From 7ffffd2d0f70ffe52c001126910cd0e241d8acf7 Mon Sep 17 00:00:00 2001
From: Marco Realacci <marco@marcorealacci.me>
Date: Wed, 12 Mar 2025 11:19:27 +0100
Subject: [PATCH] vault backup: 2025-03-12 11:19:27

---
 .obsidian/workspace.json                   |  4 ++--
 Concurrent Systems/notes/4 - Semaphores.md | 24 ++++++++++++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/.obsidian/workspace.json b/.obsidian/workspace.json
index f8b3faf..9499de3 100644
--- a/.obsidian/workspace.json
+++ b/.obsidian/workspace.json
@@ -34,9 +34,9 @@
               "type": "pdf",
               "state": {
                 "file": "Concurrent Systems/slides/class 4.pdf",
-                "page": 7,
+                "page": 8,
                 "left": -23,
-                "top": 248,
+                "top": 421,
                 "zoom": 0.652019002375297
               },
               "icon": "lucide-file-text",
diff --git a/Concurrent Systems/notes/4 - Semaphores.md b/Concurrent Systems/notes/4 - Semaphores.md
index 7c2d90a..0d586b2 100644
--- a/Concurrent Systems/notes/4 - Semaphores.md	
+++ b/Concurrent Systems/notes/4 - Semaphores.md	
@@ -130,3 +130,27 @@ Thanks to the semaphores, we are sure that while loops will not go on forever! T
 EXERCISE - will do later
 
 #### The Readers/Writers problem
+- Several processes want to access a file
+- Readers may simultaneously access the file
+- At most one writer at a time
+- Reads and writes are mutually exclusive
+
+>[!note] Remark
+>this generalizes the MUTEX problem (MUTEX = RW with only writers)
+
+The read/write operations on the file will all have the following shape:
+```
+conc_read() :=
+	begin_read()
+	read()
+	end_read()
+
+conc_write() :=
+	begin_write()
+	write()
+	end_write()
+```
+
+##### Weak priority to Readers
+- If a reader arrives during a read, it can surpass possible writers already suspended (risk of starvation for the writes)
+- When a writer terminates, it activates the first suspended process, irrispectively of whether it is a reader or a writer (so, the priority to readers is said «weak»)
\ No newline at end of file