From d9b9d82c7936cd7b513c10972d61fac18e26d700 Mon Sep 17 00:00:00 2001
From: PowerUser64 <blake@blakenorth.net>
Date: Tue, 19 Nov 2024 13:48:43 -0800
Subject: [PATCH] mutex(fix): prevents a potential race condition where threads
 could

skip waiting for the manager to give them the mutex

The solution was to remove write access from threads to the
manager_took_mutex variable
---
 main.cpp | 21 +++++++++------------
 1 file changed, 9 insertions(+), 12 deletions(-)

diff --git a/main.cpp b/main.cpp
index 2391d85..ca4be50 100644
--- a/main.cpp
+++ b/main.cpp
@@ -1,3 +1,8 @@
+/**
+ * @brief A mutex-like thread syncronization mechanism
+ * @author Blake North <blake.north@digipen.edu>
+ */
+
 #include <cstddef>  // size_t
 #include <cstring>  // memset
 #include <iostream> // cout
@@ -26,7 +31,7 @@ struct thread_data {
   size_t id;
   bool *wants_mutex;
   const bool *has_mutex;
-  bool *manager_took_mutex;
+  const bool *manager_took_mutex;
   bool *is_finished;
   void *data;
 };
@@ -50,13 +55,10 @@ void *thread_task_increment(const struct thread_data &thread) {
     // mutex
     {
 
-      // wait for the thread manager to take the mutex if it hasn't yet
-      while (!*thread.manager_took_mutex)
-        ;
       // tell the thread manager we want the mutex
       (*thread.wants_mutex) = true;
       // block until we have the mutex
-      while (!*thread.has_mutex)
+      while (!*thread.has_mutex && *thread.manager_took_mutex)
         ;
 
       // enter the mutex
@@ -68,8 +70,6 @@ void *thread_task_increment(const struct thread_data &thread) {
 
       // tell the thread manager we don't need the mutex
       (*thread.wants_mutex) = false;
-      // wait until the thread manager takes the mutex from us
-      *thread.manager_took_mutex = false;
       // mutex exit
     }
     loops++;
@@ -151,7 +151,9 @@ void do_threading(struct thread_group threads) {
 
               // take the mutex from the thread that has it
               threads.has_mutex[tid_has] = false;
+              threads.manager_took_mutex[tid_has] = true;
               // give the mutex to the thread that wants it
+              threads.manager_took_mutex[tid_wants] = false;
               threads.has_mutex[tid_wants] = true;
             }
 
@@ -166,11 +168,6 @@ void do_threading(struct thread_group threads) {
       }
     }
 
-    // tell all threads we're done taking the mutex from whoever had it
-    for (size_t tid = 0; tid < threads.total_threads; ++tid) {
-      threads.manager_took_mutex[tid] = true;
-    }
-
     // find how many threads are done with the mutex
     finished_threads = 0;
     for (size_t tid = 0; tid < threads.total_threads; ++tid) {