From b130d96445bf1551ae88bde9a9dda18a47c41049 Mon Sep 17 00:00:00 2001
From: tatehaga <tatehaga@gmail.com>
Date: Tue, 23 Oct 2018 15:19:27 -0400
Subject: [PATCH] barrier implementation using semaphores (#186)

---
 nx/include/switch.h                |  1 +
 nx/include/switch/kernel/barrier.h | 30 ++++++++++++++++++++++++++++++
 nx/source/kernel/barrier.c         | 29 +++++++++++++++++++++++++++++
 3 files changed, 60 insertions(+)
 create mode 100644 nx/include/switch/kernel/barrier.h
 create mode 100644 nx/source/kernel/barrier.c

diff --git a/nx/include/switch.h b/nx/include/switch.h
index dd22df23..6c5d8423 100644
--- a/nx/include/switch.h
+++ b/nx/include/switch.h
@@ -34,6 +34,7 @@ extern "C" {
 #include "switch/kernel/random.h"
 #include "switch/kernel/jit.h"
 #include "switch/kernel/ipc.h"
+#include "switch/kernel/barrier.h"
 
 #include "switch/services/sm.h"
 #include "switch/services/smm.h"
diff --git a/nx/include/switch/kernel/barrier.h b/nx/include/switch/kernel/barrier.h
new file mode 100644
index 00000000..befa6e93
--- /dev/null
+++ b/nx/include/switch/kernel/barrier.h
@@ -0,0 +1,30 @@
+/**
+ * @file barrier.h
+ * @brief Multi-threading Barrier
+ * @author tatehaga
+ * @copyright libnx Authors
+ */
+#pragma once
+#include "semaphore.h"
+
+/// Barrier structure.
+typedef struct Barrier {
+    u64 count;             ///< Number of threads to reach the barrier.
+    u64 thread_total;      ///< Number of threads to wait on.
+    Semaphore throttle;    ///< Semaphore to make sure threads release to scheduler one at a time.
+    Semaphore lock;        ///< Semaphore to lock barrier to prevent multiple operations by threads at once.
+    Semaphore thread_wait; ///< Semaphore to force a thread to wait if count < thread_total.
+} Barrier;
+
+/**
+ * @brief Initializes a barrier and the number of threads to wait on.
+ * @param b Barrier object.
+ * @param thread_count Initial value for the number of threads the barrier must wait for.
+ */
+void barrierInit(Barrier *b, u64 thread_count);
+
+/**
+ * @brief Forces threads to wait until all threads have called barrierWait.
+ * @param b Barrier object.
+ */
+void barrierWait(Barrier *b);
diff --git a/nx/source/kernel/barrier.c b/nx/source/kernel/barrier.c
new file mode 100644
index 00000000..d0c9af1b
--- /dev/null
+++ b/nx/source/kernel/barrier.c
@@ -0,0 +1,29 @@
+#include "kernel/barrier.h"
+
+void barrierInit(Barrier *b, u64 thread_count) {
+    b->count = 0;
+    b->thread_total = thread_count;
+    semaphoreInit(&b->throttle, 0);
+    semaphoreInit(&b->lock, 1);
+    semaphoreInit(&b->thread_wait, 0);
+}
+
+void barrierWait(Barrier *b) {
+    semaphoreWait(&b->lock);
+    if(b->count < b->thread_total) {
+        b->count++;
+    }
+    if(b->count < b->thread_total) {
+        semaphoreSignal(&b->lock);
+        semaphoreWait(&b->thread_wait);
+        semaphoreSignal(&b->throttle);
+    }
+    else if(b->count == b->thread_total) {
+        for(int i = 0; i < b->thread_total-1; i++) {
+            semaphoreSignal(&b->thread_wait);
+            semaphoreWait(&b->throttle);
+        }
+        b->count = 0;
+        semaphoreSignal(&b->lock);
+    }
+}