From b4fa80ad676217ab28bd1da20d25542cc9089957 Mon Sep 17 00:00:00 2001
From: "Remzi H. Arpaci-Dusseau" <remzi.arpacidusseau@gmail.com>
Date: Mon, 22 Apr 2019 13:14:08 -0500
Subject: [PATCH] reader writer lock + small cleanups

---
 threads-sema/Makefile                         |  1 +
 threads-sema/README.md                        |  8 ++
 threads-sema/dining_philosophers_deadlock.c   |  5 +-
 .../dining_philosophers_no_deadlock.c         |  5 +-
 threads-sema/rwlock.c                         | 94 +++++++++++++++++++
 5 files changed, 109 insertions(+), 4 deletions(-)
 create mode 100644 threads-sema/rwlock.c

diff --git a/threads-sema/Makefile b/threads-sema/Makefile
index 32c63fa..0d7aba7 100644
--- a/threads-sema/Makefile
+++ b/threads-sema/Makefile
@@ -8,6 +8,7 @@ SRCS   := dining_philosophers_deadlock.c \
 	join.c \
 	binary.c \
 	producer_consumer_works.c \
+	rwlock.c \
 	zemaphore.c
 
 OBJS   := ${SRCS:c=o}
diff --git a/threads-sema/README.md b/threads-sema/README.md
index 9683b9c..652e1e5 100644
--- a/threads-sema/README.md
+++ b/threads-sema/README.md
@@ -43,6 +43,9 @@ prompt> ./producer_consumer 1 1000 1
 The output should print each produced item once, and show which
 consumer consumed each produced item.
 
+# Reader/Writer Locks
+
+Code in `rwlock.c`. Build via `make`, run via `rwlock`.
 
 # Dining Philosophers
 
@@ -55,3 +58,8 @@ different forms:
 
 Run `make` to build all of them with the highly primitive `Makefile`.
 
+
+# Zemaphores
+
+Code in `zemaphore.c`. We bet you can figure out the rest. This is just
+a small test of the Zemaphore with the fork/join problem.
diff --git a/threads-sema/dining_philosophers_deadlock.c b/threads-sema/dining_philosophers_deadlock.c
index 28c3170..84db8d6 100644
--- a/threads-sema/dining_philosophers_deadlock.c
+++ b/threads-sema/dining_philosophers_deadlock.c
@@ -40,13 +40,14 @@ void eat() {
 
 void *philosopher(void *arg) {
     arg_t *args = (arg_t *) arg;
+    int p = args->thread_id;
 
     int i;
     for (i = 0; i < args->num_loops; i++) {
 	think();
-	get_forks(args->thread_id);
+	get_forks(p);
 	eat();
-	put_forks(args->thread_id);
+	put_forks(p);
     }
     return NULL;
 }
diff --git a/threads-sema/dining_philosophers_no_deadlock.c b/threads-sema/dining_philosophers_no_deadlock.c
index c9d409f..6840410 100644
--- a/threads-sema/dining_philosophers_no_deadlock.c
+++ b/threads-sema/dining_philosophers_no_deadlock.c
@@ -45,13 +45,14 @@ void eat() {
 
 void *philosopher(void *arg) {
     arg_t *args = (arg_t *) arg;
+    int p = args->thread_id;
 
     int i;
     for (i = 0; i < args->num_loops; i++) {
 	think();
-	get_forks(args->thread_id);
+	get_forks(p);
 	eat();
-	put_forks(args->thread_id);
+	put_forks(p);
     }
     return NULL;
 }
diff --git a/threads-sema/rwlock.c b/threads-sema/rwlock.c
new file mode 100644
index 0000000..6b2bc10
--- /dev/null
+++ b/threads-sema/rwlock.c
@@ -0,0 +1,94 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <pthread.h>
+#include <semaphore.h>
+#include <unistd.h>
+
+#include "common.h"
+#include "common_threads.h"
+
+typedef struct _rwlock_t {
+    sem_t writelock;
+    sem_t lock;
+    int readers;
+} rwlock_t;
+
+void rwlock_init(rwlock_t *lock) {
+    lock->readers = 0;
+    Sem_init(&lock->lock, 1); 
+    Sem_init(&lock->writelock, 1); 
+}
+
+void rwlock_acquire_readlock(rwlock_t *lock) {
+    Sem_wait(&lock->lock);
+    lock->readers++;
+    if (lock->readers == 1)
+	Sem_wait(&lock->writelock);
+    Sem_post(&lock->lock);
+}
+
+void rwlock_release_readlock(rwlock_t *lock) {
+    Sem_wait(&lock->lock);
+    lock->readers--;
+    if (lock->readers == 0)
+	Sem_post(&lock->writelock);
+    Sem_post(&lock->lock);
+}
+
+void rwlock_acquire_writelock(rwlock_t *lock) {
+    Sem_wait(&lock->writelock);
+}
+
+void rwlock_release_writelock(rwlock_t *lock) {
+    Sem_post(&lock->writelock);
+}
+
+int read_loops;
+int write_loops;
+int counter = 0;
+
+rwlock_t mutex;
+
+void *reader(void *arg) {
+    int i;
+    int local = 0;
+    for (i = 0; i < read_loops; i++) {
+	rwlock_acquire_readlock(&mutex);
+	local = counter;
+	rwlock_release_readlock(&mutex);
+	printf("read %d\n", local);
+    }
+    printf("read done: %d\n", local);
+    return NULL;
+}
+
+void *writer(void *arg) {
+    int i;
+    for (i = 0; i < write_loops; i++) {
+	rwlock_acquire_writelock(&mutex);
+	counter++;
+	rwlock_release_writelock(&mutex);
+    }
+    printf("write done\n");
+    return NULL;
+}
+
+int main(int argc, char *argv[]) {
+    if (argc != 3) {
+	fprintf(stderr, "usage: rwlock readloops writeloops\n");
+	exit(1);
+    }
+    read_loops = atoi(argv[1]);
+    write_loops = atoi(argv[2]);
+    
+    rwlock_init(&mutex); 
+    pthread_t c1, c2;
+    Pthread_create(&c1, NULL, reader, NULL);
+    Pthread_create(&c2, NULL, writer, NULL);
+    Pthread_join(c1, NULL);
+    Pthread_join(c2, NULL);
+    printf("all done\n");
+    return 0;
+}
+    
+