Android存储系统之源码篇

/framework/base/services/java/com/android/server/SystemServer.java
/framework/base/services/core/java/com/android/server/MountService.java
/framework/base/services/core/java/com/android/server/NativeDaemonConnector.java
/framework/base/services/core/java/com/android/server/NativeDaemonEvent.java

/system/vold/Main.cpp
/system/vold/VolumeManager.cpp
/system/vold/NetlinkManager.cpp
/system/vold/NetlinkHandler.cpp
/system/vold/CommandListener.cpp
/system/vold/VoldCommand.cpp
/system/vold/VolumeBase.cpp
/system/vold/PublicVolume.cpp
/system/vold/EmulatedVolume.cpp
/system/vold/PublicVolume.cpp
/system/vold/Disk.cpp

/system/core/libsysutils/src/NetlinkListener.cpp
/system/core/libsysutils/src/SocketListener.cpp
/system/core/libsysutils/src/FrameworkListener.cpp
/system/core/libsysutils/src/FrameworkCommand.cpp
/system/core/include/sysutils/NetlinkListener.h
/system/core/include/sysutils/SocketListener.h
/system/core/include/sysutils/FrameworkListener.h
/system/core/include/sysutils/FrameworkCommand.h

一、概述

本文主要介绍跟存储相关的模块MountService和Vold，并不涉及底层文件系统。

• MountService：Android Binder服务，运行在system_server进程，用于跟Vold进行消息通信，比如 MountService 向 Vold 发送挂载SD卡的命令,或者接收到来自 Vold 的外设热插拔事件。
• Vold(全称为Volume Daemon)，用于管理外部存储设备的Native守护进程，这是一个非常重要的守护进程，由NetlinkManager，VolumeManager，CommandListener这3部分组成。

MountService的NativeDaemonConnector(Client端)和 Vold的CL模块(Server端)建立socket通信。

1. Linux Kernel:通过uevent向Vold的NetlinkManager发送Uevent事件；
2. NetlinkManager:接收来自Kernel的 Uevent 事件，再转发给VolumeManager；
3. VolumeManager: 接收来自NetlinkManager的事件，再转发给CommandListener进行处理；
4. CommandListener: 接收来自VolumeManager的事件，通过 socket 通信方式发送给MountService；
5. MountService： 接收来自CommandListener的事件。

1.1 存储架构设计

存储架构从进程/线程视角：

• Java层采用1个system_server主线程+3个子线程; 另外还会使用到系统进程中的两个线程”android.fg”和”android.io”。
• Native采用1个vold主线程+3个子线程+1子进程的架构；

先看看Java层的线程：

root@gityuan:/ # ps -t | grep 1212
system    1212  557   2334024 160340 SyS_epoll_ 7faedddbe4 S system_server
system    2662  1212  2334024 160340 SyS_epoll_ 7faedddbe4 S MountService
system    2663  1212  2334024 160340 unix_strea 7faedde73c S VoldConnector
system    2664  1212  2334024 160340 unix_strea 7faedde73c S CryptdConnector
...

再看看Native层的线程：

root@gityuan:/ # ps -t | grep " 387 "
USER      PID   PPID  VSIZE  RSS   WCHAN              PC  NAME
root      387   1     13572  2912  hrtimer_na 7fa34755d4 S /system/bin/vold
root      397   387   13572  2912  poll_sched 7fa3474d1c S vold
root      399   387   13572  2912  poll_sched 7fa3474d1c S vold
root      400   387   13572  2912  poll_sched 7fa3474d1c S vold
media_rw  2702  387   7140   2036  inotify_re 7f84b1d6ac S /system/bin/sdcard

小技巧：有读者可能会好奇，为什么/system/bin/sdcard是子进程，而非子线程呢？要回答这个问题，有两个方法，其一就是直接看撸源码，会发现这是通过 fork 方式创建的，而其他子线程都是通过 pthread_create 方式创建的。当然其实还有个更快捷的小技巧，就是直接看上图中的第4列，这一列的含义是 VSIZE ，代表的是进程虚拟地址空间大小，是否共享地址空间，这是进程与线程最大的区别，再来看看/sdcard的VSIZE大小跟父进程不一样，基本可以确实/sdcard是子进程。

Tips: 同一个模块可以运行在各个不同的进程/线程， 同一个进程可以运行不同模块的代码。

1.2 NativeDaemonEvent

例如当操作执行成功，VoldConnector线程能收到类似“RCV <- {200 3 Command succeeded}”的响应事件。

这里在详细列举远程Vold进程的那些”不请自来”的事件，也就是指底层触发的响应码，范围为[600,700)

这些命令主要是针对disk，volume的一系列操作，比如设备创建，状态、路径改变，以及文件类型、uid、标签改变等事件都是底层直接触发。

二、MountService

MountService运行在system_server进程，在系统启动到阶段PHASE_WAIT_FOR_DEFAULT_DISPLAY后，进入startOtherServices会启动MountService.

2.1 启动

[-> SystemServer.java]

private void startOtherServices() {
    ...
    IMountService mountService = null;
    //启动MountService服务，【见小节2.2】
    mSystemServiceManager.startService(MOUNT_SERVICE_CLASS);
    //等价new IMountService.Stub.Proxy()，即获取MountService的proxy对象
    mountService = IMountService.Stub.asInterface(
            ServiceManager.getService("mount"));
    ...

    mActivityManagerService.systemReady(new Runnable() {
        public void run() {
            //启动到阶段550【见小节2.7】
            mSystemServiceManager.startBootPhase(
                        SystemService.PHASE_ACTIVITY_MANAGER_READY);
        ...
    });
}

NotificationManagerService依赖于MountService，比如media/usb通知事件，所以需要先启动MountService。此处MOUNT_SERVICE_CLASS=”com.android.server.MountService$Lifecycle”

2.2 startService

mSystemServiceManager.startService(MOUNT_SERVICE_CLASS)主要完成3件事：

• 创建MOUNT_SERVICE_CLASS所指类的Lifecycle对象；
• 将该对象添加SystemServiceManager的 mServices 服务列表；
• 最后调用Lifecycle的onStart()方法，主要工作量这个过程，如下：

