diff --git a/libs/ui/SharedBufferStack.cpp b/libs/ui/SharedBufferStack.cpp
new file mode 100644
index 0000000..5995af5
--- /dev/null
+++ b/libs/ui/SharedBufferStack.cpp
@@ -0,0 +1,352 @@
+/*
+ * 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.
+ */
+
+#define LOG_TAG "SharedBufferStack"
+
+#include <stdint.h>
+#include <sys/types.h>
+
+#include <utils/Debug.h>
+#include <utils/Log.h>
+#include <utils/threads.h>
+
+#include <private/ui/SharedBufferStack.h>
+
+#include <ui/Rect.h>
+#include <ui/Region.h>
+
+#define DEBUG_ATOMICS 0
+
+namespace android {
+// ----------------------------------------------------------------------------
+
+SharedClient::SharedClient()
+    : lock(Mutex::SHARED)
+{
+}
+
+SharedClient::~SharedClient() {
+}
+
+
+// these functions are used by the clients
+status_t SharedClient::validate(size_t i) const {
+    if (uint32_t(i) >= uint32_t(NUM_LAYERS_MAX))
+        return BAD_INDEX;
+    return surfaces[i].status;
+}
+
+uint32_t SharedClient::getIdentity(size_t token) const {
+    return uint32_t(surfaces[token].identity);
+}
+
+status_t SharedClient::setIdentity(size_t token, uint32_t identity) {
+    if (token >= NUM_LAYERS_MAX)
+        return BAD_INDEX;
+    surfaces[token].identity = identity;
+    return NO_ERROR;
+}
+
+// ----------------------------------------------------------------------------
+
+
+SharedBufferStack::SharedBufferStack()
+    : inUse(-1), identity(-1), status(NO_ERROR)
+{
+}
+
+status_t SharedBufferStack::setDirtyRegion(int buffer, const Region& dirty)
+{
+    if (uint32_t(buffer) >= NUM_BUFFER_MAX)
+        return BAD_INDEX;
+
+    // in the current implementation we only send a single rectangle
+    const Rect bounds(dirty.getBounds());
+    FlatRegion& reg(dirtyRegion[buffer]);
+    reg.count = 1;
+    reg.rects[0] = uint16_t(bounds.left);
+    reg.rects[1] = uint16_t(bounds.top);
+    reg.rects[2] = uint16_t(bounds.right);
+    reg.rects[3] = uint16_t(bounds.bottom);
+    return NO_ERROR;
+}
+
+Region SharedBufferStack::getDirtyRegion(int buffer) const
+{
+    Region res;
+    if (uint32_t(buffer) >= NUM_BUFFER_MAX)
+        return res;
+
+    const FlatRegion& reg(dirtyRegion[buffer]);
+    res.set(Rect(reg.rects[0], reg.rects[1], reg.rects[2], reg.rects[3]));
+    return res;
+}
+
+// ----------------------------------------------------------------------------
+
+SharedBufferBase::SharedBufferBase(SharedClient* sharedClient,
+        int surface, int num)
+    : mSharedClient(sharedClient), 
+      mSharedStack(sharedClient->surfaces + surface),
+      mNumBuffers(num)
+{
+}
+
+SharedBufferBase::~SharedBufferBase()
+{
+}
+
+uint32_t SharedBufferBase::getIdentity()
+{
+    SharedBufferStack& stack( *mSharedStack );
+    return stack.identity;
+}
+
+size_t SharedBufferBase::getFrontBuffer() const
+{
+    SharedBufferStack& stack( *mSharedStack );
+    return size_t( stack.head );
+}
+
+String8 SharedBufferBase::dump(char const* prefix) const
+{
+    const size_t SIZE = 1024;
+    char buffer[SIZE];
+    String8 result;
+    SharedBufferStack& stack( *mSharedStack );
+    snprintf(buffer, SIZE, 
+            "%s[ head=%2d, available=%2d, queued=%2d ] "
+            "reallocMask=%08x, inUse=%2d, identity=%d, status=%d\n",
+            prefix, stack.head, stack.available, stack.queued, 
+            stack.reallocMask, stack.inUse, stack.identity, stack.status);
+    result.append(buffer);
+    return result;
+}
+
+
+// ============================================================================
+// conditions and updates
+// ============================================================================
+
+SharedBufferClient::DequeueCondition::DequeueCondition(
