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理解Binder通信原理及常见问题3

猪小花1号2018-08-30 09:48

作者:李尚


Client端线程睡眠在哪个队列上,唤醒Server端哪个等待队列上的线程

先看第一部分:发送端线程睡眠在哪个队列上?

发送端线程一定睡眠在自己binder_thread的等待队列上,并且,该队列上有且只有自己一个睡眠线程

再看第二部分:在Binder驱动去唤醒线程的时候,唤醒的是哪个等待队列上的线程?

理解这个问题需要理解binder_thread中的 struct binder_transaction transaction_stack栈,这个栈规定了transaction的执行顺序:*栈顶的一定先于栈内执行

如果本地操作是BC_REPLY,一定是唤醒之前发送等待的线程,这个是100%的,但是如果是BC_TRANSACTION,那就不一定了,尤其是当两端互为服务相互请求的时候,场景如下:

  • 进程A的普通线程AT1请求B进程的B1服务,唤醒B进程的Binder线程,AT1睡眠等待服务结束
  • B进程的B1服务在执行的的时候,需要请求进程A的A1服务,则B进程的Binder线程BT1睡眠,唤醒A进程的Binder线程,等待服务结束
  • A进程的A1服务在执行的时候,又需要B进程的B2服务,则A进程的binder线程AT2睡眠,唤醒B进程的Binder线程,等待服务结束

这个时候就会遇到一个问题:唤醒哪个线程比较合适?是睡眠在进程队列上的线程,还是之前睡眠的线程BT1?答案是:之前睡眠的线程BT1,具体看下面的图解分析

首先第一步A普通线程去请求B进程的B1服务,这个时候在A进程的AT1线程的binder_ref中会将binder_transaction1入栈,而同样B的Binder线程在读取binder_work之后,也会将binder_transaction1加入自己的堆栈,如下图:

而当B的Binder线程被唤醒后,执行Binder实体中的服务时,发现服务函数需要反过来去请求A端的A1服务,那就需要通过Binder向A进程发送请求,并新建binder_transaction2压入自己的binder_transaction堆栈,而A进程的Binder线程被唤醒后也会将binder_transaction2加入自己的堆栈,会后效果如下: 这个时候,还是没有任何问题,但是恰好在执行A1服务的时候,又需要请求B2服务,这个时候,A1线程重复上述压栈过程,新建binder_transaction3压入自己的栈,不过在写入到目标端B的时候,会面临一个抉择,写入那个队列,是binder_proc上的队列,还是正在等候A返回的BT1线程的队列? 结果已经说过,是BT1的队列,为什么呢?因为BT1队列上的之前的binder_transaction2在等待A进程执行完,但是A端的binder_transaction3同样要等待binder_transaction3在B进程中执行完毕,也就是说,binder_transaction3在B端一定是先于binder_transaction2执行的,因此唤醒BT1线程,并将binder_transaction3压入BT2的栈,等binder_transaction3执行完毕,出栈后,binder_transaction2才能执行,这样,既不妨碍binder_transaction2的执行,同样也能让睡眠的BT1进程提高利用率,因为最终的堆栈效果就是:

而当binder_transaction3完成,出栈的过程其实就简单了,

  • BT1 执行binder_transaction3,唤醒A端AT2 Binder线程,并且BT1继续睡眠(因为还有等待的transaction)
  • AT2 执行binder_transaction2,唤醒BT1
  • BT1 执行binder_transaction1,唤醒AT1
  • 执行结束

从这里可以看出,其实设计的还是很巧妙的,让线程复用,提高了效率,还避免了新建不必要的Binder线程,在binder驱动中岛实现代码,其实就是根据binder_transaction中堆栈记录查询, static void binder_transaction(struct binder_proc proc, struct binder_thread thread, struct binder_transaction_data *tr, int reply) {.. while (tmp) { // 找到对方正在等待自己进程的线程,如果线程没有在等待自己进程的返回,就不要找了

                    // 判断是不target_proc中,是不是有线程,等待当前线程
                    // thread->transaction_stack,这个时候,
                    // 是binder线程的,不是普通线程 B去请求A服务,
                    // 在A服务的时候,又请求了B,这个时候,A的服务一定要等B处理完,才能再返回B,可以放心用B
                        if (tmp->from && tmp->from->proc == target_proc)
                            target_thread = tmp->from;
                        tmp = tmp->from_parent;
          ...            }
        } }

Binder协议中BC与BR的区别

BC与BR主要是标志数据及Transaction流向,其中BC是从用户空间流向内核,而BR是从内核流线用户空间,比如Client向Server发送请求的时候,用的是BC_TRANSACTION,当数据被写入到目标进程后,target_proc所在的进程被唤醒,在内核空间中,会将BC转换为BR,并将数据与操作传递该用户空间。