[-> MountService.java]

class MountService extends IMountService.Stub
        implements INativeDaemonConnectorCallbacks, Watchdog.Monitor {
    public static class Lifecycle extends SystemService {
        public void onStart() {
            //创建MountService对象【见小节2.3】
            mMountService = new MountService(getContext());
            //登记Binder服务
            publishBinderService("mount", mMountService);
        }
        ...
    }
    ...
}

创建MountService对象，并向Binder服务的大管家ServiceManager登记，该服务名为“mount”，对应服务对象为mMountService。登记之后，其他地方当需要MountService的服务时便可以通过服务名来向ServiceManager来查询具体的MountService服务。

2.3 MountService

public MountService(Context context) {
    sSelf = this;

    mContext = context;
    //FgThread线程名为“"android.fg"，创建IMountServiceListener回调方法【见小节2.4】
    mCallbacks = new Callbacks(FgThread.get().getLooper());
    //获取PKMS的Client端对象
    mPms = (PackageManagerService) ServiceManager.getService("package");
    //创建“MountService”线程
    HandlerThread hthread = new HandlerThread(TAG);
    hthread.start();

    mHandler = new MountServiceHandler(hthread.getLooper());
    //IoThread线程名为"android.io"，创建OBB操作的handler
    mObbActionHandler = new ObbActionHandler(IoThread.get().getLooper());

    File dataDir = Environment.getDataDirectory();
    File systemDir = new File(dataDir, "system");
    mLastMaintenanceFile = new File(systemDir, LAST_FSTRIM_FILE);
    //判断/data/system/last-fstrim文件，不存在则创建，存在则更新最后修改时间
    if (!mLastMaintenanceFile.exists()) {
        (new FileOutputStream(mLastMaintenanceFile)).close();
        ...
    } else {
        mLastMaintenance = mLastMaintenanceFile.lastModified();
    }
    ...
    //将MountServiceInternalImpl登记到sLocalServiceObjects
    LocalServices.addService(MountServiceInternal.class, mMountServiceInternal);
    //创建用于VoldConnector的NDC对象【见小节2.5】
    mConnector = new NativeDaemonConnector(this, "vold", MAX_CONTAINERS * 2, VOLD_TAG, 25,
            null);
    mConnector.setDebug(true);
    //创建线程名为"VoldConnector"的线程，用于跟vold通信【见小节2.6】
    Thread thread = new Thread(mConnector, VOLD_TAG);
    thread.start();

    //创建用于CryptdConnector工作的NDC对象
    mCryptConnector = new NativeDaemonConnector(this, "cryptd",
            MAX_CONTAINERS * 2, CRYPTD_TAG, 25, null);
    mCryptConnector.setDebug(true);
    //创建线程名为"CryptdConnector"的线程，用于加密
    Thread crypt_thread = new Thread(mCryptConnector, CRYPTD_TAG);
    crypt_thread.start();

    //注册监听用户添加、删除的广播
    final IntentFilter userFilter = new IntentFilter();
    userFilter.addAction(Intent.ACTION_USER_ADDED);
    userFilter.addAction(Intent.ACTION_USER_REMOVED);
    mContext.registerReceiver(mUserReceiver, userFilter, null, mHandler);

    //内部私有volume的路径为/data，该volume通过dumpsys mount是不会显示的
    addInternalVolume();

    //默认为false
    if (WATCHDOG_ENABLE) {
        Watchdog.getInstance().addMonitor(this);
    }
}

主要功能依次是：

1. FgThread线程名为”android.fg”，创建IMountServiceListener回调方法；
2. 创建并启动线程名为”MountService”的handlerThread；
3. IoThread线程名为”android.io”，创建OBB操作的handler；
4. 创建NativeDaemonConnector对象
5. 创建并启动线程名为”VoldConnector”的线程；
6. 创建并启动线程名为”CryptdConnector”的线程；
7. 注册监听用户添加、删除的广播；

该过程总共创建了3个线程：”MountService”,”VoldConnector”,”CryptdConnector”，另外还会使用到系统进程中的两个线程”android.fg”和”android.io”

2.4 Callbacks

class MountService {
    ...
    private static class Callbacks extends Handler {
        private final RemoteCallbackList<IMountServiceListener>
                        mCallbacks = new RemoteCallbackList<>();
        public Callbacks(Looper looper) {
            super(looper);
        }
        ...
    }
}

创建Callbacks时的Looper为FgThread.get().getLooper()，其中 FgThread 采用单例模式，是一个线程名为”android.fg”的HandlerThread。另外，Callbacks对象有一个成员变量 mCallbacks ，如下：

[-> RemoteCallbackList.java]

public class RemoteCallbackList<E extends IInterface> {
    ArrayMap<IBinder, Callback> mCallbacks
            = new ArrayMap<IBinder, Callback>();

    //Binder死亡通知
    private final class Callback implements IBinder.DeathRecipient {
        public void binderDied() {
            ...
        }
    }

    //注册死亡回调
    public boolean register(E callback, Object cookie) {
        synchronized (mCallbacks) {
            ...
            IBinder binder = callback.asBinder();
            Callback cb = new Callback(callback, cookie);
            binder.linkToDeath(cb, 0);
            mCallbacks.put(binder, cb);
            ...
        }
    }
    ...
}

通过 register() 方法添加IMountServiceListener对象信息到 mCallbacks 成员变量。RemoteCallbackList的内部类Callback继承于IBinder.DeathRecipient，很显然这是死亡通知，当binder服务端进程死亡后，回调binderDied方法通知binder客户端进行相应地处理。

2.5 NativeDaemonConnector

[-> NativeDaemonConnector.java]

NativeDaemonConnector(INativeDaemonConnectorCallbacks callbacks, String socket,
        int responseQueueSize, String logTag, int maxLogSize, PowerManager.WakeLock wl) {
    this(callbacks, socket, responseQueueSize, logTag, maxLogSize, wl,
            FgThread.get().getLooper());
}