+        SharedBufferClient* sbc) : ConditionBase(sbc)  { 
+}
+bool SharedBufferClient::DequeueCondition::operator()() {
+    return stack.available > 0;
+}
+
+SharedBufferClient::LockCondition::LockCondition(
+        SharedBufferClient* sbc, int buf) : ConditionBase(sbc), buf(buf) { 
+}
+bool SharedBufferClient::LockCondition::operator()() {
+    return (buf != stack.head || 
+            (stack.queued > 0 && stack.inUse != buf));
+}
+
+SharedBufferServer::ReallocateCondition::ReallocateCondition(
+        SharedBufferBase* sbb, int buf) : ConditionBase(sbb), buf(buf) { 
+}
+bool SharedBufferServer::ReallocateCondition::operator()() {
+    // TODO: we should also check that buf has been dequeued
+    return (buf != stack.head);
+}
+
+// ----------------------------------------------------------------------------
+
+SharedBufferClient::QueueUpdate::QueueUpdate(SharedBufferBase* sbb)
+    : UpdateBase(sbb) {    
+}
+ssize_t SharedBufferClient::QueueUpdate::operator()() {
+    android_atomic_inc(&stack.queued);
+    return NO_ERROR;
+}
+
+SharedBufferClient::UndoDequeueUpdate::UndoDequeueUpdate(SharedBufferBase* sbb)
+    : UpdateBase(sbb) {    
+}
+ssize_t SharedBufferClient::UndoDequeueUpdate::operator()() {
+    android_atomic_inc(&stack.available);
+    return NO_ERROR;
+}
+
+SharedBufferServer::UnlockUpdate::UnlockUpdate(
+        SharedBufferBase* sbb, int lockedBuffer)
+    : UpdateBase(sbb), lockedBuffer(lockedBuffer) {
+}
+ssize_t SharedBufferServer::UnlockUpdate::operator()() {
+    if (stack.inUse != lockedBuffer) {
+        LOGE("unlocking %d, but currently locked buffer is %d",
+                lockedBuffer, stack.inUse);
+        return BAD_VALUE;
+    }
+    android_atomic_write(-1, &stack.inUse);
+    return NO_ERROR;
+}
+
+SharedBufferServer::RetireUpdate::RetireUpdate(
+        SharedBufferBase* sbb, int numBuffers)
+    : UpdateBase(sbb), numBuffers(numBuffers) {
+}
+ssize_t SharedBufferServer::RetireUpdate::operator()() {
+    // head is only written in this function, which is single-thread.
+    int32_t head = stack.head;
+
+    // Preventively lock the current buffer before updating queued.
+    android_atomic_write(head, &stack.inUse);
+
+    // Decrement the number of queued buffers 
+    int32_t queued;
+    do {
+        queued = stack.queued;
+        if (queued == 0) {
+            return NOT_ENOUGH_DATA;
+        }
+    } while (android_atomic_cmpxchg(queued, queued-1, &stack.queued));
+    
+    // update the head pointer
+    head = ((head+1 >= numBuffers) ? 0 : head+1);
+
+    // lock the buffer before advancing head, which automatically unlocks
+    // the buffer we preventively locked upon entering this function
+    android_atomic_write(head, &stack.inUse);
+
+    // advance head
+    android_atomic_write(head, &stack.head);
+    
+    // now that head has moved, we can increment the number of available buffers
+    android_atomic_inc(&stack.available);
+    return head;
+}
+
+// ============================================================================
+
+SharedBufferClient::SharedBufferClient(SharedClient* sharedClient,
+        int surface, int num)
+    : SharedBufferBase(sharedClient, surface, num), tail(0)
+{
+}
+
+ssize_t SharedBufferClient::dequeue()
+{
+    //LOGD("[%d] about to dequeue a buffer",
+    //        mSharedStack->identity);
+    DequeueCondition condition(this);
+    status_t err = waitForCondition(condition);
+    if (err != NO_ERROR)
+        return ssize_t(err);
+
+
+    SharedBufferStack& stack( *mSharedStack );
+    // NOTE: 'stack.available' is part of the conditions, however
+    // decrementing it, never changes any conditions, so we don't need
+    // to do this as part of an update.
