vault backup: 2025-03-12 11:44:27

Concurrent Systems/notes/4 - Semaphores.md

@@ -205,7 +205,8 @@ end_write() :=
 
 ##### Weak priority to Writers
 ```
-GLOB_MUTEX, PRIO_MUTEX, R_MUTEX and W_MUTEX semaphores init. at 1 R and W shared registers init. at 0
+GLOB_MUTEX, PRIO_MUTEX (to prioritize the writers), R_MUTEX and W_MUTEX semaphores init. at 1
+R and W shared registers init. at 0
 
 begin_read() :=
 	PRIO_MUTEX.down()
@@ -217,5 +218,27 @@ begin_read() :=
 	PRIO_MUTEX.up()
 	return
 
-end_read() :=
-	
+end_read() := # like weak priority to readers
+	R_MUTEX.down()
+	R--
+	if R = 0 then GLOB_MUTEX.up()
+	R_MUTEX.up()
+	return
+
+def begin_write() :=
+	W_MUTEX.down()
+	W++
+	if W = 1 then
+		PRIO_MUTEX.down()
+	W_MUTEX.up()
+	GLOB_MUTEX.down()
+	return
+
+def end_write() :=
+	GLOB_MUTEX.up()
+	W_MUTEX.down()
+	W--
+	if W = 0 then
+		PRIO_MUTEX.up()
+	W_MUTEX.up()
+	return