NativeDaemonConnector(INativeDaemonConnectorCallbacks callbacks, String socket,
        int responseQueueSize, String logTag, int maxLogSize, PowerManager.WakeLock wl,
        Looper looper) {
    mCallbacks = callbacks;
    //socket名为"vold"
    mSocket = socket;
    //对象响应个数为500
    mResponseQueue = new ResponseQueue(responseQueueSize);
    mWakeLock = wl;
    if (mWakeLock != null) {
        mWakeLock.setReferenceCounted(true);
    }
    mLooper = looper;
    mSequenceNumber = new AtomicInteger(0);
    //TAG为"VoldConnector"
    TAG = logTag != null ? logTag : "NativeDaemonConnector";
    mLocalLog = new LocalLog(maxLogSize);
}

• mLooper为FgThread.get().getLooper()，即运行在”android.fg”线程；
• mResponseQueue对象中成员变量 mPendingCmds 数据类型为LinkedList，记录着vold进程上报的响应事件，事件个数上限为500。

2.6 NDC.run

[-> NativeDaemonConnector.java]

final class NativeDaemonConnector implements Runnable, Handler.Callback, Watchdog.Monitor {
    public void run() {
        mCallbackHandler = new Handler(mLooper, this);

        while (true) {
            try {
                //监听vold的socket【见小节2.7】
                listenToSocket();
            } catch (Exception e) {
                loge("Error in NativeDaemonConnector: " + e);
                SystemClock.sleep(5000);
            }
        }
    }
}

“VoldConnector”线程建立了”vold“的socket的客户端，通过循环方式不断监听Vold服务端发送过来的消息。 另外，同理还有一个线程“CryptdConnector”也采用类似的方式，建立了“cryptd”的socket客户端，监听Vold中另个线程发送过来的消息。

MountService与NDC都启动，那么接下来到系统启动到达阶段PHASE_ACTIVITY_MANAGER_READY，则调用到onBootPhase方法。

2.7 onBootPhase

[-> MountService.java ::Lifecycle]

由于MountService的内部Lifecycle已添加SystemServiceManager的 mServices 服务列表；故到系统启动到 PHASE_ACTIVITY_MANAGER_READY 时会回调 mServices 中的 onBootPhase 方法

public static class Lifecycle extends SystemService {
    public void onBootPhase(int phase) {
        if (phase == SystemService.PHASE_ACTIVITY_MANAGER_READY) {
            mMountService.systemReady();
        }
    }
}

再调用MountService.systemReady方法，该方法主要是通过mHandler发送消息。

private void systemReady() {
    mSystemReady = true;
    mHandler.obtainMessage(H_SYSTEM_READY).sendToTarget();
}

此处mHandler = new MountServiceHandler(hthread.getLooper())可知采用的是线程”MountService”中的Looper。到此system_server主线程通过handler向线程”MountService”发送H_SYSTEM_READY消息，接下来进入线程”MountService”的MountServiceHandler对象(简称MSH)的handleMessage()来处理相关的消息。

2.8 MSH.handleMessage

[-> MountService.java ::MountServiceHandler]

class MountServiceHandler extends Handler {
    public void handleMessage(Message msg) {
        switch (msg.what) {
            case H_SYSTEM_READY: {
                handleSystemReady(); //【见小节2.9】
                break;
            }
            ...
        }
    }
}

2.9 handleSystemReady

[-> MountService.java]

private void handleSystemReady() {
    synchronized (mLock) {
        //【见小节2.10】
        resetIfReadyAndConnectedLocked();
    }

    //计划执行日常的fstrim操作【】
    MountServiceIdler.scheduleIdlePass(mContext);
}

2.10 resetIfReadyAndConnectedLocked

[-> MountService.java]

private void resetIfReadyAndConnectedLocked() {
    Slog.d(TAG, "Thinking about reset, mSystemReady=" + mSystemReady
            + ", mDaemonConnected=" + mDaemonConnected);
    //当系统启动到阶段550，并且已经与vold守护进程建立连接，则执行reset
    if (mSystemReady && mDaemonConnected) {
        killMediaProvider();
        mDisks.clear();
        mVolumes.clear();

        //将/data为路径的private volume添加到mVolumes
        addInternalVolume();

        try {
            //【见小节2.11】
            mConnector.execute("volume", "reset");

            //告知所有已经存在和启动的users
            final UserManager um = mContext.getSystemService(UserManager.class);
            final List<UserInfo> users = um.getUsers();
            for (UserInfo user : users) {
                mConnector.execute("volume", "user_added", user.id, user.serialNumber);
            }
            for (int userId : mStartedUsers) {
                mConnector.execute("volume", "user_started", userId);
            }
        } catch (NativeDaemonConnectorException e) {
            Slog.w(TAG, "Failed to reset vold", e);
        }
    }
}

2.11 NDC.execute

[-> NativeDaemonConnector.java]

public NativeDaemonEvent execute(String cmd, Object... args)
        throws NativeDaemonConnectorException {
    return execute(DEFAULT_TIMEOUT, cmd, args);
}

其中DEFAULT_TIMEOUT等于1分钟，即命令执行超时时长为1分钟。经过层层调用，executeForList()

public NativeDaemonEvent[] executeForList(long timeoutMs, String cmd, Object... args)
        throws NativeDaemonConnectorException {
    final long startTime = SystemClock.elapsedRealtime();

    final ArrayList<NativeDaemonEvent> events = Lists.newArrayList();

    final StringBuilder rawBuilder = new StringBuilder();
    final StringBuilder logBuilder = new StringBuilder();

    //mSequenceNumber初始化值为0，每执行一次该方法则进行加1操作
    final int sequenceNumber = mSequenceNumber.incrementAndGet();

    makeCommand(rawBuilder, logBuilder, sequenceNumber, cmd, args);

    //例如：“3 volume reset”
    final String rawCmd = rawBuilder.toString();
    final String logCmd = logBuilder.toString();

    log("SND -> {" + logCmd + "}");

    synchronized (mDaemonLock) {
        //将cmd写入到socket的输出流
        mOutputStream.write(rawCmd.getBytes(StandardCharsets.UTF_8));
        ...
    }

    NativeDaemonEvent event = null;
    do {
        //【见小节2.12】
        event = mResponseQueue.remove(sequenceNumber, timeoutMs, logCmd);
        events.add(event);
    //当收到的事件响应码属于[100,200)区间，则继续等待后续事件上报
    } while (event.isClassContinue());

    final long endTime = SystemClock.elapsedRealtime();
    //对于执行时间超过500ms则会记录到log
    if (endTime - startTime > WARN_EXECUTE_DELAY_MS) {
        loge("NDC Command {" + logCmd + "} took too long (" + (endTime - startTime) + "ms)");
    }
    ...
    return events.toArray(new NativeDaemonEvent[events.size()]);
}

首先，将带执行的命令mSequenceNumber执行加1操作，再将cmd(例如 3 volume reset )写入到socket的输出流，通过循环与poll机制等待执行底层响应该操作结果，否则直到1分钟超时才结束该方法。即便收到底层的响应码，如果响应码属于[100,200)区间，则继续阻塞等待后续事件上报。

2.12 ResponseQueue.remove

[-> MountService.java ::ResponseQueue]

private static class ResponseQueue {
    public NativeDaemonEvent remove(int cmdNum, long timeoutMs, String logCmd) {
        PendingCmd found = null;
        synchronized (mPendingCmds) {
            //从mPendingCmds查询cmdNum
            for (PendingCmd pendingCmd : mPendingCmds) {
                if (pendingCmd.cmdNum == cmdNum) {
                    found = pendingCmd;
                    break;
                }
            }
            //如果已有的mPendingCmds中查询不到，则创建一个新的PendingCmd
            if (found == null) {
                found = new PendingCmd(cmdNum, logCmd);
                mPendingCmds.add(found);
            }
            found.availableResponseCount--;
            if (found.availableResponseCount == 0) mPendingCmds.remove(found);
        }
        NativeDaemonEvent result = null;
        try {
            //采用poll轮询方式等待底层上报该事件，直到1分钟超时
            result = found.responses.poll(timeoutMs, TimeUnit.MILLISECONDS);
        } catch (InterruptedException e) {}
        return result;
    }
}

mPendingCmds中的内容是如何添加的呢？其实是在NDC.listenToSocket循环监听到消息时添加的，则接下来看看监听过程。 这里用到poll，先来看看 responses = new ArrayBlockingQueue<NativeDaemonEvent>(10) ,这是一个长度为10的可阻塞队列。 这里的poll也是阻塞的方式来轮询事件。

responses.poll

[-> ArrayBlockingQueue.java]

public E poll(long timeout, TimeUnit unit) throws InterruptedException {
    long nanos = unit.toNanos(timeout);
    final ReentrantLock lock = this.lock;
    //可中断的锁等待
    lock.lockInterruptibly();
    try {
        //当队列长度为空，循环等待
        while (count == 0) {
            if (nanos <= 0)
                return null;
            nanos = notEmpty.awaitNanos(nanos);
        }
        return extract();
    } finally {
        lock.unlock();
    }
}

小知识：这里用到了ReentrantLock同步锁，该锁跟synchronized有功能有很相似，用于多线程并发访问。那么ReentrantLock与synchronized相比，ReentrantLock优势：

• ReentrantLock功能更为强大，比如有时间锁等候，可中断锁等候(lockInterruptibly)，锁投票等功能；
• ReentrantLock性能更好些；
• ReentrantLock提供可轮询的锁请求(tryLock)，相对不容易产生死锁；而synchronized只要进入，要么成功获取，要么一直阻塞等待。

ReentrantLock的劣势：

• lock必须在finally块显式地释放，否则如果代码抛出Exception，锁将一直得不到释放；对于synchronized而言，JVM或者ART虚拟机都会确保该锁能自动释放。
• synchronized锁，在dump线程转储时会记录锁信息，对于分析调试大有裨益；对于Lock来说，只是普通类，虚拟机无法识别。

2.13 listenToSocket

[-> NativeDaemonConnector.java]

private void listenToSocket() throws IOException {
    LocalSocket socket = null;

    try {
        socket = new LocalSocket();
        LocalSocketAddress address = determineSocketAddress();
        //建立与"/dev/socket/vold"的socket连接
        socket.connect(address);

        InputStream inputStream = socket.getInputStream();
        synchronized (mDaemonLock) {
            mOutputStream = socket.getOutputStream();
        }
        //建立连接后，回调MS.onDaemonConnected【见小节2.15】
        mCallbacks.onDaemonConnected();

        byte[] buffer = new byte[BUFFER_SIZE];
        int start = 0;

        while (true) {
            int count = inputStream.read(buffer, start, BUFFER_SIZE - start);
            ...

            for (int i = 0; i < count; i++) {
                if (buffer[i] == 0) {
                    final String rawEvent = new String(
                            buffer, start, i - start, StandardCharsets.UTF_8);

                    boolean releaseWl = false;
                    try {
                        //解析socket服务端发送的event
                        final NativeDaemonEvent event = NativeDaemonEvent.parseRawEvent(
                                rawEvent);

                        log("RCV <- {" + event + "}");
                        //当事件的响应码区间为[600,700)，则发送消息交由mCallbackHandler处理
                        if (event.isClassUnsolicited()) {
                            if (mCallbacks.onCheckHoldWakeLock(event.getCode())
                                    && mWakeLock != null) {
                                mWakeLock.acquire();
                                releaseWl = true;
                            }
                            if (mCallbackHandler.sendMessage(mCallbackHandler.obtainMessage(
                                    event.getCode(), event.getRawEvent()))) {
                                releaseWl = false;
                            }
                        } else {
                            //对于其他的响应码则添加到mResponseQueue队列【见小节2.14】
                            mResponseQueue.add(event.getCmdNumber(), event);
                        }
                    } catch (IllegalArgumentException e) {
                        log("Problem parsing message " + e);
                    } finally {
                        if (releaseWl) {
                            mWakeLock.acquire();
                        }
                    }
                    start = i + 1;
                }
            }
            ...
        }
    } catch (IOException ex) {
        throw ex;
    } finally {
        //收尾清理类工作，关闭mOutputStream, socket
        ...
    }
}

这里有一个动作是mResponseQueue.add()，通过该方法便能触发ResponseQueue.poll阻塞操作继续往下执行。

2.14 ResponseQueue.add

[-> NativeDaemonConnector.java]

private static class ResponseQueue {
    public void add(int cmdNum, NativeDaemonEvent response) {
       PendingCmd found = null;
       synchronized (mPendingCmds) {
           for (PendingCmd pendingCmd : mPendingCmds) {
               if (pendingCmd.cmdNum == cmdNum) {
                   found = pendingCmd;
                   break;
               }
           }
            //没有找到则创建相应的PendingCmd
           if (found == null) {
               while (mPendingCmds.size() >= mMaxCount) {
                   PendingCmd pendingCmd = mPendingCmds.remove();
               }
               found = new PendingCmd(cmdNum, null);
               mPendingCmds.add(found);
           }
           found.availableResponseCount++;
           if (found.availableResponseCount == 0) mPendingCmds.remove(found);
       }
       try {
           found.responses.put(response);
       } catch (InterruptedException e) { }
   }
}

responses.put

[-> ArrayBlockingQueue.java]

public void put(E e) throws InterruptedException {
    checkNotNull(e);
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly();
    try {
        //当队列满了则等待
        while (count == items.length)
            notFull.await();
        insert(e);
    } finally {
        lock.unlock();
    }
}

2.15 MS.onDaemonConnected

[-> MountService.java]

public void onDaemonConnected() {
    mDaemonConnected = true;
    mHandler.obtainMessage(H_DAEMON_CONNECTED).sendToTarget();
}

当前主线程发送消息H_DAEMON_CONNECTED给线程“MountService”，该线程收到消息后调用MountServiceHandler的handleMessage()相应分支后，进而调用handleDaemonConnected()方法。

private void handleDaemonConnected() {
    synchronized (mLock) {
        resetIfReadyAndConnectedLocked();
    }

    //类型为CountDownLatch，用于多线程同步，阻塞await()直到计数器为零
    mConnectedSignal.countDown();
    if (mConnectedSignal.getCount() != 0) {
        return;
    }

    //调用PMS来加载ASECs
    mPms.scanAvailableAsecs();

    //用于通知ASEC扫描已完成
    mAsecsScanned.countDown();
}

这里的PMS.scanAvailableAsecs()经过层层调用，最终核心工作还是通过MountService.getSecureContainerList。

[-> MountService.java]

public String[] getSecureContainerList() {
    enforcePermission(android.Manifest.permission.ASEC_ACCESS);
    //等待mConnectedSignal计数锁达到零
    waitForReady();
    //当没有挂载Primary卷设备，则弹出警告
    warnOnNotMounted();

    try {
        //向vold进程发送asec list命令
        return NativeDaemonEvent.filterMessageList(
                mConnector.executeForList("asec", "list"), VoldResponseCode.AsecListResult);
    } catch (NativeDaemonConnectorException e) {
        return new String[0];
    }
}

三、Vold

Vold是由开机过程中解析init.rc时启动：

on post-fs-data
    start vold

Vold的service定义如下：

service vold /system/bin/vold
    class core
    socket vold stream 0660 root mount
    socket cryptd stream 0660 root mount
    ioprio be 2

接下来便进入main（）方法：

3.1 main

[-> system/vold/Main.cpp]

int main(int argc, char** argv) {
    setenv("ANDROID_LOG_TAGS", "*:v", 1);
    android::base::InitLogging(argv, android::base::LogdLogger(android::base::SYSTEM));

    VolumeManager *vm;
    CommandListener *cl;
    CryptCommandListener *ccl;
    NetlinkManager *nm;

    //解析参数，设置contenxt
    parse_args(argc, argv);
    ...

    fcntl(android_get_control_socket("vold"), F_SETFD, FD_CLOEXEC);
    fcntl(android_get_control_socket("cryptd"), F_SETFD, FD_CLOEXEC);

    mkdir("/dev/block/vold", 0755);

    //用于cryptfs检查，并mount加密的文件系统
    klog_set_level(6);

    //创建单例对象VolumeManager 【见小节3.2.1】
    if (!(vm = VolumeManager::Instance())) {
        exit(1);
    }

    //创建单例对象NetlinkManager 【见小节3.3.1】
    if (!(nm = NetlinkManager::Instance())) {
        exit(1);
    }

    if (property_get_bool("vold.debug", false)) {
        vm->setDebug(true);
    }

    // 创建CommandListener对象 【见小节3.4.1】
    cl = new CommandListener();
    // 创建CryptCommandListener对象 【见小节3.5.1】
    ccl = new CryptCommandListener();

    //【见小节3.2.2】
    vm->setBroadcaster((SocketListener *) cl);
    //【见小节3.3.2】
    nm->setBroadcaster((SocketListener *) cl);

    if (vm->start()) { //【见小节3.2.3】
        exit(1);
    }

    process_config(vm)； //【见小节3.2.4】

    if (nm->start()) {  //【见小节3.3.3】
        exit(1);
    }

    coldboot("/sys/block");

    //启动响应命令的监听器 //【见小节3.4.2】
    if (cl->startListener()) {
        exit(1);
    }

    if (ccl->startListener()) {
        exit(1);
    }

    //Vold成为监听线程
    while(1) {
        sleep(1000);
    }

    exit(0);
}

该方法的主要功能是创建下面4个对象并启动

• VolumeManager
• NetlinkManager （NetlinkHandler）
• CommandListener
• CryptCommandListener

接下来分别说说几个类：

3.2 VolumeManager

3.2.1 创建

[-> VolumeManager.cpp]

VolumeManager *VolumeManager::Instance() {
    if (!sInstance)
        sInstance = new VolumeManager();
    return sInstance;
}

创建单例模式的VolumeManager对象

VolumeManager::VolumeManager() {
    mDebug = false;
    mActiveContainers = new AsecIdCollection();
    mBroadcaster = NULL;
    mUmsSharingCount = 0;
    mSavedDirtyRatio = -1;
    //当UMS获取时，则设置dirty ratio为0
    mUmsDirtyRatio = 0;
}

3.2.2 vm->setBroadcaster

void setBroadcaster(SocketListener *sl) {
     mBroadcaster = sl;
 }

将新创建的 CommandListener 对象sl赋值给vm对象的成员变量 mBroadcaster

3.2.3 vm->start

int VolumeManager::start() {
    //卸载所有设备，以提供最干净的环境
    unmountAll();

    //创建Emulated内部存储
    mInternalEmulated = std::shared_ptr<android::vold::VolumeBase>(
            new android::vold::EmulatedVolume("/data/media"));
    mInternalEmulated->create();
    return 0;
}

mInternalEmulated的据类型为 EmulatedVolume ，设备路径为 /data/media ，id和label为“emulated”，mMountFlags=0。 EmulatedVolume 继承于 VolumeBase

3.2.3.1 unmountAll

int VolumeManager::unmountAll() {
    std::lock_guard<std::mutex> lock(mLock);

    //卸载内部存储
    if (mInternalEmulated != nullptr) {
        mInternalEmulated->unmount();
    }
    //卸载外部存储
    for (auto disk : mDisks) {
        disk->unmountAll();
    }

    FILE* fp = setmntent("/proc/mounts", "r");
    if (fp == NULL) {
        return -errno;
    }

    std::list<std::string> toUnmount;
    mntent* mentry;
    while ((mentry = getmntent(fp)) != NULL) {
        if (strncmp(mentry->mnt_dir, "/mnt/", 5) == 0
                || strncmp(mentry->mnt_dir, "/storage/", 9) == 0) {
            toUnmount.push_front(std::string(mentry->mnt_dir));
        }
    }
    endmntent(fp);

    for (auto path : toUnmount) {
        //强制卸载
        android::vold::ForceUnmount(path);
    }

    return 0;
}

此处打开的”/proc/mounts”每一行内容依次是文件名，目录，类型，操作。例如：

/dev/fuse /mnt/runtime/default/emulated fuse rw,nosuid,nodev,...

该方法的主要工作是卸载：

• 内部存储mInternalEmulated；
• 外部存储mDisks，比如sdcard等；
• “/proc/mounts”中目录包含mnt或者storage的路径；

卸载内部存储:

status_t EmulatedVolume::doUnmount() {
    if (mFusePid > 0) {
        kill(mFusePid, SIGTERM);
        TEMP_FAILURE_RETRY(waitpid(mFusePid, nullptr, 0));
        mFusePid = 0;
    }

    KillProcessesUsingPath(getPath());
    //强制卸载fuse路径
    ForceUnmount(mFuseDefault);
    ForceUnmount(mFuseRead);
    ForceUnmount(mFuseWrite);

    rmdir(mFuseDefault.c_str());
    rmdir(mFuseRead.c_str());
    rmdir(mFuseWrite.c_str());

    mFuseDefault.clear();
    mFuseRead.clear();
    mFuseWrite.clear();

    return OK;
}

KillProcessesUsingPath 的功能很强大，通过文件path来查看其所在进程，并杀掉相应进程。当以下5处任意一处存在与path相同的地方，则会杀掉相应的进程：

• proc/<pid>/fd ，打开文件；
• proc/<pid>/maps 打开文件映射；
• proc/<pid>/cwd 链接文件；
• proc/<pid>/root 链接文件；
• proc/<pid>/exe 链接文件；

3.2.3.2 EV.create

[-> VolumeBase.cpp]

status_t VolumeBase::create() {
    mCreated = true;
    status_t res = doCreate();
    //通知VolumeCreated事件
    notifyEvent(ResponseCode::VolumeCreated,
            StringPrintf("%d /"%s/" /"%s/"", mType, mDiskId.c_str(), mPartGuid.c_str()));
    //设置为非挂载状态
    setState(State::kUnmounted);
    return res;
}


void VolumeBase::notifyEvent(int event, const std::string& value) {
    if (mSilent) return;
    //通过socket向MountService发送创建volume的命令(650)
    VolumeManager::Instance()->getBroadcaster()->sendBroadcast(event,
            StringPrintf("%s %s", getId().c_str(), value.c_str()).c_str(), false);
}

3.2.4 process_config(vm)

[-> system/vold/Main.cpp]

static int process_config(VolumeManager *vm) {
    //获取Fstab路径
    std::string path(android::vold::DefaultFstabPath());
    fstab = fs_mgr_read_fstab(path.c_str());
    ...

    bool has_adoptable = false;
    for (int i = 0; i < fstab->num_entries; i++) {
        if (fs_mgr_is_voldmanaged(&fstab->recs[i])) {
            if (fs_mgr_is_nonremovable(&fstab->recs[i])) {
                LOG(WARNING) << "nonremovable no longer supported; ignoring volume";
                continue;
            }

            std::string sysPattern(fstab->recs[i].blk_device);
            std::string nickname(fstab->recs[i].label);
            int flags = 0;

            if (fs_mgr_is_encryptable(&fstab->recs[i])) {
                flags |= android::vold::Disk::Flags::kAdoptable;
                has_adoptable = true;
            }
            if (fs_mgr_is_noemulatedsd(&fstab->recs[i])
                    || property_get_bool("vold.debug.default_primary", false)) {
                flags |= android::vold::Disk::Flags::kDefaultPrimary;
            }

            vm->addDiskSource(std::shared_ptr<VolumeManager::DiskSource>(
                    new VolumeManager::DiskSource(sysPattern, nickname, flags)));
        }
    }
    property_set("vold.has_adoptable", has_adoptable ? "1" : "0");
    return 0;
}

Fstab路径：首先通过 getprop ro.hardware ，比如高通芯片则为 qcom 那么Fstab路径就是 /fstab.qcom ，那么该文件的具体内容，例如(当然这个不同手机会有所不同)：

3.3 NetlinkManager

3.3.1 创建

[-> NetlinkManager.cpp]

NetlinkManager *NetlinkManager::Instance() {
    if (!sInstance)
        sInstance = new NetlinkManager();
    return sInstance;
}

3.3.2 nm->setBroadcaster

void setBroadcaster(SocketListener *sl) {
    mBroadcaster = sl;
}

3.3.3 nm->start

int NetlinkManager::start() {
    struct sockaddr_nl nladdr;
    int sz = 64 * 1024;
    int on = 1;

    memset(&nladdr, 0, sizeof(nladdr));
    nladdr.nl_family = AF_NETLINK;
    nladdr.nl_pid = getpid(); //记录当前进程的pid
    nladdr.nl_groups = 0xffffffff;

