fix semaphore spinlock bug

c_os/kernel/threads/Scheduler.cc

@@ -182,6 +182,7 @@ void Scheduler::block() {
 
     cpu.disable_int();
     Thread& next = *(Thread*)this->readyQueue.dequeue();
+    // Current thread is not added to readyQueue, gets managed by semaphore
     this->dispatch(next);
 }
 

c_os/lib/Semaphore.cc

@@ -8,13 +8,13 @@ void Semaphore::p() {
     if (this->counter > 0) {
         // Semaphore can be acquired
         this->counter = this->counter - 1;
+        this->lock.release();
     } else {
-        // Block and manage thread in semaphore queue until it's woken up by v()
+        // Block and manage thread in semaphore queue until it's woken up by v() again
         this->waitQueue.enqueue(scheduler.get_active());
-        scheduler.block();
+        this->lock.release();
+        scheduler.block(); // Moves to next thread
     }
-
-    this->lock.release();
 }
 
 void Semaphore::v() {
@@ -22,11 +22,13 @@ void Semaphore::v() {
 
     if (!this->waitQueue.isEmpty()) {
         // Semaphore stays busy and unblocks next thread to work in critical section
-        scheduler.deblock((Thread*)this->waitQueue.dequeue());
+        Thread* next = (Thread*)this->waitQueue.dequeue();
+        this->lock.release();
+        scheduler.deblock(next);
     } else {
         // No more threads want to work so free semaphore
         this->counter = this->counter + 1;
+        this->lock.release();
     }
 
-    this->lock.release();
 }