diff --git a/include/media/AudioRecord.h b/include/media/AudioRecord.h
index ca57f9e..437a89c 100644
--- a/include/media/AudioRecord.h
+++ b/include/media/AudioRecord.h
@@ -341,7 +341,7 @@
     private:
         friend class AudioRecord;
         virtual bool        threadLoop();
-        virtual status_t    readyToRun() { return NO_ERROR; }
+        virtual status_t    readyToRun();
         virtual void        onFirstRef() {}
         AudioRecord& mReceiver;
     };
@@ -359,7 +359,9 @@
     sp<IAudioRecord>        mAudioRecord;
     sp<IMemory>             mCblkMemory;
     sp<ClientRecordThread>  mClientRecordThread;
+    status_t                mReadyToRun;
     Mutex                   mLock;
+    Condition               mCondition;
 
     uint32_t                mFrameCount;
 
diff --git a/include/media/IAudioRecord.h b/include/media/IAudioRecord.h
index 46735de..7869020 100644
--- a/include/media/IAudioRecord.h
+++ b/include/media/IAudioRecord.h
@@ -37,8 +37,9 @@
 
     /* After it's created the track is not active. Call start() to
      * make it active. If set, the callback will start being called.
+     * tid identifies the client callback thread, or 0 if not needed.
      */
-    virtual status_t    start() = 0;
+    virtual status_t    start(pid_t tid) = 0;
 
     /* Stop a track. If set, the callback will cease being called and
      * obtainBuffer will return an error. Buffers that are already released 
diff --git a/include/media/IAudioTrack.h b/include/media/IAudioTrack.h
index b83e552..b346722 100644
--- a/include/media/IAudioTrack.h
+++ b/include/media/IAudioTrack.h
@@ -40,8 +40,9 @@
 
     /* After it's created the track is not active. Call start() to
      * make it active. If set, the callback will start being called.
+     * tid identifies the client callback thread, or 0 if not needed.
      */
-    virtual status_t    start() = 0;
+    virtual status_t    start(pid_t tid) = 0;
 
     /* Stop a track. If set, the callback will cease being called and
      * obtainBuffer will return an error. Buffers that are already released 
diff --git a/media/libmedia/AudioRecord.cpp b/media/libmedia/AudioRecord.cpp
index b74b3e3..a4068ff 100644
--- a/media/libmedia/AudioRecord.cpp
+++ b/media/libmedia/AudioRecord.cpp
@@ -304,10 +304,25 @@
     if (mActive == 0) {
         mActive = 1;
 
+        pid_t tid;
+        if (t != 0) {
+            mReadyToRun = WOULD_BLOCK;
+            t->run("ClientRecordThread", ANDROID_PRIORITY_AUDIO);
+            tid = t->getTid();  // pid_t is unknown until run()
+            ALOGV("getTid=%d", tid);
+            if (tid == -1) {
+                tid = 0;
+            }
+            // thread blocks in readyToRun()
+        } else {
+            tid = 0;    // not gettid()
+        }
+
         cblk->lock.lock();
         if (!(cblk->flags & CBLK_INVALID_MSK)) {
             cblk->lock.unlock();
-            ret = mAudioRecord->start();
+            ALOGV("mAudioRecord->start(tid=%d)", tid);
+            ret = mAudioRecord->start(tid);
             cblk->lock.lock();
             if (ret == DEAD_OBJECT) {
                 android_atomic_or(CBLK_INVALID_ON, &cblk->flags);
@@ -322,7 +337,9 @@
             cblk->bufferTimeoutMs = MAX_RUN_TIMEOUT_MS;
             cblk->waitTimeMs = 0;
             if (t != 0) {
-                t->run("ClientRecordThread", ANDROID_PRIORITY_AUDIO);
+                // thread unblocks in readyToRun() and returns NO_ERROR
+                mReadyToRun = NO_ERROR;
+                mCondition.signal();
             } else {
                 mPreviousPriority = getpriority(PRIO_PROCESS, 0);
                 mPreviousSchedulingGroup = androidGetThreadSchedulingGroup(0);
@@ -330,6 +347,9 @@
             }
         } else {
             mActive = 0;
+            // thread unblocks in readyToRun() and returns NO_INIT
+            mReadyToRun = NO_INIT;
+            mCondition.signal();
         }
     }
 
