mutex: comments and name clarity

In addition to the previous commit, this commit makes threads need to
wait for the thread manager to say it took the mutex from them before
they can ask for it again.

main.cpp

@@ -18,10 +18,10 @@ typedef void *(*ThreadTask)(const struct thread_data &);
 typedef void *(*PthreadFun)(void *arg);
 
 struct thread_group {
-  bool wants_mutex[NUM_THREADS] = {0};
+  bool wants_mutex[NUM_THREADS] = {0};       // set by the thread
-  bool has_mutex[NUM_THREADS] = {0};
+  bool has_mutex[NUM_THREADS] = {0};         // set by us, read by thread
-  bool manager_took_mutex[NUM_THREADS] = {0};
+  bool can_ask_for_mutex[NUM_THREADS] = {1}; // set by us, read by thread
-  bool threads_finished[NUM_THREADS] = {0};
+  bool threads_finished[NUM_THREADS] = {0};  // set by thread
   ThreadTask task;
   size_t total_threads;
   void *data;
@@ -31,7 +31,7 @@ struct thread_data {
   size_t id;
   bool *wants_mutex;
   const bool *has_mutex;
-  const bool *manager_took_mutex;
+  const bool *can_ask_for_mutex;
   bool *is_finished;
   void *data;
 };
@@ -55,8 +55,10 @@ void *thread_task_increment(const struct thread_data &thread) {
     // mutex
     {
 
-      // tell the thread manager we want the mutex
+      // wait for permission to ask for the mutex
-      while (!*thread.manager_took_mutex)
+      while (!*thread.can_ask_for_mutex)
+        ;
+      // say we want the mutex
       (*thread.wants_mutex) = true;
       // block until we have the mutex
       while (!*thread.has_mutex)
@@ -103,8 +105,8 @@ void do_threading(struct thread_group threads) {
          threads.total_threads * sizeof(*threads.threads_finished));
 
   // at the start, we have the mutex and haven't given it
-  memset(threads.manager_took_mutex, 1,
+  memset(threads.can_ask_for_mutex, 1,
-         threads.total_threads * sizeof(*threads.manager_took_mutex));
+         threads.total_threads * sizeof(*threads.can_ask_for_mutex));
 
   // create thread data
   for (size_t tid = 0; tid < threads.total_threads; ++tid) {
@@ -112,7 +114,7 @@ void do_threading(struct thread_group threads) {
         .id = tid,
         .wants_mutex = &threads.wants_mutex[tid],
         .has_mutex = &threads.has_mutex[tid],
-        .manager_took_mutex = &threads.manager_took_mutex[tid],
+        .can_ask_for_mutex = &threads.can_ask_for_mutex[tid],
         .is_finished = &threads.threads_finished[tid],
         .data = threads.data,
     };
@@ -152,9 +154,9 @@ 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;
+              threads.can_ask_for_mutex[tid_has] = true;
               // give the mutex to the thread that wants it
-              threads.manager_took_mutex[tid_wants] = false;
+              threads.can_ask_for_mutex[tid_wants] = false;
               threads.has_mutex[tid_wants] = true;
             }