Detailed gem5 analysis of how data races happen

And pass niters as a thread argument to all threading implementations...
otherwise every loop has to do a memory load from the global!

userland/c/atomic.c

@@ -1,4 +1,4 @@
-/* https://cirosantilli.com/linux-kernel-module-cheat#c-multithreading */
+/* https://cirosantilli.com/linux-kernel-module-cheat#atomic-c */
 
 #if __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_THREADS__)
 #include <stdatomic.h>
@@ -9,10 +9,9 @@
 
 atomic_int acnt;
 int cnt;
-size_t niters;
 
-int f(void *thr_data) {
-    (void)thr_data;
+int my_thread_main(void *thr_data) {
+    size_t niters = *(size_t *)thr_data;
     for (size_t i = 0; i < niters; ++i) {
         ++cnt;
         ++acnt;
@@ -23,7 +22,7 @@ int f(void *thr_data) {
 
 int main(int argc, char **argv) {
 #if __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_THREADS__)
-    size_t nthreads;
+    size_t niters, nthreads;
     thrd_t *threads;
     if (argc > 1) {
         nthreads = strtoull(argv[1], NULL, 0);
@@ -37,7 +36,7 @@ int main(int argc, char **argv) {
     }
     threads = malloc(sizeof(thrd_t) * nthreads);
     for(size_t i = 0; i < nthreads; ++i)
-        thrd_create(threads + i, f, NULL);
+        thrd_create(threads + i, my_thread_main, &niters);
     for(size_t i = 0; i < nthreads; ++i)
         thrd_join(threads[i], NULL);
     free(threads);

userland/c/snprintf.c

@@ -39,14 +39,18 @@ int main(void) {
     /* Less common case where string does not fit. Error handling would
      * normally follow in a real program. */
     {
-        int in = 1234;
-        char out[6];
         /* The return here is the same as before.
          *
          * Because it is >= than the imposed limit of 6, we know that
          * the write failed to fully complete. */
+#if 0
+        /* GCC 8.3.0 with -O3 actually detects this and breaks the build.
+         * error: ‘cd’ directive output truncated writing 2 bytes into a region of size 0 [-Werror=format-truncation=] */
+        int in = 1234;
+        char out[6];
         assert(snprintf(out, sizeof(out), "ab%dcd", in) == 8);
         assert(strcmp(out, "ab123") == 0);
+#endif
     }
 #endif
     return EXIT_SUCCESS;

userland/cpp/atomic/main.hpp

@@ -8,8 +8,6 @@
 #include <thread>
 #include <vector>
 
-size_t niters;
-
 #if LKMC_USERLAND_ATOMIC_STD_ATOMIC
 std::atomic_ulong global(0);
 #else
@@ -20,7 +18,7 @@ uint64_t global = 0;
 std::mutex mutex;
 #endif
 
-void threadMain() {
+void threadMain(size_t niters) {
     for (size_t i = 0; i < niters; ++i) {
 #if LKMC_USERLAND_ATOMIC_MUTEX
         mutex.lock();
@@ -97,7 +95,7 @@ void threadMain() {
 
 int main(int argc, char **argv) {
 #if __cplusplus >= 201103L
-    size_t nthreads;
+    size_t niters, nthreads;
     if (argc > 1) {
         nthreads = std::stoull(argv[1], NULL, 0);
     } else {
@@ -110,7 +108,7 @@ int main(int argc, char **argv) {
     }
     std::vector<std::thread> threads(nthreads);
     for (size_t i = 0; i < nthreads; ++i)
-        threads[i] = std::thread(threadMain);
+        threads[i] = std::thread(threadMain, niters);
     for (size_t i = 0; i < nthreads; ++i)
         threads[i].join();
     uint64_t expect = nthreads * niters;

userland/kernel_modules/ioctl.c

@@ -25,9 +25,13 @@ int main(int argc, char **argv) {
     request = strtol(argv[2], NULL, 10);
     if (argc > 3) {
         arg0 = strtol(argv[3], NULL, 10);
+    } else {
+        arg0 = 0;
     }
     if (argc > 4) {
         arg1 = strtol(argv[4], NULL, 10);
+    } else {
+        arg1 = 0;
     }
 
     fd = open(ioctl_path, O_RDONLY);

userland/posix/pthread_mutex.c

@@ -7,12 +7,11 @@
 #include <sys/types.h>
 #include <unistd.h>
 
-unsigned long long niters;
 unsigned long long global = 0;
 pthread_mutex_t main_thread_mutex = PTHREAD_MUTEX_INITIALIZER;
 
 void* main_thread(void *arg) {
-    (void)arg;
+    unsigned long long niters = *(unsigned long long *)arg;
     unsigned long long i;
     for (i = 0; i < niters; ++i) {
         pthread_mutex_lock(&main_thread_mutex);
@@ -24,7 +23,7 @@ void* main_thread(void *arg) {
 
 int main(int argc, char **argv) {
     pthread_t *threads;
-    unsigned long long i, nthreads;
+    unsigned long long i, niters, nthreads;
 
     /* CLI arguments. */
     if (argc > 1) {
@@ -41,7 +40,7 @@ int main(int argc, char **argv) {
 
     /* Action */
     for (i = 0; i < nthreads; ++i)
-        pthread_create(&threads[i], NULL, main_thread, NULL);
+        pthread_create(&threads[i], NULL, main_thread, &niters);
     for (i = 0; i < nthreads; ++i)
         pthread_join(threads[i], NULL);
     assert(global == nthreads * niters);