Binder在传输数据的时候是如何层层封装的--不同层次使用的数据结构(命令的封装)

内核中,与用户空间对应的结构体对象都需要新建,但传输数据的数据只拷贝一次,就是一次拷贝的时候。

从Client端请求开始分析,暂不考虑java层,只考虑Native,以ServiceManager的addService为例,具体看一下

MediaPlayerService::instantiate();

MediaPlayerService会新建Binder实体,并将其注册到ServiceManager中:

void MediaPlayerService::instantiate() {
    defaultServiceManager()->addService(
            String16("media.player"), new MediaPlayerService());
}    

这里defaultServiceManager其实就是获取ServiceManager的远程代理:

sp<IServiceManager> defaultServiceManager()
{
    if (gDefaultServiceManager != NULL) return gDefaultServiceManager;

    {
        AutoMutex _l(gDefaultServiceManagerLock);
        if (gDefaultServiceManager == NULL) {
            gDefaultServiceManager = interface_cast<IServiceManager>(
                ProcessState::self()->getContextObject(NULL));
        }
    }

    return gDefaultServiceManager;
}

如果将代码简化其实就是

return gDefaultServiceManager = BpServiceManager (new BpBinder(0));

addService就是调用BpServiceManager的addService,

virtual status_t addService(const String16& name, const sp<IBinder>& service,
        bool allowIsolated)
{
    Parcel data, reply;
    data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
    data.writeString16(name);
    data.writeStrongBinder(service);
    data.writeInt32(allowIsolated ? 1 : 0);
    status_t err = remote()->transact(ADD_SERVICE_TRANSACTION, data, &reply);
    return err == NO_ERROR ? reply.readExceptionCode() : err;
}

这里会开始第一步的封装,数据封装,其实就是讲具体的传输数据写入到Parcel对象中,与Parcel对应是ADD_SERVICE_TRANSACTION等具体操作。比较需要注意的就是data.writeStrongBinder,这里其实就是把Binder实体压扁:

status_t Parcel::writeStrongBinder(const sp<IBinder>& val)
{
    return flatten_binder(ProcessState::self(), val, this);
}

具体做法就是转换成flat_binder_object,以传递Binder的类型、指针之类的信息:

status_t flatten_binder(const sp<ProcessState>& proc,
    const sp<IBinder>& binder, Parcel* out)
{
    flat_binder_object obj;

    obj.flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS;
    if (binder != NULL) {
        IBinder *local = binder->localBinder();
        if (!local) {
            BpBinder *proxy = binder->remoteBinder();
            if (proxy == NULL) {
                ALOGE("null proxy");
            }
            const int32_t handle = proxy ? proxy->handle() : 0;
            obj.type = BINDER_TYPE_HANDLE;
            obj.handle = handle;
            obj.cookie = NULL;
        } else {
            obj.type = BINDER_TYPE_BINDER;
            obj.binder = local->getWeakRefs();
            obj.cookie = local;
        }
    } else {
        obj.type = BINDER_TYPE_BINDER;
        obj.binder = NULL;
        obj.cookie = NULL;
    }

    return finish_flatten_binder(binder, obj, out);
}

接下来看 remote()->transact(ADD_SERVICE_TRANSACTION, data, &reply); 在上面的环境中,remote()函数返回的就是BpBinder(0),

status_t BpBinder::transact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    // Once a binder has died, it will never come back to life.
    if (mAlive) {
        status_t status = IPCThreadState::self()->transact(
            mHandle, code, data, reply, flags);
        if (status == DEAD_OBJECT) mAlive = 0;
        return status;
    }

    return DEAD_OBJECT;
}

之后通过 IPCThreadState::self()->transact( mHandle, code, data, reply, flags)进行进一步封装:

status_t IPCThreadState::transact(int32_t handle,
                uint32_t code, const Parcel& data,
                Parcel* reply, uint32_t flags){
    if ((flags & TF_ONE_WAY) == 0) {
        if (err == NO_ERROR) {
            err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);
        }
        if (reply) {
            err = waitForResponse(reply);
        } 
        ..
    return err;
    }

writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);是进一步封装的入口,在这个函数中Parcel& data、handle、code、被进一步封装成binder_transaction_data对象,并拷贝到mOut的data中去,同时也会将BC_TRANSACTION命令也写入mOut,这里与binder_transaction_data对应的CMD是BC_TRANSACTION,binder_transaction_data也存储了数据的指引新信息:

