为什么这个 Vulkan 代码有写后写的危险？

Question

我正在尝试新的 Vulkan synchronization validation tool，它在我的纹理加载代码中发现了写写危险：


    mipLevelCount = mipmaps == Mipmaps::Generate ? MathUtil::GetMipmapCount( width, height ) : 1;

    VkImageSubresourceRange range = {};
    range.aspectMask = VK_IMAGE_ASPECT_COLOR_BIT;
    range.baseMipLevel = 0;
    range.levelCount = mipLevelCount;
    range.baseArrayLayer = 0;
    range.layerCount = 1;

    imageMemoryBarrier = {};
    imageMemoryBarrier.sType = VK_STRUCTURE_TYPE_IMAGE_MEMORY_BARRIER;
    imageMemoryBarrier.srcQueueFamilyIndex = VK_QUEUE_FAMILY_IGNORED;
    imageMemoryBarrier.dstQueueFamilyIndex = VK_QUEUE_FAMILY_IGNORED;
    imageMemoryBarrier.srcAccessMask = 0;
    imageMemoryBarrier.dstAccessMask = VK_ACCESS_TRANSFER_WRITE_BIT;
    imageMemoryBarrier.oldLayout = VK_IMAGE_LAYOUT_TRANSFER_DST_OPTIMAL;
    imageMemoryBarrier.newLayout = VK_IMAGE_LAYOUT_GENERAL;
    imageMemoryBarrier.image = image;
    imageMemoryBarrier.subresourceRange = range;

    vkCmdPipelineBarrier(
        GfxDeviceGlobal::texCmdBuffer,
        VK_PIPELINE_STAGE_TRANSFER_BIT,
        VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT,
        0,
        0, nullptr,
        0, nullptr,
        1, &imageMemoryBarrier );

    for (int i = 1; i < mipLevelCount; ++i)
    {
        const std::int32_t mipWidth = MathUtil::Max( width >> i, 1 );
        const std::int32_t mipHeight = MathUtil::Max( height >> i, 1 );

        VkImageBlit imageBlit = {};
        imageBlit.srcSubresource.aspectMask = VK_IMAGE_ASPECT_COLOR_BIT;
        imageBlit.srcSubresource.baseArrayLayer = 0;
        imageBlit.srcSubresource.layerCount = 1;
        imageBlit.srcSubresource.mipLevel = 0;
        imageBlit.srcOffsets[ 0 ] = { 0, 0, 0 };
        imageBlit.srcOffsets[ 1 ] = { width, height, 1 };

        imageBlit.dstSubresource.aspectMask = VK_IMAGE_ASPECT_COLOR_BIT;
        imageBlit.dstSubresource.baseArrayLayer = 0;
        imageBlit.dstSubresource.layerCount = 1;
        imageBlit.dstSubresource.mipLevel = i;
        imageBlit.dstOffsets[ 0 ] = { 0, 0, 0 };
        imageBlit.dstOffsets[ 1 ] = { mipWidth, mipHeight, 1 };

        vkCmdBlitImage( GfxDeviceGlobal::texCmdBuffer, image, VK_IMAGE_LAYOUT_GENERAL, image,
            VK_IMAGE_LAYOUT_GENERAL, 1, &imageBlit, VK_FILTER_LINEAR );
    }

    imageMemoryBarrier.srcAccessMask = VK_ACCESS_TRANSFER_WRITE_BIT;
    imageMemoryBarrier.dstAccessMask = VK_ACCESS_SHADER_READ_BIT;
    imageMemoryBarrier.oldLayout = VK_IMAGE_LAYOUT_GENERAL;
    imageMemoryBarrier.newLayout = VK_IMAGE_LAYOUT_SHADER_READ_ONLY_OPTIMAL;
    imageMemoryBarrier.image = image;
    imageMemoryBarrier.subresourceRange = range;
    imageMemoryBarrier.subresourceRange.baseMipLevel = 0;
    imageMemoryBarrier.subresourceRange.levelCount = mipLevelCount;

    vkCmdPipelineBarrier(                   // <-- The error happens at this call.
            GfxDeviceGlobal::texCmdBuffer,
            VK_PIPELINE_STAGE_TRANSFER_BIT,
            VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT,
            0,
            0, nullptr,
            0, nullptr,
            1, &imageMemoryBarrier );

错误是：

ERROR: Validation Error: [ SYNC-HAZARD-WRITE_AFTER_WRITE ] Object 0: handle = 0x559307e35610, type = VK_OBJECT_TYPE_IMAGE; | MessageID = 0xfdf9f5e1 | vkCmdPipelineBarrier: Hazard WRITE_AFTER_WRITE for image barrier 0 VkImage 0x559307e35610[]. Access info (usage: SYNC_IMAGE_LAYOUT_TRANSITION, prior_usage: SYNC_IMAGE_LAYOUT_TRANSITION, write_barriers: 0, command: vkCmdPipelineBarrier, seq_no: 1, reset_no: 3).

如何解决？

Answer 1

请记住，Layout Transition 是 happens-between srcStage 和 dstStage.

的读写操作

首先提交布局转换。它happens-before VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT.

然后你做一个 vkCmdBlitImage，这将是 mis-synchronized，因为那是 STAGE_TRANSFER，而不是 STAGE_FRAGMENT_SHADER。尽管你的 mipLevelCount == 1，所以整个循环都是死代码。

然后你有另一个布局转换。它happens-after VK_PIPELINE_STAGE_TRANSFER_BIT.

因此您有两个可能同时发生的布局转换。

我认为设置 dstStageMask = VK_PIPELINE_STAGE_TRANSFER_BIT 应该可以解决这个问题。

为什么这个 Vulkan 代码有写后写的危险？

Why does this Vulkan code have a write-after-write hazard?

synchronization

vulkan