Updated the spinlock semaphore to a cooperative semaphore

coder137 · coder137 · commit b0d68ee0c879 · 2021-01-04T01:38:54.000-08:00
diff --git a/Semaphore/README.md b/Semaphore/README.md
@@ -15,7 +15,7 @@
 
 # Spinlock Semaphore
 
-- Wait for a signal to be raised to gain access to a shared resource
+- Wait for a signal to be sent to gain access to a shared resource
 - During a **wait** this shared resource burns CPU cycles till it gets a signal from another thread.
 
 ![Spinlock Semaphore for UART](images/example2_spinlock_semaphore.PNG)
@@ -27,4 +27,7 @@
 
 # Cooperative Semaphore
 
-> TODO,
+- Wait for a signal to be sent to gain access to a shared resource
+- During a **wait** this shared resource **yields** to a different thread if it does not have access to the shared resource.
+
+**NOTE** The output and working of the Cooperative Semaphore is similar to the Spinlock Semaphore. However the cooperative semaphore is more efficient since it does not burn unnecessary CPU cycles.
diff --git a/Semaphore/application/osSemaphore.c b/Semaphore/application/osSemaphore.c
@@ -1,5 +1,7 @@
 #include "osSemaphore.h"
 
+#include "osKernel.h"
+
 #include "stm32l4xx.h"
 
 void osSemaphoreInit(uint32_t *semaphore, uint32_t value) {
@@ -8,6 +10,11 @@ void osSemaphoreInit(uint32_t *semaphore, uint32_t value) {
 
 void osSemaphoreWait(uint32_t *semaphore) {
   while (*semaphore <= 0) {
+    // This yield operation is required for a `cooperative semaphore`
+    // If the 3 lines below are disabled we have a `spinlock semaphore`
+    __disable_irq();
+    osKernelYield();
+    __enable_irq();
   }
 
   __disable_irq();
