Use shared_first_mutex (#3179)

src/common/shared_first_mutex.h

@@ -0,0 +1,46 @@
+// SPDX-FileCopyrightText: Copyright 2025 shadPS4 Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include <condition_variable>
+#include <mutex>
+
+namespace Common {
+
+// Like std::shared_mutex, but reader has priority over writer.
+class SharedFirstMutex {
+public:
+    void lock() {
+        std::unique_lock<std::mutex> lock(mtx);
+        cv.wait(lock, [this]() { return !writer_active && readers == 0; });
+        writer_active = true;
+    }
+
+    void unlock() {
+        std::lock_guard<std::mutex> lock(mtx);
+        writer_active = false;
+        cv.notify_all();
+    }
+
+    void lock_shared() {
+        std::unique_lock<std::mutex> lock(mtx);
+        cv.wait(lock, [this]() { return !writer_active; });
+        ++readers;
+    }
+
+    void unlock_shared() {
+        std::lock_guard<std::mutex> lock(mtx);
+        if (--readers == 0) {
+            cv.notify_all();
+        }
+    }
+
+private:
+    std::mutex mtx;
+    std::condition_variable cv;
+    int readers = 0;
+    bool writer_active = false;
+};
+
+} // namespace Common