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Protect savestates while the threaded software renderer is running (#1864)

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* First crack at ensuring the render thread doesn't touch GPU state while it's being serialized

* Get rid of the semaphore wait

* Add some extra fields into GPU3D's serialization

* Oops, TempVertexBuffer is already serialized

* Move vertex serialization into its own method

* Lock the GPU3D state when rendering on the render thread or serializing it

* Revert "Lock the GPU3D state when rendering on the render thread or serializing it"

This reverts commit 2f49a551c1.

* Add comments that describe the synchronization within GPU3D_Soft

- I need to understand it before I can solve my actual problem
- Now I do

* Revert "Revert "Lock the GPU3D state when rendering on the render thread or serializing it""

This reverts commit 1977566a6d.

* Let's try locking the GPU3D state throughout NDS::RunFrame

- Just to see what happens

* Slim down the lock's scope

* Narrow the lock's scope some more

* Remove the lock entirely

* Try protecting the GPU3D state with just a mutex

- I'll clean this up once I know it works

* Remove a duplicate method definition

* Add a missing `noexcept` specifier

* Remove an unused function

* Cut some non-hardware state from `GPU3D`'s savestate

* Assume that the next frame after loading a savestate won't be identical

* Actually, it _is_ worth it

* Don't serialize the clip matrix

- It's recalculated anyway

* Serialize `RenderPolygonRAM` as an array of indexes

* Clean up some comments

- I liked the dialogue style, but oh well

* Try restarting the render thread instead of using the lock

- Let's see what happens

* Put the lock back

* Fix some polygon and vertex indexes being saved incorrectly

- Taking the difference between two pointers results in the number of elements, not the number of bytes

