From e87d0dbde2f1534169b0ac61ae6f361b85c03b88 Mon Sep 17 00:00:00 2001
From: Jet Hughes <jethughes0@gmail.com>
Date: Sun, 30 Oct 2022 15:25:53 +1300
Subject: [PATCH] vault backup: 2022-10-30 15:25:53

---
 content/notes/20-sync-deadlock.md | 31 +++++++++++++++++++++++++++++--
 content/notes/cosc-204.md         |  1 +
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/content/notes/20-sync-deadlock.md b/content/notes/20-sync-deadlock.md
index 6a559f517..151b6ade0 100644
--- a/content/notes/20-sync-deadlock.md
+++ b/content/notes/20-sync-deadlock.md
@@ -107,7 +107,34 @@ again:
 
 ```
 
-## for mutual exclusion
-![slide|400](https://i.imgur.com/fkBl7PR.png)
+semaphore without busy waiting ![](https://i.imgur.com/bLzY5q3.png)
+![mutex|400](https://i.imgur.com/fkBl7PR.png)
+![producer consumer again](https://i.imgur.com/ZN8bonW.png)
 
+range of semaphore init vals
+- 1 for mutex
+- 0 for process sync
+- >1 for concurrent processes/threads
 
+- negative values tell us how many processes are waiting
+
+# Deadlock
+Definition of deadlock: a set of processes is in a deadlocked state if every process is waiting for an event that can only be caused by another process in the set.
+
+![slide|400](https://i.imgur.com/XTSVwVI.png)
+
+conditions
+- mutual exclusion: at least one of the held resources must be nonsharable
+- hold and wait: must be at least one process holding a resource and one waiting
+- no preemption: a resource can only be released by the process thats holding it
+- circular wait: 
+	- P1 is waiting on a resource held by P2; 
+	- P2 is waiting on . . . 
+	- Pn is waiting on a resource held by P1.
+
+typical solution
+- break wait by acquiring lock in the same order
+- ![](https://i.imgur.com/tDCTXWu.png)
+
+os solution
+- ![](https://i.imgur.com/1qHM2w0.png)
diff --git a/content/notes/cosc-204.md b/content/notes/cosc-204.md
index f9ab8e5ad..b56e1a6fb 100644
--- a/content/notes/cosc-204.md
+++ b/content/notes/cosc-204.md
@@ -50,6 +50,7 @@ tags:
 - [17-processes-communication](notes/17-processes-communication.md)
 - [18-processes-sharing-and-threads](notes/18-processes-sharing-and-threads.md)
 - [19-cpu-scheduling](notes/19-cpu-scheduling.md)
+- [20-sync-deadlock](notes/20-sync-deadlock.md)
 
 
 # Archive