status_t BufferItemConsumer::releaseBuffer(const BufferItem &item,
        const sp<Fence>& releaseFence) {
    status_t err;

    Mutex::Autolock _l(mMutex);

    err = addReleaseFenceLocked(item.mSlot, item.mGraphicBuffer, releaseFence);
    if (err != OK) {
        BI_LOGE("Failed to addReleaseFenceLocked");
    }

    err = releaseBufferLocked(item.mSlot, item.mGraphicBuffer, EGL_NO_DISPLAY,
            EGL_NO_SYNC_KHR);
    if (err != OK && err != IGraphicBufferConsumer::STALE_BUFFER_SLOT) {
        BI_LOGE("Failed to release buffer: %s (%d)",
                strerror(-err), err);
    }
    return err;
}

• 调用 addReleaseFenceLocked() 设置 releaseFence，与 BufferStateLayer::addReleaseFence() 类似，如果不存在 fence，则直接设置，否则 merge()。
• 调用 ConsumerBase::releaseBufferLocked() 释放 buffer。