From 012f68838bac7e69e54df9d0d2f13d90dcdce329 Mon Sep 17 00:00:00 2001
From: ChUrl <christoph.urlacher@protonmail.com>
Date: Mon, 11 Jul 2022 15:00:05 +0200
Subject: [PATCH] implement semaphore

---
 c_os/kernel/threads/Scheduler.cc |  1 -
 c_os/lib/Semaphore.cc            | 32 ++++++++++++++++++++++++++++++++
 c_os/lib/SpinLock.cc             |  2 ++
 c_os/main.cc                     |  5 +++++
 4 files changed, 39 insertions(+), 1 deletion(-)
 create mode 100644 c_os/lib/Semaphore.cc

diff --git a/c_os/kernel/threads/Scheduler.cc b/c_os/kernel/threads/Scheduler.cc
index 8765496..ee3cd49 100644
--- a/c_os/kernel/threads/Scheduler.cc
+++ b/c_os/kernel/threads/Scheduler.cc
@@ -181,7 +181,6 @@ void Scheduler::block() {
     }
 
     cpu.disable_int();
-
     Thread& next = *(Thread*)this->readyQueue.dequeue();
     this->dispatch(next);
 }
diff --git a/c_os/lib/Semaphore.cc b/c_os/lib/Semaphore.cc
new file mode 100644
index 0000000..9d1bb7a
--- /dev/null
+++ b/c_os/lib/Semaphore.cc
@@ -0,0 +1,32 @@
+#include "Semaphore.h"
+#include "kernel/Globals.h"
+
+void Semaphore::p() {
+    // Lock to allow deterministic operations on counter
+    this->lock.acquire();
+
+    if (this->counter > 0) {
+        // Semaphore can be acquired
+        this->counter = this->counter - 1;
+    } else {
+        // Block and manage thread in semaphore queue until it's woken up by v()
+        this->waitQueue.enqueue(scheduler.get_active());
+        scheduler.block();
+    }
+
+    this->lock.release();
+}
+
+void Semaphore::v() {
+    this->lock.acquire();
+
+    if (!this->waitQueue.isEmpty()) {
+        // Semaphore stays busy and unblocks next thread to work in critical section
+        scheduler.deblock((Thread*)this->waitQueue.dequeue());
+    } else {
+        // No more threads want to work so free semaphore
+        this->counter = this->counter + 1;
+    }
+
+    this->lock.release();
+}
diff --git a/c_os/lib/SpinLock.cc b/c_os/lib/SpinLock.cc
index 6f2d5f5..cfcf279 100644
--- a/c_os/lib/SpinLock.cc
+++ b/c_os/lib/SpinLock.cc
@@ -50,6 +50,8 @@ static inline unsigned long CAS(unsigned long* ptr, unsigned long old, unsigned
  * Beschreibung:    Lock belegen.                                            *
  *****************************************************************************/
 void SpinLock::acquire() {
+    // If lock == 0 the SpinLock can be aquired without waiting
+    // If lock == 1 the  while loop blocks until aquired
     while (CAS(ptr, 0, 1) != 0) {}
 }
 
diff --git a/c_os/main.cc b/c_os/main.cc
index 21428e6..08f2e2c 100755
--- a/c_os/main.cc
+++ b/c_os/main.cc
@@ -84,6 +84,11 @@ int main() {
     scheduler.schedule();
 
     // TODO: Use templates for queue so threads don't have to be casted down from chain
+    // TODO: Change scheduler to only use references instead of pointers
+    // TODO: Rewrite all demos to threads
+    // TODO: Make menu for demos
+    // TODO: Unify debug output format
+    // TODO: Serial output
 
     // Scheduler doesn't return
     return 0;