3 files changed, 173 insertions, 0 deletions

diff --git a/src/core/CMakeLists.txt b/src/core/CMakeLists.txt
index 843bce150..397cc028f 100644
--- a/src/core/CMakeLists.txt
+++ b/src/core/CMakeLists.txt
@@ -160,6 +160,8 @@ add_library(core STATIC
     hle/kernel/k_affinity_mask.h
     hle/kernel/k_condition_variable.cpp
     hle/kernel/k_condition_variable.h
+    hle/kernel/k_light_lock.cpp
+    hle/kernel/k_light_lock.h
     hle/kernel/k_priority_queue.h
     hle/kernel/k_scheduler.cpp
     hle/kernel/k_scheduler.h
diff --git a/src/core/hle/kernel/k_light_lock.cpp b/src/core/hle/kernel/k_light_lock.cpp
new file mode 100644
index 000000000..08fa65fd5
--- /dev/null
+++ b/src/core/hle/kernel/k_light_lock.cpp
@@ -0,0 +1,130 @@
+// Copyright 2021 yuzu Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#include "core/hle/kernel/k_light_lock.h"
+#include "core/hle/kernel/k_scheduler.h"
+#include "core/hle/kernel/k_thread.h"
+#include "core/hle/kernel/kernel.h"
+
+namespace Kernel {
+
+void KLightLock::Lock() {
+    const uintptr_t cur_thread = reinterpret_cast<uintptr_t>(GetCurrentThreadPointer(kernel));
+    const uintptr_t cur_thread_tag = (cur_thread | 1);
+
+    while (true) {
+        uintptr_t old_tag = tag.load(std::memory_order_relaxed);
+
+        while (!tag.compare_exchange_weak(old_tag, (old_tag == 0) ? cur_thread : old_tag | 1,
+                                          std::memory_order_acquire)) {
+            if ((old_tag | 1) == cur_thread_tag) {
+                return;
+            }
+        }
+
+        if ((old_tag == 0) || ((old_tag | 1) == cur_thread_tag)) {
+            break;
+        }
+
+        LockSlowPath(old_tag | 1, cur_thread);
+    }
+}
+
+void KLightLock::Unlock() {
+    const uintptr_t cur_thread = reinterpret_cast<uintptr_t>(GetCurrentThreadPointer(kernel));
+    uintptr_t expected = cur_thread;
+    do {
+        if (expected != cur_thread) {
+            return UnlockSlowPath(cur_thread);
+        }
+    } while (!tag.compare_exchange_weak(expected, 0, std::memory_order_release));
+}
+
+void KLightLock::LockSlowPath(uintptr_t _owner, uintptr_t _cur_thread) {
+    KThread* cur_thread = reinterpret_cast<KThread*>(_cur_thread);
+
+    // Pend the current thread waiting on the owner thread.
+    {
+        KScopedSchedulerLock sl{kernel};
+
+        // Ensure we actually have locking to do.
+        if (tag.load(std::memory_order_relaxed) != _owner) {
+            return;
+        }
+
+        // Add the current thread as a waiter on the owner.
+        KThread* owner_thread = reinterpret_cast<KThread*>(_owner & ~1ul);
+        cur_thread->SetAddressKey(reinterpret_cast<uintptr_t>(std::addressof(tag)));
+        owner_thread->AddWaiter(cur_thread);
+
+        // Set thread states.
+        if (cur_thread->GetState() == ThreadState::Runnable) {
+            cur_thread->SetState(ThreadState::Waiting);
+        } else {
+            KScheduler::SetSchedulerUpdateNeeded(kernel);
+        }
+
+        if (owner_thread->IsSuspended()) {
+            owner_thread->ContinueIfHasKernelWaiters();
+            KScheduler::SetSchedulerUpdateNeeded(kernel);
+        }
+    }
+
+    // We're no longer waiting on the lock owner.
+    {
+        KScopedSchedulerLock sl{kernel};
+        KThread* owner_thread = cur_thread->GetLockOwner();
+        if (owner_thread) {
+            owner_thread->RemoveWaiter(cur_thread);
+        }
+    }
+}
+
+void KLightLock::UnlockSlowPath(uintptr_t _cur_thread) {
+    KThread* owner_thread = reinterpret_cast<KThread*>(_cur_thread);
+
+    // Unlock.
+    {
+        KScopedSchedulerLock sl{kernel};
+
+        // Get the next owner.
+        s32 num_waiters = 0;
+        KThread* next_owner = owner_thread->RemoveWaiterByKey(
+            std::addressof(num_waiters), reinterpret_cast<uintptr_t>(std::addressof(tag)));
+
+        // Pass the lock to the next owner.
+        uintptr_t next_tag = 0;
+        if (next_owner) {
+            next_tag = reinterpret_cast<uintptr_t>(next_owner);
+            if (num_waiters > 1) {
+                next_tag |= 0x1;
+            }
+
+            if (next_owner->GetState() == ThreadState::Waiting) {
+                next_owner->SetState(ThreadState::Runnable);
+            } else {
+                KScheduler::SetSchedulerUpdateNeeded(kernel);
+            }
+
+            if (next_owner->IsSuspended()) {
+                next_owner->ContinueIfHasKernelWaiters();
+            }
+        }
+
+        // We may have unsuspended in the process of acquiring the lock, so we'll re-suspend now if
+        // so.
+        if (owner_thread->IsSuspended()) {
+            owner_thread->TrySuspend();
+        }
+
+        // Write the new tag value.
+        tag.store(next_tag);
+    }
+}
+
+bool KLightLock::IsLockedByCurrentThread() const {
+    return (tag | 0x1ul) == (reinterpret_cast<uintptr_t>(GetCurrentThreadPointer(kernel)) | 0x1ul);
+}
+
+} // namespace Kernel
diff --git a/src/core/hle/kernel/k_light_lock.h b/src/core/hle/kernel/k_light_lock.h
new file mode 100644
index 000000000..f4c45f76a
--- /dev/null
+++ b/src/core/hle/kernel/k_light_lock.h
@@ -0,0 +1,41 @@
+// Copyright 2021 yuzu Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <atomic>
+
+#include "common/common_types.h"
+#include "core/hle/kernel/k_scoped_lock.h"
+
+namespace Kernel {
+
+class KernelCore;
+
+class KLightLock {
+public:
+    explicit KLightLock(KernelCore& kernel_) : kernel{kernel_} {}
+
+    void Lock();
+
+    void Unlock();
+
+    void LockSlowPath(uintptr_t owner, uintptr_t cur_thread);
+
+    void UnlockSlowPath(uintptr_t cur_thread);
+
+    bool IsLocked() const {
+        return tag != 0;
+    }
+
+    bool IsLockedByCurrentThread() const;
+
+private:
+    std::atomic<uintptr_t> tag{};
+    KernelCore& kernel;
+};
+
+using KScopedLightLock = KScopedLock<KLightLock>;
+
+} // namespace Kernel