vault backup: 2025-03-03 09:47:47

Concurrent Systems/notes/Lezione1.md

@@ -116,7 +116,7 @@ Atomic R/W registers are storage units that can be accessed through two operatio
 ### Peterson algorithm (for two processes)
 Let's try to enforce MUTEX with just 2 processes.
 
-1st attempt:
+##### 1st attempt:
 ```
 lock(i) :=
 	AFTER_YOU <- i
@@ -128,14 +128,30 @@ unlock(i) :=
 ```
 This protocol satisfies MUTEX, but suffers from deadlock (if one process never locks).
 
-2nd attempt:
+##### 2nd attempt:
 ```
 Initialize FLAG[0] and FLAG[1] to down
 lock(i) :=
-	AFTER_YOU <- i
-	wait AFTER_YOU != i
+	FLAG[i] <- up
+	wait FLAG[1-i] = down
 	return
 
 unlock(i) :=
+	FLAG[i] <- down
+	return
+```
+Still suffers from deadlock if both processes simultaneously raise their flag.
+
+##### Correct solution:
+```
+Initialize FLAG[0] and FLAG[1] to down
+lock(i) :=
+	FLAG[i] <- up
+	AFTER_YOU <- i
+	wait FLAG[1-i] = down OR AFTER_YOU != i
+	return
+
+unlock(i) :=
+	FLAG[i] <- down
 	return
 ```