Avoid locking Texture/Buffer in trap handler

We generally don't need to lock the `Texture`/`Buffer` in the trap handler, this is particularly problematic now as we hold the lock for the duration of a submission of any workloads. This leads to a large amount of contention for the lock and stalling in the signal handler when the resource may be `Clean` and can simply be switched over to `CpuDirty` without locking and utilizing atomics which is what this commit addresses.

app/src/main/cpp/skyline/gpu/buffer.cpp

@@ -20,6 +20,10 @@ namespace skyline::gpu {
             SynchronizeGuest(true); // We can skip trapping since the caller will do it
             WaitOnFence();
         }, [this] {
+            DirtyState expectedState{DirtyState::Clean};
+            if (dirtyState.compare_exchange_strong(expectedState, DirtyState::CpuDirty, std::memory_order_relaxed) || expectedState == DirtyState::CpuDirty)
+                return; // If we can transition the buffer to CPU dirty (from Clean) or if it already is CPU dirty then we can just return, we only need to do the lock and corresponding sync if the buffer is GPU dirty
+
             std::scoped_lock lock{*this};
             SynchronizeGuest(true);
             dirtyState = DirtyState::CpuDirty; // We need to assume the buffer is dirty since we don't know what the guest is writing
@@ -97,12 +101,17 @@ namespace skyline::gpu {
     }
 
     void Buffer::MarkGpuDirty() {
-        if (dirtyState == DirtyState::GpuDirty || !guest)
+        if (!guest)
             return;
 
+        auto currentState{dirtyState.load(std::memory_order_relaxed)};
+        do {
+            if (currentState == DirtyState::GpuDirty)
+                return;
+        } while (!dirtyState.compare_exchange_strong(currentState, DirtyState::GpuDirty, std::memory_order_relaxed));
+
         AdvanceSequence(); // The GPU will modify buffer contents so advance to the next sequence
         gpu.state.nce->RetrapRegions(*trapHandle, false);
-        dirtyState = DirtyState::GpuDirty;
     }
 
     void Buffer::WaitOnFence() {
@@ -123,42 +132,47 @@ namespace skyline::gpu {
     }
 
     void Buffer::SynchronizeHost(bool rwTrap) {
-        if (dirtyState != DirtyState::CpuDirty || !guest)
+        if (!guest)
-            return; // If the buffer has not been modified on the CPU or there's no guest buffer, there is no need to synchronize it
+            return;
 
-        WaitOnFence();
+        auto currentState{dirtyState.load(std::memory_order_relaxed)};
+        do {
+            if (currentState != DirtyState::CpuDirty || !guest)
+                return; // If the buffer has not been modified on the CPU, there is no need to synchronize it
+        } while (!dirtyState.compare_exchange_strong(currentState, rwTrap ? DirtyState::GpuDirty : DirtyState::Clean, std::memory_order_relaxed));
 
         TRACE_EVENT("gpu", "Buffer::SynchronizeHost");
 
         AdvanceSequence(); // We are modifying GPU backing contents so advance to the next sequence
-        std::memcpy(backing.data(), mirror.data(), mirror.size());
 
-        if (rwTrap) {
+        WaitOnFence();
-            gpu.state.nce->RetrapRegions(*trapHandle, false);
+
-            dirtyState = DirtyState::GpuDirty;
+        std::memcpy(backing.data(), mirror.data(), mirror.size());
-        } else {
+        gpu.state.nce->RetrapRegions(*trapHandle, !rwTrap); // Trap any future CPU reads (optionally) + writes to this buffer
-            gpu.state.nce->RetrapRegions(*trapHandle, true);
-            dirtyState = DirtyState::Clean;
-        }
     }
 
     void Buffer::SynchronizeGuest(bool skipTrap, bool nonBlocking) {
-        if (dirtyState != DirtyState::GpuDirty || !guest)
+        if (!guest)
-            return; // If the buffer has not been used on the GPU or there's no guest buffer, there is no need to synchronize it
+            return;
+
+        auto currentState{dirtyState.load(std::memory_order_relaxed)};
+        do {
+            if (currentState != DirtyState::GpuDirty)
+                return; // If the buffer has not been used on the GPU, there is no need to synchronize it
+        } while (!dirtyState.compare_exchange_strong(currentState, DirtyState::Clean, std::memory_order_relaxed));
 
         if (nonBlocking && !PollFence())
             return;
-        else if (!nonBlocking)
-            WaitOnFence();
 