@@ -522,7 +542,7 @@
                     ALOGW(   "obtainBuffer timed out (is the CPU pegged?) "
                             "user=%08x, server=%08x", cblk->user, cblk->server);
                     cblk->lock.unlock();
-                    result = mAudioRecord->start();
+                    result = mAudioRecord->start(0);    // callback thread hasn't changed
                     cblk->lock.lock();
                     if (result == DEAD_OBJECT) {
                         android_atomic_or(CBLK_INVALID_ON, &cblk->flags);
@@ -760,7 +780,7 @@
         result = openRecord_l(cblk->sampleRate, mFormat, mChannelMask,
                 mFrameCount, mFlags, getInput_l());
         if (result == NO_ERROR) {
-            result = mAudioRecord->start();
+            result = mAudioRecord->start(0);    // callback thread hasn't changed
         }
         if (result != NO_ERROR) {
             mActive = false;
@@ -811,6 +831,15 @@
     return mReceiver.processAudioBuffer(this);
 }
 
+status_t AudioRecord::ClientRecordThread::readyToRun()
+{
+    AutoMutex(mReceiver.mLock);
+    while (mReceiver.mReadyToRun == WOULD_BLOCK) {
+        mReceiver.mCondition.wait(mReceiver.mLock);
+    }
+    return mReceiver.mReadyToRun;
+}
+
 // -------------------------------------------------------------------------
 
 }; // namespace android
diff --git a/media/libmedia/AudioTrack.cpp b/media/libmedia/AudioTrack.cpp
index 087d7b2..2518921 100644
--- a/media/libmedia/AudioTrack.cpp
+++ b/media/libmedia/AudioTrack.cpp
@@ -362,18 +362,26 @@
         cblk->bufferTimeoutMs = MAX_STARTUP_TIMEOUT_MS;
         cblk->waitTimeMs = 0;
         android_atomic_and(~CBLK_DISABLED_ON, &cblk->flags);
+        pid_t tid;
         if (t != 0) {
             t->run("AudioTrackThread", ANDROID_PRIORITY_AUDIO);
+            tid = t->getTid();  // pid_t is unknown until run()
+            ALOGV("getTid=%d", tid);
+            if (tid == -1) {
+                tid = 0;
+            }
         } else {
             mPreviousPriority = getpriority(PRIO_PROCESS, 0);
             mPreviousSchedulingGroup = androidGetThreadSchedulingGroup(0);
             androidSetThreadPriority(0, ANDROID_PRIORITY_AUDIO);
+            tid = 0;    // not gettid()
         }
 
         ALOGV("start %p before lock cblk %p", this, mCblk);
         if (!(cblk->flags & CBLK_INVALID_MSK)) {
             cblk->lock.unlock();
-            status = mAudioTrack->start();
+            ALOGV("mAudioTrack->start(tid=%d)", tid);
+            status = mAudioTrack->start(tid);
             cblk->lock.lock();
             if (status == DEAD_OBJECT) {
                 android_atomic_or(CBLK_INVALID_ON, &cblk->flags);
@@ -895,7 +903,7 @@
                                 "user=%08x, server=%08x", this, cblk->user, cblk->server);
                         //unlock cblk mutex before calling mAudioTrack->start() (see issue #1617140)
                         cblk->lock.unlock();
-                        result = mAudioTrack->start();
+                        result = mAudioTrack->start(0); // callback thread hasn't changed
                         cblk->lock.lock();
                         if (result == DEAD_OBJECT) {
                             android_atomic_or(CBLK_INVALID_ON, &cblk->flags);
@@ -927,7 +935,7 @@
     if (mActive && (cblk->flags & CBLK_DISABLED_MSK)) {
         android_atomic_and(~CBLK_DISABLED_ON, &cblk->flags);
         ALOGW("obtainBuffer() track %p disabled, restarting", this);
-        mAudioTrack->start();
+        mAudioTrack->start(0);  // callback thread hasn't changed
     }
 
     cblk->waitTimeMs = 0;
@@ -1218,7 +1226,7 @@
                 }
             }
             if (mActive) {
-                result = mAudioTrack->start();
+                result = mAudioTrack->start(0); // callback thread hasn't changed
                 ALOGW_IF(result != NO_ERROR, "restoreTrack_l() start() failed status %d", result);
             }
             if (fromStart && result == NO_ERROR) {
diff --git a/media/libmedia/IAudioRecord.cpp b/media/libmedia/IAudioRecord.cpp
index 8c7a960..6b473c9 100644
--- a/media/libmedia/IAudioRecord.cpp
+++ b/media/libmedia/IAudioRecord.cpp
@@ -42,10 +42,11 @@
     {
     }
     