status_t IPCThreadState::writeTransactionData(int32_t cmd, uint32_t binderFlags,
    int32_t handle, uint32_t code, const Parcel& data, status_t* statusBuffer)
{
    binder_transaction_data tr;
    tr.target.handle = handle;
    tr.code = code;
    tr.flags = binderFlags;
    tr.cookie = 0;
    tr.sender_pid = 0;
    tr.sender_euid = 0;
    const status_t err = data.errorCheck();
    if (err == NO_ERROR) {
        tr.data_size = data.ipcDataSize();
        tr.data.ptr.buffer = data.ipcData();
        tr.offsets_size = data.ipcObjectsCount()*sizeof(size_t);
        tr.data.ptr.offsets = data.ipcObjects();
    } ..
    mOut.writeInt32(cmd);
    mOut.write(&tr, sizeof(tr));
    return NO_ERROR;
}

mOut封装结束后,会通过waitForResponse调用talkWithDriver继续封装:

status_t IPCThreadState::talkWithDriver(bool doReceive)
{
    binder_write_read bwr;
    // Is the read buffer empty? 这里会有同时返回两个命令的情况 BR_NOOP、BR_COMPLETE
    const bool needRead = mIn.dataPosition() >= mIn.dataSize();
    // We don't want to write anything if we are still reading
    // from data left in the input buffer and the caller
    // has requested to read the next data.
    const size_t outAvail = (!doReceive || needRead) ? mOut.dataSize() : 0;
    bwr.write_size = outAvail;
    bwr.write_buffer = (long unsigned int)mOut.data();        // This is what we'll read.
    if (doReceive && needRead) {
        bwr.read_size = mIn.dataCapacity();
        bwr.read_buffer = (long unsigned int)mIn.data();
    } else {
        bwr.read_size = 0;
        bwr.read_buffer = 0;
    }
    // Return immediately if there is nothing to do.
    if ((bwr.write_size == 0) && (bwr.read_size == 0)) return NO_ERROR;
    bwr.write_consumed = 0;
    bwr.read_consumed = 0;
    status_t err;
    do {
        。。
        if (ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr) >= 0)
            err = NO_ERROR;
        if (mProcess->mDriverFD <= 0) {
            err = -EBADF;
        }
    } while (err == -EINTR);

    if (err >= NO_ERROR) {
        if (bwr.write_consumed > 0) {
            if (bwr.write_consumed < (ssize_t)mOut.dataSize())
                mOut.remove(0, bwr.write_consumed);
            else
                mOut.setDataSize(0);
        }
        if (bwr.read_consumed > 0) {
            mIn.setDataSize(bwr.read_consumed);
            mIn.setDataPosition(0);
        }
        return NO_ERROR;
    }
    return err;
}

talkWithDriver会将mOut中的数据与命令继续封装成binder_write_read对象,其中bwr.write_buffer就是mOut中的data(binder_transaction_data+BC_TRRANSACTION),之后就会通过ioctl与binder驱动交互,进入内核,这里与binder_write_read对象对应的CMD是BINDER_WRITE_READ,进入驱动后,是先写后读的顺序,所以才叫BINDER_WRITE_READ命令,与BINDER_WRITE_READ层级对应的几个命令码一般都是跟线程、进程、数据整体传输相关的操作,不涉及具体的业务处理,比如BINDER_SET_CONTEXT_MGR是将线程编程ServiceManager线程,并创建0号Handle对应的binder_node、BINDER_SET_MAX_THREADS是设置最大的非主Binder线程数,而BINDER_WRITE_READ就是表示这是一次读写操作:

#define BINDER_CURRENT_PROTOCOL_VERSION 7
#define BINDER_WRITE_READ _IOWR('b', 1, struct binder_write_read)
#define BINDER_SET_IDLE_TIMEOUT _IOW('b', 3, int64_t)
#define BINDER_SET_MAX_THREADS _IOW('b', 5, size_t)
/* WARNING: DO NOT EDIT, AUTO-GENERATED CODE - SEE TOP FOR INSTRUCTIONS */
#define BINDER_SET_IDLE_PRIORITY _IOW('b', 6, int)
#define BINDER_SET_CONTEXT_MGR _IOW('b', 7, int)
#define BINDER_THREAD_EXIT _IOW('b', 8, int)
#define BINDER_VERSION _IOWR('b', 9, struct binder_version)

