From bb639b166c10368810cbec7477696c1780a276f9 Mon Sep 17 00:00:00 2001
From: Remzi Arpaci-Dusseau <remzi.arpacidusseau@gmail.com>
Date: Wed, 8 May 2019 14:35:39 -0500
Subject: [PATCH] solution for atomicity bug

---
 threads-bugs/Makefile          |  3 +-
 threads-bugs/atomicity_fixed.c | 66 ++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+), 1 deletion(-)
 create mode 100644 threads-bugs/atomicity_fixed.c

diff --git a/threads-bugs/Makefile b/threads-bugs/Makefile
index fff9c59..3e04898 100644
--- a/threads-bugs/Makefile
+++ b/threads-bugs/Makefile
@@ -7,7 +7,8 @@ ifeq ($(OS),Linux)
 	LIBS += -pthread
 endif
 
-SRCS   := atomicity.c 
+SRCS   := atomicity.c \
+	atomicity_fixed.c
 
 OBJS   := ${SRCS:c=o}
 PROGS  := ${SRCS:.c=}
diff --git a/threads-bugs/atomicity_fixed.c b/threads-bugs/atomicity_fixed.c
new file mode 100644
index 0000000..cb71dc9
--- /dev/null
+++ b/threads-bugs/atomicity_fixed.c
@@ -0,0 +1,66 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <pthread.h>
+#include <unistd.h>
+
+#include "common.h"
+#include "common_threads.h"
+
+pthread_mutex_t proc_info_lock = PTHREAD_MUTEX_INITIALIZER;
+
+typedef struct {
+    int pid;
+} proc_t;
+
+typedef struct {
+    proc_t *proc_info;
+} thread_info_t; 
+
+proc_t p;
+thread_info_t th;
+thread_info_t *thd;
+
+void *thread1(void *arg) {
+    printf("t1: before check\n");
+    Pthread_mutex_lock(&proc_info_lock);
+    if (thd->proc_info) {
+	printf("t1: after check\n");
+	sleep(2);
+	printf("t1: use!\n");
+	printf("%d\n", thd->proc_info->pid);
+    }
+    Pthread_mutex_unlock(&proc_info_lock);
+    return NULL;
+}
+
+void *thread2(void *arg) {
+    printf("                 t2: begin\n");
+    sleep(1); // change to 5 to make the code "work"...
+    Pthread_mutex_lock(&proc_info_lock);
+    printf("                 t2: set to NULL\n");
+    thd->proc_info = NULL;
+    Pthread_mutex_unlock(&proc_info_lock);
+    return NULL;
+}
+
+int main(int argc, char *argv[]) {                    
+    if (argc != 1) {
+	fprintf(stderr, "usage: main\n");
+	exit(1);
+    }
+    thread_info_t t;
+    p.pid = 100;
+    t.proc_info = &p;
+    thd = &t;
+
+    pthread_t p1, p2;
+    printf("main: begin\n");
+    Pthread_create(&p1, NULL, thread1, NULL); 
+    Pthread_create(&p2, NULL, thread2, NULL);
+    // join waits for the threads to finish
+    Pthread_join(p1, NULL); 
+    Pthread_join(p2, NULL); 
+    printf("main: end\n");
+    return 0;
+}
+