-    virtual status_t start()
+    virtual status_t start(pid_t tid)
     {
         Parcel data, reply;
         data.writeInterfaceToken(IAudioRecord::getInterfaceDescriptor());
+        data.writeInt32(tid);
         status_t status = remote()->transact(START, data, &reply);
         if (status == NO_ERROR) {
             status = reply.readInt32();
@@ -90,7 +91,7 @@
         } break;
         case START: {
             CHECK_INTERFACE(IAudioRecord, data, reply);
-            reply->writeInt32(start());
+            reply->writeInt32(start(data.readInt32()));
             return NO_ERROR;
         } break;
         case STOP: {
diff --git a/media/libmedia/IAudioTrack.cpp b/media/libmedia/IAudioTrack.cpp
index e618619..3724642 100644
--- a/media/libmedia/IAudioTrack.cpp
+++ b/media/libmedia/IAudioTrack.cpp
@@ -58,10 +58,11 @@
         return cblk;
     }
 
-    virtual status_t start()
+    virtual status_t start(pid_t tid)
     {
         Parcel data, reply;
         data.writeInterfaceToken(IAudioTrack::getInterfaceDescriptor());
+        data.writeInt32(tid);
         status_t status = remote()->transact(START, data, &reply);
         if (status == NO_ERROR) {
             status = reply.readInt32();
@@ -130,7 +131,7 @@
         } break;
         case START: {
             CHECK_INTERFACE(IAudioTrack, data, reply);
-            reply->writeInt32(start());
+            reply->writeInt32(start(data.readInt32()));
             return NO_ERROR;
         } break;
         case STOP: {
diff --git a/services/audioflinger/AudioFlinger.cpp b/services/audioflinger/AudioFlinger.cpp
index 5c964b2..fb163d1 100644
--- a/services/audioflinger/AudioFlinger.cpp
+++ b/services/audioflinger/AudioFlinger.cpp
@@ -3494,11 +3494,11 @@
     return false;
 }
 
-status_t AudioFlinger::PlaybackThread::Track::start()
+status_t AudioFlinger::PlaybackThread::Track::start(pid_t tid)
 {
     status_t status = NO_ERROR;
-    ALOGV("start(%d), calling pid %d session %d",
-            mName, IPCThreadState::self()->getCallingPid(), mSessionId);
+    ALOGV("start(%d), calling pid %d session %d tid %d",
+            mName, IPCThreadState::self()->getCallingPid(), mSessionId, tid);
     sp<ThreadBase> thread = mThread.promote();
     if (thread != 0) {
         Mutex::Autolock _l(thread->mLock);
@@ -3717,12 +3717,12 @@
     return NOT_ENOUGH_DATA;
 }
 
-status_t AudioFlinger::RecordThread::RecordTrack::start()
+status_t AudioFlinger::RecordThread::RecordTrack::start(pid_t tid)
 {
     sp<ThreadBase> thread = mThread.promote();
     if (thread != 0) {
         RecordThread *recordThread = (RecordThread *)thread.get();
-        return recordThread->start(this);
+        return recordThread->start(this, tid);
     } else {
         return BAD_VALUE;
     }
@@ -3789,9 +3789,9 @@
     clearBufferQueue();
 }
 
-status_t AudioFlinger::PlaybackThread::OutputTrack::start()
+status_t AudioFlinger::PlaybackThread::OutputTrack::start(pid_t tid)
 {
-    status_t status = Track::start();
+    status_t status = Track::start(tid);
     if (status != NO_ERROR) {
         return status;
     }
@@ -3821,7 +3821,7 @@
     uint32_t waitTimeLeftMs = mSourceThread->waitTimeMs();
 
     if (!mActive && frames != 0) {
-        start();
+        start(0);
         sp<ThreadBase> thread = mThread.promote();
         if (thread != 0) {
             MixerThread *mixerThread = (MixerThread *)thread.get();
@@ -4056,8 +4056,8 @@
     return mTrack->getCblk();
 }
 