* Remove `SoftRenderer::StateBusy` since it turns out we don't need it

- The real synchronization was the friends we made along the way
This commit is contained in:
Jesse Talavera
2024-01-07 17:39:43 -05:00
committed by GitHub
parent f68f55d002
commit 8143f54956
5 changed files with 128 additions and 53 deletions
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57
src/GPU3D_Soft.cpp
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@ -34,8 +34,11 @@ void SoftRenderer::StopRenderThread()
{
if (RenderThreadRunning.load(std::memory_order_relaxed))
{
// Tell the render thread to stop drawing new frames, and finish up the current one.
RenderThreadRunning = false;
Platform::Semaphore_Post(Sema_RenderStart);
Platform::Thread_Wait(RenderThread);
Platform::Thread_Free(RenderThread);
RenderThread = nullptr;
@ -47,24 +50,36 @@ void SoftRenderer::SetupRenderThread(GPU& gpu)
if (Threaded)
{
if (!RenderThreadRunning.load(std::memory_order_relaxed))
{
RenderThreadRunning = true;
{ // If the render thread isn't already running...
RenderThreadRunning = true; // "Time for work, render thread!"
RenderThread = Platform::Thread_Create([this, &gpu]() {
RenderThreadFunc(gpu);
});
}
// otherwise more than one frame can be queued up at once
// "Be on standby, but don't start rendering until I tell you to!"
Platform::Semaphore_Reset(Sema_RenderStart);
// "Oh, sorry, were you already in the middle of a frame from the last iteration?"
if (RenderThreadRendering)
// "Tell me when you're done, I'll wait here."
Platform::Semaphore_Wait(Sema_RenderDone);
Platform::Semaphore_Reset(Sema_RenderDone);
Platform::Semaphore_Reset(Sema_RenderStart);
Platform::Semaphore_Reset(Sema_ScanlineCount);
// "All good? Okay, let me give you your training."
// "(Maybe you're still the same thread, but I have to tell you this stuff anyway.)"
Platform::Semaphore_Post(Sema_RenderStart);
// "This is the signal you'll send when you're done with a frame."
// "I'll listen for it when I need to show something to the frontend."
Platform::Semaphore_Reset(Sema_RenderDone);
// "This is the signal I'll send when I want you to start rendering."
// "Don't do anything until you get the message."
Platform::Semaphore_Reset(Sema_RenderStart);
// "This is the signal you'll send every time you finish drawing a line."
// "I might need some of your scanlines before you finish the whole buffer,"
// "so let me know as soon as you're done with each one."
Platform::Semaphore_Reset(Sema_ScanlineCount);
}
else
{
@ -72,6 +87,13 @@ void SoftRenderer::SetupRenderThread(GPU& gpu)
}
}
void SoftRenderer::EnableRenderThread()
{
if (Threaded && Sema_RenderStart)
{
Platform::Semaphore_Post(Sema_RenderStart);
}
}
SoftRenderer::SoftRenderer(bool threaded) noexcept
: Renderer3D(false), Threaded(threaded)
@ -103,6 +125,7 @@ void SoftRenderer::Reset(GPU& gpu)
PrevIsShadowMask = false;
SetupRenderThread(gpu);
EnableRenderThread();
}
void SoftRenderer::SetThreaded(bool threaded, GPU& gpu) noexcept
@ -111,6 +134,7 @@ void SoftRenderer::SetThreaded(bool threaded, GPU& gpu) noexcept
{
Threaded = threaded;
SetupRenderThread(gpu);
EnableRenderThread();
}
}
@ -1692,12 +1716,14 @@ void SoftRenderer::RenderPolygons(const GPU& gpu, bool threaded, Polygon** polyg
ScanlineFinalPass(gpu.GPU3D, y-1);
if (threaded)
// Notify the main thread that we're done with a scanline.
Platform::Semaphore_Post(Sema_ScanlineCount);
}
ScanlineFinalPass(gpu.GPU3D, 191);
if (threaded)
// If this renderer is threaded, notify the main thread that we're done with the frame.
Platform::Semaphore_Post(Sema_ScanlineCount);
}
@ -1719,6 +1745,7 @@ void SoftRenderer::RenderFrame(GPU& gpu)
if (RenderThreadRunning.load(std::memory_order_relaxed))
{
// "Render thread, you're up! Get moving."
Platform::Semaphore_Post(Sema_RenderStart);
}
else if (!FrameIdentical)
@ -1731,18 +1758,26 @@ void SoftRenderer::RenderFrame(GPU& gpu)
void SoftRenderer::RestartFrame(GPU& gpu)
{
SetupRenderThread(gpu);
EnableRenderThread();
}
void SoftRenderer::RenderThreadFunc(GPU& gpu)
{
for (;;)
{
// Wait for a notice from the main thread to start rendering (or to stop entirely).
Platform::Semaphore_Wait(Sema_RenderStart);
if (!RenderThreadRunning) return;
// Protect the GPU state from the main thread.
// Some melonDS frontends (though not ours)
// will repeatedly save or load states;
// if they do so while the render thread is busy here,
// the ensuing race conditions may cause a crash
// (since some of the GPU state includes pointers).
RenderThreadRendering = true;
if (FrameIdentical)
{
{ // If no rendering is needed, just say we're done.
Platform::Semaphore_Post(Sema_ScanlineCount, 192);
}
else
@ -1751,7 +1786,10 @@ void SoftRenderer::RenderThreadFunc(GPU& gpu)
RenderPolygons(gpu, true, &gpu.GPU3D.RenderPolygonRAM[0], gpu.GPU3D.RenderNumPolygons);
}
// Tell the main thread that we're done rendering
// and that it's safe to access the GPU state again.
Platform::Semaphore_Post(Sema_RenderDone);
RenderThreadRendering = false;
}
}
@ -1761,6 +1799,9 @@ u32* SoftRenderer::GetLine(int line)
if (RenderThreadRunning.load(std::memory_order_relaxed))
{
if (line < 192)
// We need a scanline, so let's wait for the render thread to finish it.
// (both threads process scanlines from top-to-bottom,
// so we don't need to wait for a specific row)
Platform::Semaphore_Wait(Sema_ScanlineCount);
}