    // Signal the condition variable, allowing one thread to continue.
    void signal();
    // Signal the condition variable, allowing one or all threads to continue.
    void signal(WakeUpType type) {
        if (type == WAKE_UP_ONE) {
            signal();
        } else {
            broadcast();
        }
    }
    // Signal the condition variable, allowing all threads to continue.
    void broadcast();

private:
#if !defined(_WIN32)
    pthread_cond_t mCond;
#else
    void*   mState;
#endif
};

// ---------------------------------------------------------------------------

#if !defined(_WIN32)

inline Condition::Condition() : Condition(PRIVATE) {
}
inline Condition::Condition(int type) {
    pthread_condattr_t attr;
    pthread_condattr_init(&attr);
#if defined(__linux__)
    pthread_condattr_setclock(&attr, CLOCK_MONOTONIC);
#endif

    if (type == SHARED) {
        pthread_condattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
    }

    pthread_cond_init(&mCond, &attr);
    pthread_condattr_destroy(&attr);

}
inline Condition::~Condition() {
    pthread_cond_destroy(&mCond);
}
inline status_t Condition::wait(Mutex& mutex) {
    return -pthread_cond_wait(&mCond, &mutex.mMutex);
}
inline status_t Condition::waitRelative(Mutex& mutex, nsecs_t reltime) {
    struct timespec ts;
#if defined(__linux__)
    clock_gettime(CLOCK_MONOTONIC, &ts);
#else // __APPLE__
    // Apple doesn't support POSIX clocks.
    struct timeval t;
    gettimeofday(&t, NULL);
    ts.tv_sec = t.tv_sec;
    ts.tv_nsec = t.tv_usec*1000;
#endif

    // On 32-bit devices, tv_sec is 32-bit, but `reltime` is 64-bit.
    int64_t reltime_sec = reltime/1000000000;

    ts.tv_nsec += static_cast<long>(reltime%1000000000);
    if (reltime_sec < INT64_MAX && ts.tv_nsec >= 1000000000) {
        ts.tv_nsec -= 1000000000;
        ++reltime_sec;
    }

    int64_t time_sec = ts.tv_sec;
    if (time_sec > INT64_MAX - reltime_sec) {
        time_sec = INT64_MAX;
    } else {
        time_sec += reltime_sec;
    }

    ts.tv_sec = (time_sec > LONG_MAX) ? LONG_MAX : static_cast<long>(time_sec);

    return -pthread_cond_timedwait(&mCond, &mutex.mMutex, &ts);
}
inline void Condition::signal() {
    pthread_cond_signal(&mCond);
}
inline void Condition::broadcast() {
    pthread_cond_broadcast(&mCond);
}

#endif // !defined(_WIN32)

// ---------------------------------------------------------------------------
}; // namespace android
// ---------------------------------------------------------------------------

#endif // _LIBS_UTILS_CONDITON_H

 

　　同样，Condition也是基于 pthread_cond 实现的。

4. pthread_mutex，pthrad_cond

　　TODO