Extract value-barrier-norace example from silver searcher

tests/regression/03-practical/38-value-barrier-norace.c

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+// Race-free due to value-based barrier, main thread only writes before.
+// Extracted from silver searcher.
+#include <stdlib.h>
+#include <stdbool.h>
+#include <pthread.h>
+#include <goblint.h>
+extern int __VERIFIER_nondet_int();
+
+bool ready = false;
+pthread_mutex_t ready_mutex = PTHREAD_MUTEX_INITIALIZER;
+
+int data = 0;
+
+void *thread(void *arg) {
+  // wait for main thread to be ready
+  pthread_mutex_lock(&ready_mutex);
+  while (!ready) { // NORACE
+    pthread_mutex_unlock(&ready_mutex);
+    // busy loop for simplicity
+    pthread_mutex_lock(&ready_mutex);
+  }
+  pthread_mutex_unlock(&ready_mutex);
+
+  int x = data; // NORACE (main thread wrote before ready)
+  return NULL;
+}
+
+int main() {
+  int threads_total = __VERIFIER_nondet_int();
+  __goblint_assume(threads_total >= 0);
+
+  pthread_t *tids = malloc(threads_total * sizeof(pthread_t));
+
+  // create threads
+  for (int i = 0; i < threads_total; i++) {
+    pthread_create(&tids[i], NULL, &thread, NULL); // may fail but doesn't matter
+  }
+
+  data = __VERIFIER_nondet_int(); // NORACE (write before ready)
+
+  // become ready
+  pthread_mutex_lock(&ready_mutex);
+  ready = true; // NORACE
+  pthread_mutex_unlock(&ready_mutex);
+
+  // join threads
+  for (int i = 0; i < threads_total; i++) {
+    pthread_join(tids[i], NULL);
+  }
+
+  free(tids);
+
+  return 0;
+}