         TRACE_EVENT("gpu", "Buffer::SynchronizeGuest");
 
-        std::memcpy(mirror.data(), backing.data(), mirror.size());
-
         if (!skipTrap)
             gpu.state.nce->RetrapRegions(*trapHandle, true);
 
-        dirtyState = DirtyState::Clean;
+        if (!nonBlocking)
+            WaitOnFence();
+
+        std::memcpy(mirror.data(), backing.data(), mirror.size());
     }
 
     void Buffer::SynchronizeGuestImmediate(bool isFirstUsage, const std::function<void()> &flushHostCallback) {
@@ -186,8 +200,7 @@ namespace skyline::gpu {
         else if (dirtyState == DirtyState::GpuDirty)
             SynchronizeGuestImmediate(isFirstUsage, flushHostCallback);
 
-        if (dirtyState != DirtyState::Clean)
+        // It's possible that the guest will arbitrarily modify the buffer contents on the CPU after the syncs and trigger the signal handler which would set the dirty state to CPU dirty, this is acceptable as there is no requirement to make writes visible immediately
-            Logger::Error("Attempting to write to a dirty buffer"); // This should never happen since we do syncs in both directions above
 
         std::memcpy(mirror.data() + offset, data.data(), data.size()); // Always copy to mirror since any CPU side reads will need the up-to-date contents
 
@@ -267,7 +280,7 @@ namespace skyline::gpu {
 
     bool Buffer::BufferDelegate::LockWithTag(ContextTag pTag) {
         bool result{};
-        buffer.Lock([pTag, &result](Buffer* pBuffer) {
+        buffer.Lock([pTag, &result](Buffer *pBuffer) {
             result = pBuffer->LockWithTag(pTag);
         });
         return result;

app/src/main/cpp/skyline/gpu/buffer.h

@@ -36,7 +36,9 @@ namespace skyline::gpu {
             Clean, //!< The CPU mappings are in sync with the GPU buffer
             CpuDirty, //!< The CPU mappings have been modified but the GPU buffer is not up to date
             GpuDirty, //!< The GPU buffer has been modified but the CPU mappings have not been updated
-        } dirtyState{DirtyState::CpuDirty}; //!< The state of the CPU mappings with respect to the GPU buffer
+        };
+        std::atomic<DirtyState> dirtyState{DirtyState::CpuDirty}; //!< The state of the CPU mappings with respect to the GPU buffer
+        static_assert(std::atomic<DirtyState>::is_always_lock_free);
 
         bool everHadInlineUpdate{}; //!< Whether the buffer has ever had an inline update since it was created, if this is set then megabuffering will be attempted by views to avoid the cost of inline GPU updates
 

app/src/main/cpp/skyline/gpu/texture/texture.cpp

@@ -98,7 +98,7 @@ namespace skyline::gpu {
 
     bool TextureView::LockWithTag(ContextTag tag) {
         bool result{};
-        texture.Lock([tag, &result](Texture* pTexture) {
+        texture.Lock([tag, &result](Texture *pTexture) {
             result = pTexture->LockWithTag(tag);
         });
         return result;
@@ -148,6 +148,10 @@ namespace skyline::gpu {
             SynchronizeGuest(true); // We can skip trapping since the caller will do it
             WaitOnFence();
         }, [this] {
+            DirtyState expectedState{DirtyState::Clean};
+            if (dirtyState.compare_exchange_strong(expectedState, DirtyState::CpuDirty, std::memory_order_relaxed) || expectedState == DirtyState::CpuDirty)
+                return; // If we can transition the texture to CPU dirty (from Clean) or if it already is CPU dirty then we can just return, we only need to do the lock and corresponding sync if the texture is GPU dirty
+
             std::scoped_lock lock{*this};
             SynchronizeGuest(true);
             dirtyState = DirtyState::CpuDirty; // We need to assume the texture is dirty since we don't know what the guest is writing
@@ -156,9 +160,7 @@ namespace skyline::gpu {
     }
 
     std::shared_ptr<memory::StagingBuffer> Texture::SynchronizeHostImpl() {
-        if (!guest)
+        if (guest->dimensions != dimensions)
-            throw exception("Synchronization of host textures requires a valid guest texture to synchronize from");
-        else if (guest->dimensions != dimensions)
             throw exception("Guest and host dimensions being different is not supported currently");
 
         auto pointer{mirror.data()};
@@ -576,10 +578,16 @@ namespace skyline::gpu {
     }
 
