vault backup: 2025-03-03 09:17:47

Concurrent Systems/notes/Lezione1.md

@@ -61,7 +61,7 @@ While `read()` and `write()` may be considered as atomic, their sequential compo
 
 ![[Pasted image 20250303090135.png]]
 
-#### Mutual Exclusion (MUTEX)
+### Mutual Exclusion (MUTEX)
 Ensure that some parts of the code are executed as *atomic*.
 This is needed in competition but also in cooperation.
 
@@ -75,3 +75,19 @@ Design an entry protocol (lock) and an exit protocol (unlock) such that, when us
 
 We assume that all C.S.s terminate and that the code is well-formed (*lock ; critical section ; unlock*).
 
+#### Safety and liveness
+Every solution to a problem should satisfy at least:
+- **Safety**: *nothing bad ever happens*
+- **Liveness:** *something good eventually happens*
+
+*Liveness without safety:* allow anything, something good may eventually happen but also something terrible!
+
+*Safety without liveness:* forbid anything (no activity in the system)
+
+So safety is necessary for correctness, liveness for meaningfulness.
+
+###### In the context of MUTEX:
+- **Safety:** there is at most one process at a time in a C.S.
+- **Liveness:**
+	- **Deadlock freedom:** if there is at least one invocation of lock, eventually after at least one process enters a C.S.
+	- **Starvation freedom:** every invocation of lock eve