[Core] Use a mutex guard for split shared memory (#16647)

platforms/chibios/synchronization_util.c

@@ -0,0 +1,26 @@
+// Copyright 2022 Stefan Kerkmann
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "synchronization_util.h"
+#include "ch.h"
+
+#if defined(SPLIT_KEYBOARD)
+static MUTEX_DECL(SPLIT_SHARED_MEMORY_MUTEX);
+
+/**
+ * @brief Acquire exclusive access to the split keyboard shared memory, by
+ * locking the mutex guarding it. If the mutex is already held, the calling
+ * thread will be suspended until the mutex currently owning thread releases the
+ * mutex again.
+ */
+void split_shared_memory_lock(void) {
+    chMtxLock(&SPLIT_SHARED_MEMORY_MUTEX);
+}
+
+/**
+ * @brief Release the split shared memory mutex that has been acquired before.
+ */
+void split_shared_memory_unlock(void) {
+    chMtxUnlock(&SPLIT_SHARED_MEMORY_MUTEX);
+}
+#endif