     void Texture::MarkGpuDirty() {
-        if (dirtyState == DirtyState::GpuDirty || !guest || format != guest->format)
+        if (!guest || format != guest->format)
-            return; // In addition to other checks, we also need to skip GPU dirty if the host format and guest format differ as we don't support re-encoding compressed textures which is when this generally occurs
+            return; // We need to skip GPU dirty if the host format and guest format differ as we don't support re-encoding compressed textures which is when this generally occurs
+
+        auto currentState{dirtyState.load(std::memory_order_relaxed)};
+        do {
+            if (currentState == DirtyState::GpuDirty)
+                return;
+        } while (!dirtyState.compare_exchange_strong(currentState, DirtyState::GpuDirty, std::memory_order_relaxed));
+
         gpu.state.nce->RetrapRegions(*trapHandle, false);
-        dirtyState = DirtyState::GpuDirty;
     }
 
     bool Texture::WaitOnBacking() {
@@ -642,8 +650,14 @@ namespace skyline::gpu {
     }
 
     void Texture::SynchronizeHost(bool rwTrap) {
-        if (dirtyState != DirtyState::CpuDirty || !guest)
+        if (!guest)
-            return; // If the texture has not been modified on the CPU or has no mappings, there is no need to synchronize it
+            return;
+
+        auto currentState{dirtyState.load(std::memory_order_relaxed)};
+        do {
+            if (currentState != DirtyState::CpuDirty)
+                return; // If the texture has not been modified on the CPU or has no mappings, there is no need to synchronize it
+        } while (!dirtyState.compare_exchange_strong(currentState, rwTrap ? DirtyState::GpuDirty : DirtyState::Clean, std::memory_order_relaxed));
 
         TRACE_EVENT("gpu", "Texture::SynchronizeHost");
 
@@ -657,19 +671,19 @@ namespace skyline::gpu {
             cycle = lCycle;
         }
 
-        if (rwTrap) {
+        gpu.state.nce->RetrapRegions(*trapHandle, !rwTrap); // Trap any future CPU reads (optionally) + writes to this texture
-            gpu.state.nce->RetrapRegions(*trapHandle, false);
-            dirtyState = DirtyState::GpuDirty;
-        } else {
-            gpu.state.nce->RetrapRegions(*trapHandle, true); // Trap any future CPU writes to this texture
-            dirtyState = DirtyState::Clean;
-        }
     }
 
     void Texture::SynchronizeHostWithBuffer(const vk::raii::CommandBuffer &commandBuffer, const std::shared_ptr<FenceCycle> &pCycle, bool rwTrap) {
-        if (dirtyState != DirtyState::CpuDirty || !guest)
+        if (!guest)
             return;
 
+        auto currentState{dirtyState.load(std::memory_order_relaxed)};
+        do {
+            if (currentState != DirtyState::CpuDirty)
+                return;
+        } while (!dirtyState.compare_exchange_strong(currentState, rwTrap ? DirtyState::GpuDirty : DirtyState::Clean, std::memory_order_relaxed));
+
         TRACE_EVENT("gpu", "Texture::SynchronizeHostWithBuffer");
 
         auto stagingBuffer{SynchronizeHostImpl()};
@@ -680,35 +694,29 @@ namespace skyline::gpu {
             cycle = pCycle;
         }
 
-        if (rwTrap) {
+        gpu.state.nce->RetrapRegions(*trapHandle, !rwTrap); // Trap any future CPU reads (optionally) + writes to this texture
-            gpu.state.nce->RetrapRegions(*trapHandle, false);
-            dirtyState = DirtyState::GpuDirty;
-        } else {
-            gpu.state.nce->RetrapRegions(*trapHandle, true); // Trap any future CPU writes to this texture
-            dirtyState = DirtyState::Clean;
-        }
     }
 
     void Texture::SynchronizeGuest(bool skipTrap) {
-        if (dirtyState != DirtyState::GpuDirty || layout == vk::ImageLayout::eUndefined || !guest) {
+        if (!guest)
-            // We can skip syncing in three cases:
-            // * If the texture has not been used on the GPU, there is no need to synchronize it
-            // * If the state of the host texture is undefined then so can the guest
-            // * If there is no guest texture to synchronise
             return;
-        }
 