+    if (android_atomic_dec(&stack.available) == 0) {
+        LOGW("dequeue probably called from multiple threads!");
+    }
+
+    int dequeued = tail;
+    tail = ((tail+1 >= mNumBuffers) ? 0 : tail+1);
+    LOGD_IF(DEBUG_ATOMICS, "dequeued=%d, tail=%d, %s",
+            dequeued, tail, dump("").string());
+    return dequeued;
+}
+
+status_t SharedBufferClient::undoDequeue(int buf)
+{
+    UndoDequeueUpdate update(this);
+    status_t err = updateCondition( update );
+    return err;
+}
+
+status_t SharedBufferClient::lock(int buf)
+{
+    LockCondition condition(this, buf);
+    status_t err = waitForCondition(condition);    
+    return err;
+}
+
+status_t SharedBufferClient::queue(int buf)
+{
+    QueueUpdate update(this);
+    status_t err = updateCondition( update );
+    LOGD_IF(DEBUG_ATOMICS, "queued=%d, %s", buf, dump("").string());
+    return err;
+}
+
+bool SharedBufferClient::needNewBuffer(int buffer) const
+{
+    SharedBufferStack& stack( *mSharedStack );
+    const uint32_t mask = 1<<buffer;
+    return (android_atomic_and(~mask, &stack.reallocMask) & mask) != 0;
+}
+
+status_t SharedBufferClient::setDirtyRegion(int buffer, const Region& reg)
+{
+    SharedBufferStack& stack( *mSharedStack );
+    return stack.setDirtyRegion(buffer, reg);
+}
+
+// ----------------------------------------------------------------------------
+
+SharedBufferServer::SharedBufferServer(SharedClient* sharedClient,
+        int surface, int num)
+    : SharedBufferBase(sharedClient, surface, num)
+{
+    mSharedStack->head = num-1;
+    mSharedStack->available = num;
+    mSharedStack->queued = 0;
+    mSharedStack->reallocMask = 0;
+    memset(mSharedStack->dirtyRegion, 0, sizeof(mSharedStack->dirtyRegion));
+}
+
+ssize_t SharedBufferServer::retireAndLock()
+{
+    RetireUpdate update(this, mNumBuffers);
+    ssize_t buf = updateCondition( update );
+    LOGD_IF(DEBUG_ATOMICS, "retire=%d, %s", int(buf), dump("").string());
+    return buf;
+}
+
+status_t SharedBufferServer::unlock(int buffer)
+{
+    UnlockUpdate update(this, buffer);
+    status_t err = updateCondition( update );
+    return err;
+}
+
+status_t SharedBufferServer::reallocate()
+{
+    SharedBufferStack& stack( *mSharedStack );
+    uint32_t mask = (1<<mNumBuffers)-1;
+    android_atomic_or(mask, &stack.reallocMask); 
+    return NO_ERROR;
+}
+
+status_t SharedBufferServer::assertReallocate(int buffer)
+{
+    ReallocateCondition condition(this, buffer);
+    status_t err = waitForCondition(condition);
+    return err;
+}
+
+Region SharedBufferServer::getDirtyRegion(int buffer) const
+{
+    SharedBufferStack& stack( *mSharedStack );
+    return stack.getDirtyRegion(buffer);
+}
+
+// ---------------------------------------------------------------------------
+}; // namespace android