    //创建event socket
    if ((mSock = socket(PF_NETLINK, SOCK_DGRAM | SOCK_CLOEXEC,
            NETLINK_KOBJECT_UEVENT)) < 0) {
        return -1;
    }

    //设置uevent的SO_RCVBUFFORCE选项
    if (setsockopt(mSock, SOL_SOCKET, SO_RCVBUFFORCE, &sz, sizeof(sz)) < 0) {
        goto out;
    }

    //设置uevent的SO_PASSCRED选项
    if (setsockopt(mSock, SOL_SOCKET, SO_PASSCRED, &on, sizeof(on)) < 0) {
        goto out;
    }
    //绑定uevent socket
    if (bind(mSock, (struct sockaddr *) &nladdr, sizeof(nladdr)) < 0) {
        goto out;
    }

    //创建NetlinkHandler【见小节3.3.4】
    mHandler = new NetlinkHandler(mSock);
    //启动NetlinkHandler【见小节3.3.5】
    if (mHandler->start()) {
        goto out;
    }

    return 0;

out:
    close(mSock);
    return -1;
}

3.3.4 NetlinkHandler

NetlinkHandler继承于 NetlinkListener ， NetlinkListener 继承于 SocketListener 。new NetlinkHandler(mSock)中参数mSock是用于与Kernel进行通信的socket对象。由于这个继承关系，当NetlinkHandler初始化时会调用基类的初始化，最终调用到：

[-> SocketListener.cpp]

SocketListener::SocketListener(int socketFd, bool listen) {
    //listen=false
    init(NULL, socketFd, listen, false);
}

void SocketListener::init(const char *socketName, int socketFd, bool listen, bool useCmdNum) {
    mListen = listen;
    mSocketName = socketName;
    //用于监听Kernel发送过程的uevent事件
    mSock = socketFd;
    mUseCmdNum = useCmdNum;
    //初始化同步锁
    pthread_mutex_init(&mClientsLock, NULL);
    //创建socket通信的client端
    mClients = new SocketClientCollection();
}

到此，mListen = false; mSocketName = NULL; mUseCmdNum = false。 另外，这里用到的同步锁，用于控制多线程并发访问。 接着在来看看start过程：

3.3.5 NH->start

[-> NetlinkHandler.cpp]

int NetlinkHandler::start() {
    return this->startListener();
}

[-> SocketListener.cpp]

int SocketListener::startListener() {
    return startListener(4);
}

int SocketListener::startListener(int backlog) {
    ...
    //mListen =false
    if (mListen && listen(mSock, backlog) < 0) {
        return -1;
    } else if (!mListen)
        //创建SocketClient对象，并加入到mClients队列
        mClients->push_back(new SocketClient(mSock, false, mUseCmdNum));

    //创建匿名管道
    if (pipe(mCtrlPipe)) {
        return -1;
    }

    //创建工作线程，线程运行函数threadStart【见小节3.3.6】
    if (pthread_create(&mThread, NULL, SocketListener::threadStart, this)) {
        return -1;
    }

    return 0;
}

mCtrlPipe是匿名管道，这是一个二元数组，mCtrlPipe[0]从管道读数据，mCtrlPipe[1]从管道写数据。

3.3.6 threadStart

[-> SocketListener.cpp]

void *SocketListener::threadStart(void *obj) {
    SocketListener *me = reinterpret_cast<SocketListener *>(obj);
    //【见小节3.3.7】
    me->runListener();
    pthread_exit(NULL); //线程退出
    return NULL;
}

3.3.7 SL->runListener

[-> SocketListener.cpp]

void SocketListener::runListener() {
    SocketClientCollection pendingList;
    while(1) {
        SocketClientCollection::iterator it;
        fd_set read_fds;
        int rc = 0;
        int max = -1;

        FD_ZERO(&read_fds);

        if (mListen) {
            max = mSock;
            FD_SET(mSock, &read_fds);
        }

        FD_SET(mCtrlPipe[0], &read_fds);
        if (mCtrlPipe[0] > max)
            max = mCtrlPipe[0];

        pthread_mutex_lock(&mClientsLock);
        for (it = mClients->begin(); it != mClients->end(); ++it) {
            // NB: calling out to an other object with mClientsLock held (safe)
            int fd = (*it)->getSocket();
            FD_SET(fd, &read_fds);
            if (fd > max) {
                max = fd;
            }
        }
        pthread_mutex_unlock(&mClientsLock);
        SLOGV("mListen=%d, max=%d, mSocketName=%s", mListen, max, mSocketName);
        if ((rc = select(max + 1, &read_fds, NULL, NULL, NULL)) < 0) {
            if (errno == EINTR)
                continue;
            SLOGE("select failed (%s) mListen=%d, max=%d", strerror(errno), mListen, max);
            sleep(1);
            continue;
        } else if (!rc)
            continue;

        if (FD_ISSET(mCtrlPipe[0], &read_fds)) {
            char c = CtrlPipe_Shutdown;
            TEMP_FAILURE_RETRY(read(mCtrlPipe[0], &c, 1));
            if (c == CtrlPipe_Shutdown) {
                break;
            }
            continue;
        }
        if (mListen && FD_ISSET(mSock, &read_fds)) {
            struct sockaddr addr;
            socklen_t alen;
            int c;

            do {
                alen = sizeof(addr);
                c = accept(mSock, &addr, &alen);
                SLOGV("%s got %d from accept", mSocketName, c);
            } while (c < 0 && errno == EINTR);
            if (c < 0) {
                SLOGE("accept failed (%s)", strerror(errno));
                sleep(1);
                continue;
            }
            fcntl(c, F_SETFD, FD_CLOEXEC);
            pthread_mutex_lock(&mClientsLock);
            mClients->push_back(new SocketClient(c, true, mUseCmdNum));
            pthread_mutex_unlock(&mClientsLock);
        }

        /* Add all active clients to the pending list first */
        pendingList.clear();
        pthread_mutex_lock(&mClientsLock);
        for (it = mClients->begin(); it != mClients->end(); ++it) {
            SocketClient* c = *it;
            // NB: calling out to an other object with mClientsLock held (safe)
            int fd = c->getSocket();
            if (FD_ISSET(fd, &read_fds)) {
                pendingList.push_back(c);
                c->incRef();
            }
        }
        pthread_mutex_unlock(&mClientsLock);