-        if (layout == vk::ImageLayout::eUndefined || format != guest->format) {
+        auto currentState{dirtyState.load(std::memory_order_relaxed)};
-            // If the state of the host texture is undefined then so can the guest
+        do {
-            // If the texture has differing formats on the guest and host, we don't support converting back in that case as it may involve recompression of a decompressed texture
+            if (currentState != DirtyState::GpuDirty)
-            if (!skipTrap)
+                return; // If the buffer has not been used on the GPU, there is no need to synchronize it
-                gpu.state.nce->RetrapRegions(*trapHandle, true);
+        } while (!dirtyState.compare_exchange_strong(currentState, DirtyState::Clean, std::memory_order_relaxed));
-            dirtyState = DirtyState::Clean;
-            return;
-        }
 
         TRACE_EVENT("gpu", "Texture::SynchronizeGuest");
 
+        if (!skipTrap)
+            gpu.state.nce->RetrapRegions(*trapHandle, true);
+
+        if (layout == vk::ImageLayout::eUndefined || format != guest->format)
+            // If the state of the host texture is undefined then so can the guest
+            // If the texture has differing formats on the guest and host, we don't support converting back in that case as it may involve recompression of a decompressed texture
+            return;
+
         WaitOnBacking();
 
         if (tiling == vk::ImageTiling::eOptimal || !std::holds_alternative<memory::Image>(backing)) {
@@ -727,25 +735,27 @@ namespace skyline::gpu {
         } else {
             throw exception("Host -> Guest synchronization of images tiled as '{}' isn't implemented", vk::to_string(tiling));
         }
-
-        if (!skipTrap)
-            gpu.state.nce->RetrapRegions(*trapHandle, true);
-        dirtyState = DirtyState::Clean;
     }
 
     void Texture::SynchronizeGuestWithBuffer(const vk::raii::CommandBuffer &commandBuffer, const std::shared_ptr<FenceCycle> &pCycle) {
-        if (dirtyState != DirtyState::GpuDirty || !guest)
+        if (!guest)
             return;
 
-        if (layout == vk::ImageLayout::eUndefined || format != guest->format) {
+        auto currentState{dirtyState.load(std::memory_order_relaxed)};
-            // If the state of the host texture is undefined then so can the guest
+        do {
-            // If the texture has differing formats on the guest and host, we don't support converting back in that case as it may involve recompression of a decompressed texture
+            if (currentState != DirtyState::GpuDirty)
-            dirtyState = DirtyState::Clean;
+                return; // If the buffer has not been used on the GPU, there is no need to synchronize it
-            return;
+        } while (!dirtyState.compare_exchange_strong(currentState, DirtyState::Clean, std::memory_order_relaxed));
-        }
 
         TRACE_EVENT("gpu", "Texture::SynchronizeGuestWithBuffer");
 
+        gpu.state.nce->RetrapRegions(*trapHandle, true);
+
+        if (layout == vk::ImageLayout::eUndefined || format != guest->format)
+            // If the state of the host texture is undefined then so can the guest
+            // If the texture has differing formats on the guest and host, we don't support converting back in that case as it may involve recompression of a decompressed texture
+            return;
+
         WaitOnBacking();
         pCycle->ChainCycle(cycle);
 
@@ -762,8 +772,6 @@ namespace skyline::gpu {
         } else {
             throw exception("Host -> Guest synchronization of images tiled as '{}' isn't implemented", vk::to_string(tiling));
         }
-
-        dirtyState = DirtyState::Clean;
     }
 
     std::shared_ptr<TextureView> Texture::GetView(vk::ImageViewType type, vk::ImageSubresourceRange range, texture::Format pFormat, vk::ComponentMapping mapping) {

app/src/main/cpp/skyline/gpu/texture/texture.h

@@ -367,7 +367,9 @@ namespace skyline::gpu {
             Clean, //!< The CPU mappings are in sync with the GPU texture
             CpuDirty, //!< The CPU mappings have been modified but the GPU texture is not up to date
             GpuDirty, //!< The GPU texture has been modified but the CPU mappings have not been updated
-        } dirtyState{DirtyState::CpuDirty}; //!< The state of the CPU mappings with respect to the GPU texture
+        };
+        std::atomic<DirtyState> dirtyState{DirtyState::CpuDirty}; //!< The state of the CPU mappings with respect to the GPU texture
+        static_assert(std::atomic<DirtyState>::is_always_lock_free);
 
         /**
          * @brief Storage for all metadata about a specific view into the buffer, used to prevent redundant view creation and duplication of VkBufferView(s)