From a8b293d935e39bea69583d9cc2006780a92ab26c Mon Sep 17 00:00:00 2001
From: Marco Realacci <marco@marcorealacci.me>
Date: Mon, 17 Mar 2025 10:09:23 +0100
Subject: [PATCH] vault backup: 2025-03-17 10:09:23

---
 .obsidian/workspace.json                      | 16 +++++++-------
 .../notes/4c - Dining Philosophers.md         | 22 +++++++++++++++++++
 2 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/.obsidian/workspace.json b/.obsidian/workspace.json
index b4675d6..0cb290e 100644
--- a/.obsidian/workspace.json
+++ b/.obsidian/workspace.json
@@ -34,10 +34,10 @@
               "type": "pdf",
               "state": {
                 "file": "Concurrent Systems/slides/class 4.pdf",
-                "page": 17,
-                "left": -36,
-                "top": 91,
-                "zoom": 0.4133016627078385
+                "page": 18,
+                "left": -26,
+                "top": 163,
+                "zoom": 0.57541567695962
               },
               "icon": "lucide-file-text",
               "title": "class 4"
@@ -196,7 +196,8 @@
       }
     ],
     "direction": "horizontal",
-    "width": 364.5
+    "width": 364.5,
+    "collapsed": true
   },
   "left-ribbon": {
     "hiddenItems": {
@@ -217,10 +218,10 @@
   },
   "active": "6edd4157a160e462",
   "lastOpenFiles": [
+    "Concurrent Systems/slides/class 4.pdf",
+    "Concurrent Systems/notes/4c - Dining Philosophers.md",
     "Concurrent Systems/notes/images/Pasted image 20250317100456.png",
     "Concurrent Systems/notes/4b - Monitors.md",
-    "Concurrent Systems/notes/4c - Dining Philosophers.md",
-    "Concurrent Systems/slides/class 4.pdf",
     "Concurrent Systems/slides/class 5.pdf",
     "HCIW/slides/4 HUI2016-6-forcefeedback.pdf",
     "HCIW/slides/3b Haptic slides.pdf",
@@ -251,7 +252,6 @@
     "Concurrent Systems/notes/images/Pasted image 20250304090219.png",
     "Concurrent Systems/notes/images/Pasted image 20250304090207.png",
     "Concurrent Systems/notes/images/Pasted image 20250304084901.png",
-    "Concurrent Systems/notes/images/Pasted image 20250304084537.png",
     "Foundation of data science/notes/1 CV Basics.md",
     "Foundation of data science/notes/7 Autoencoders.md",
     "Foundation of data science/notes/6 PCA.md",
diff --git a/Concurrent Systems/notes/4c - Dining Philosophers.md b/Concurrent Systems/notes/4c - Dining Philosophers.md
index 6bcb9f9..c0dac2b 100644
--- a/Concurrent Systems/notes/4c - Dining Philosophers.md	
+++ b/Concurrent Systems/notes/4c - Dining Philosophers.md	
@@ -5,3 +5,25 @@ The first real practical example of a concurrent system.
 - a philosopher must pick up first one chopstick, then the second one, not both at once
 ![[Pasted image 20250317100456.png|100]]
 
+**PROBLEM:** *Devise a deadlock-free algorithm for allocating these limited resources (chopsticks) among several processes (philosophers).*
+
+#### A wrong solution
+each chopstick is governed by a mutual exclusion semaphore that prevents any other philosopher from picking up the chopstick when it is already in use by another philosopher
+
+```
+semaphore chopstick[5] initialized to 1
+Philosopher(i) :=
+	while(1) do
+		chopstick[i].down()
+		chopstick[(i+1)%N].down()
+		// eat
+		chopstick[(i+1)%N].up()
+		chopstick[i].up()
+```
+No two neighbors can eat simultaneously, but we can have a deadlock if all philosophers grab their right chopstick simultaneously.
+
+#### Solution 1
+Give a number to all forks and always try with the smaller.
+All philosophers must first pick left and then right, except for the last one that first picks right and then left.
+
+So there will be  c
\ No newline at end of file