        /* Process the pending list, since it is owned by the thread,
         * there is no need to lock it */
        while (!pendingList.empty()) {
            /* Pop the first item from the list */
            it = pendingList.begin();
            SocketClient* c = *it;
            pendingList.erase(it);
            /* Process it, if false is returned, remove from list */
            if (!onDataAvailable(c)) {
                release(c, false);
            }
            c->decRef();
        }
    }
}

3.4 CommandListener

3.4.1 创建

[-> CommandListener.cpp]

CommandListener::CommandListener() :
                 FrameworkListener("vold", true) {
    registerCmd(new DumpCmd());
    registerCmd(new VolumeCmd());
    registerCmd(new AsecCmd());
    registerCmd(new ObbCmd());
    registerCmd(new StorageCmd());
    registerCmd(new FstrimCmd());
}

3.4.1.1 FrameworkListener

[-> FrameworkListener.cpp]

FrameworkListener::FrameworkListener(const char *socketName, bool withSeq) :
                            SocketListener(socketName, true, withSeq) {
    init(socketName, withSeq);
}

void FrameworkListener::init(const char *socketName UNUSED, bool withSeq) {
    mCommands = new FrameworkCommandCollection();
    errorRate = 0;
    mCommandCount = 0;
    mWithSeq = withSeq; //true
}

3.4.1.2 SocketListener

[-> SocketListener.cpp]

SocketListener::SocketListener(const char *socketName, bool listen, bool useCmdNum) {
    init(socketName, -1, listen, useCmdNum);
}

void SocketListener::init(const char *socketName, int socketFd, bool listen, bool useCmdNum) {
    mListen = listen; //true
    mSocketName = socketName; //"vold"
    mSock = socketFd; // -1
    mUseCmdNum = useCmdNum; //true
    pthread_mutex_init(&mClientsLock, NULL);
    mClients = new SocketClientCollection();
}

socket名为“vold”

3.4.1.3 registerCmd

void FrameworkListener::registerCmd(FrameworkCommand *cmd) {
    mCommands->push_back(cmd);
}

CommandListener::VolumeCmd::VolumeCmd() :
             VoldCommand("volume") {
}

创建这些对象 DumpCmd，VolumeCmd，AsecCmd，ObbCmd，StorageCmd，FstrimCmd，并都加入到mCommands队列。

3.4.2 cl->startListener

int SocketListener::startListener() {
    return startListener(4);
}

int SocketListener::startListener(int backlog) {

    if (!mSocketName && mSock == -1) {
        ...
    } else if (mSocketName) {
        //获取“vold”所对应的句柄
        if ((mSock = android_get_control_socket(mSocketName)) < 0) {
            return -1;
        }
        fcntl(mSock, F_SETFD, FD_CLOEXEC);
    }

    //CL开始监听
    if (mListen && listen(mSock, backlog) < 0) {
        return -1;
    }
    ...
    //创建匿名管道
    if (pipe(mCtrlPipe)) {
        return -1;
    }

    //创建工作线程
    if (pthread_create(&mThread, NULL, SocketListener::threadStart, this)) {
        return -1;
    }

    return 0;
}

四、 案例分析

MountService根据收到消息会发送VM处理。 例如待SD卡插入后，VM会将(来自NM的“Disk Insert”的)消息发送给MountService，而后MountService则发送“Mount”指令给Vold，指示它挂载这个SD卡。有些应用程序需要检测外部存储卡的插入/拔出事件，这些事件是由MountService通过Intent广播发出的，例如外部存储卡插入后，MountService就会发送ACTION_MEDIA_MOUNTED消息。

4.1 Kernel上报处理流程

插入SD卡动作： 通过硬件驱动会引起Kernel向NM发送uevent，NM转发消息给VM，再通过CL发送给MountService；MountService收到后则向Vold发送Mount命令，来挂载SD卡。

整个流程： Kernel Uevent -> volumeManager -> NetlinkManager -> CommandListener -> NativeDaemonConnector -> MountService

Kernel发出Uevnt事件 SocketListener::startListener SocketListener::threadStart SocketListener::runListener NetlinkListener.onDataAvailable NetlinkHandler.onEvent VM.handleBlockEvent Disk.create Disk::notifyEvent SocketListener::sendBroadcast 发送socket事件给上层

4.2 framework下发处理流程

MountServic发送socket，执行mount SocketListener::startListener SocketListener::threadStart SocketListener::runListener FrameworkListener::onDataAvailable FrameworkListener::dispatchCommand VolumeCmd.runCommand VolumeBase.mount EmulatedVolume.doMount(内置) PublicVolume.doMount(外置) vfat::Check vfat::Mount fork (/sdcard)

4.3 startUser流程

AMS.systemReady goingCallback.run(); onBootPhase(550) MountService.systemReady(); => mSystemReady = true MountService.resetIfReadyAndConnectedLocked mConnector.execute(“volume”, “reset”); RCV <- {650 emulated –> VOLUME_CREATED RCV <- {650 public –> VOLUME_CREATED mConnector.execute(“volume”, “user_added”, user.id, user.serialNumber); mConnector.execute(“volume”, “user_started”, userId);

mSystemServiceManager.startUser(mCurrentUserId);
    MountService.onStartUser
        mConnector.execute("volume", "user_started", userId);
        mHandler.obtainMessage(H_VOLUME_BROADCAST, userVol).sendToTarget();
            MountServiceHandler.handleMessage -->case H_VOLUME_BROADCAS

NDC监听流程：

NDC.istenToSocket
     NativeDaemonConnector.handleMessage -->
        MountService.onEvent
            onEventLocked -->
                case VoldResponseCode.VOLUME_CREATED
                    MountService.onVolumeCreatedLocked
                        MountServiceHandler.handleMessage -->
                            case H_VOLUME_MOUNT
                                mConnector.execute("volume", "mount");

系列文章

Android存储系统之源码篇
Android存储系统之架构篇

来自：http://gityuan.com/2016/07/17/android-io/