详细看一下binder_ioctl对于BINDER_WRITE_READ的处理,

static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
    switch (cmd) {
    case BINDER_WRITE_READ: {
        struct binder_write_read bwr;
        ..
        <!--拷贝binder_write_read对象到内核空间-->
        if (copy_from_user(&bwr, ubuf, sizeof(bwr))) {
            ret = -EFAULT;
            goto err;
        }
        <!--根据是否需要写数据处理是不是要写到目标进程中去-->
        if (bwr.write_size > 0) {
            ret = binder_thread_write(proc, thread, (void __user *)bwr.write_buffer, bwr.write_size, &bwr.write_consumed);
        }
      <!--根据是否需要写数据处理是不是要读,往自己进程里读数据-->
        if (bwr.read_size > 0) {
            ret = binder_thread_read(proc, thread, (void __user *)bwr.read_buffer, bwr.read_size, &bwr.read_consumed, filp->f_flags & O_NONBLOCK);
            <!--是不是要同时唤醒进程上的阻塞队列-->
            if (!list_empty(&proc->todo))
                wake_up_interruptible(&proc->wait);
        }
        break;
    }
    case BINDER_SET_MAX_THREADS:
        if (copy_from_user(&proc->max_threads, ubuf, sizeof(proc->max_threads))) {
        }
        break;
    case BINDER_SET_CONTEXT_MGR:
       .. break;
    case BINDER_THREAD_EXIT:
        binder_free_thread(proc, thread);
        thread = NULL;
        break;
    case BINDER_VERSION:
    ..
}

binder_thread_write(proc, thread, (void __user )bwr.write_buffer, bwr.write_size, &bwr.write_consumed)这里其实就是把解析的binder_write_read对象再剥离,*bwr.write_buffer 就是上面的(BC_TRANSACTION+ binder_transaction_data),

int binder_thread_write(struct binder_proc *proc, struct binder_thread *thread,
            void __user *buffer, int size, signed long *consumed)
{
    uint32_t cmd;
    void __user *ptr = buffer + *consumed;
    void __user *end = buffer + size;
    while (ptr < end && thread->return_error == BR_OK) {

        // binder_transaction_data  BC_XXX+binder_transaction_data
        if (get_user(cmd, (uint32_t __user *)ptr))  (BC_TRANSACTION)
            return -EFAULT;
        ptr += sizeof(uint32_t);
        switch (cmd) {
        ..
        case BC_FREE_BUFFER: {
            ...
        }
        case BC_TRANSACTION:
        case BC_REPLY: {
            struct binder_transaction_data tr;
            if (copy_from_user(&tr, ptr, sizeof(tr)))
                return -EFAULT;
            ptr += sizeof(tr);
            binder_transaction(proc, thread, &tr, cmd == BC_REPLY);
            break;
        }
        case BC_REGISTER_LOOPER:
            ..
        case BC_ENTER_LOOPER:
            ...
            thread->looper |= BINDER_LOOPER_STATE_ENTERED;
            break;
        case BC_EXIT_LOOPER:
        // 这里会修改读取的数据,
        *consumed = ptr - buffer;
    }
    return 0;
}

binder_thread_write会进一步根据CMD剥离出binder_transaction_data tr,交给binder_transaction处理,其实到binder_transaction数据几乎已经剥离极限,剩下的都是业务相关的,但是这里牵扯到一个Binder实体与Handle的转换过程,同城也牵扯两个进程在内核空间共享一些数据的问题,因此这里又进行了一次进一步的封装与拆封装,这里新封装了连个对象 binder_transaction与binder_work,有所区别的是binder_work可以看做是进程私有,但是binder_transaction是两个交互的进程共享的:binder_work是插入到线程或者进程的work todo队列上去的:

struct binder_thread {
    struct binder_proc *proc;
    struct rb_node rb_node;
    int pid;
    int looper;
    struct binder_transaction *transaction_stack;
    struct list_head todo;
    uint32_t return_error; /* Write failed, return error code in read buf */
    uint32_t return_error2; /* Write failed, return error code in read */
    wait_queue_head_t wait;
    struct binder_stats stats;
};

这里主要关心一下binder_transaction:binder_transaction主要记录了当前transaction的来源,去向,同时也为了返回做准备,buffer字段是一次拷贝后数据在Binder的内存地址。

struct binder_transaction {
int debug_id;
struct binder_work work;
struct binder_thread *from;
struct binder_transaction *from_parent;
struct binder_proc *to_proc;
struct binder_thread *to_thread;
struct binder_transaction *to_parent;
unsigned need_reply:1;
/* unsigned is_dead:1; */ /* not used at the moment */
struct binder_buffer *buffer;
unsigned int code;
unsigned int flags;
long priority;
long saved_priority;
uid_t sender_euid;
};



相关阅读:

理解Binder通信原理及常见问题1

理解Binder通信原理及常见问题2

理解Binder通信原理及常见问题3

理解Binder通信原理及常见问题4

理解Binder通信原理及常见问题5

理解Binder通信原理及常见问题6

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本文来自网易实践者社区,经作者李尚授权发布。