-status_t AudioFlinger::TrackHandle::start() {
-    return mTrack->start();
+status_t AudioFlinger::TrackHandle::start(pid_t tid) {
+    return mTrack->start(tid);
 }
 
 void AudioFlinger::TrackHandle::stop() {
@@ -4179,9 +4179,9 @@
     return mRecordTrack->getCblk();
 }
 
-status_t AudioFlinger::RecordHandle::start() {
+status_t AudioFlinger::RecordHandle::start(pid_t tid) {
     ALOGV("RecordHandle::start()");
-    return mRecordTrack->start();
+    return mRecordTrack->start(tid);
 }
 
 void AudioFlinger::RecordHandle::stop() {
@@ -4473,9 +4473,9 @@
     return track;
 }
 
-status_t AudioFlinger::RecordThread::start(RecordThread::RecordTrack* recordTrack)
+status_t AudioFlinger::RecordThread::start(RecordThread::RecordTrack* recordTrack, pid_t tid)
 {
-    ALOGV("RecordThread::start");
+    ALOGV("RecordThread::start tid=%d", tid);
     sp <ThreadBase> strongMe = this;
     status_t status = NO_ERROR;
     {
diff --git a/services/audioflinger/AudioFlinger.h b/services/audioflinger/AudioFlinger.h
index fdcd916..aa0ee76 100644
--- a/services/audioflinger/AudioFlinger.h
+++ b/services/audioflinger/AudioFlinger.h
@@ -314,7 +314,7 @@
                                         int sessionId);
             virtual             ~TrackBase();
 
-            virtual status_t    start() = 0;
+            virtual status_t    start(pid_t tid) = 0;
             virtual void        stop() = 0;
                     sp<IMemory> getCblk() const { return mCblkMemory; }
                     audio_track_cblk_t* cblk() const { return mCblk; }
@@ -586,7 +586,7 @@
             virtual             ~Track();
 
                     void        dump(char* buffer, size_t size);
-            virtual status_t    start();
+            virtual status_t    start(pid_t tid);
             virtual void        stop();
                     void        pause();
 
@@ -668,7 +668,7 @@
                                         int frameCount);
             virtual             ~OutputTrack();
 
-            virtual status_t    start();
+            virtual status_t    start(pid_t tid);
             virtual void        stop();
                     bool        write(int16_t* data, uint32_t frames);
                     bool        bufferQueueEmpty() const { return (mBufferQueue.size() == 0) ? true : false; }
@@ -905,7 +905,7 @@
                             TrackHandle(const sp<PlaybackThread::Track>& track);
         virtual             ~TrackHandle();
         virtual sp<IMemory> getCblk() const;
-        virtual status_t    start();
+        virtual status_t    start(pid_t tid);
         virtual void        stop();
         virtual void        flush();
         virtual void        mute(bool);
@@ -943,7 +943,7 @@
                                         int sessionId);
             virtual             ~RecordTrack();
 
-            virtual status_t    start();
+            virtual status_t    start(pid_t tid);
             virtual void        stop();
 
                     bool        overflow() { bool tmp = mOverflow; mOverflow = false; return tmp; }
@@ -988,6 +988,7 @@
                         status_t *status);
 
                 status_t    start(RecordTrack* recordTrack);
+                status_t    start(RecordTrack* recordTrack, pid_t tid);
                 void        stop(RecordTrack* recordTrack);
                 status_t    dump(int fd, const Vector<String16>& args);
                 AudioStreamIn* getInput() const;
@@ -1028,7 +1029,7 @@
         RecordHandle(const sp<RecordThread::RecordTrack>& recordTrack);
         virtual             ~RecordHandle();
         virtual sp<IMemory> getCblk() const;
-        virtual status_t    start();
+        virtual status_t    start(pid_t tid);
         virtual void        stop();
         virtual status_t onTransact(
             uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags);