thread-rcu: Fix incorrect grace period counting

thread-rcu/Makefile

@@ -1,6 +1,6 @@
 CFLAGS = -Wall
 CFLAGS += -g
-CFLAGS += -std=c11
+CFLAGS += -std=gnu11
 CFLAGS += -D'N_READERS=100'
 CFLAGS += -D'N_UPDATE_RUN=5'
 CFLAGS += -fsanitize=thread

thread-rcu/main.c

@@ -37,6 +37,7 @@ static inline void thread_barrier(struct barrier_struct *b, size_t n)
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <time.h>
 
 #include "rcu.h"
 
@@ -55,34 +56,42 @@ static atomic_uint grace_periods[GP_IDX_MAX];
 static void *reader_func(void *argv)
 {
     struct test *tmp;
+    unsigned int local_gp_idx;
     unsigned int old_prev_count;
+    struct timespec ts = {
+        .tv_sec = 0,
+        .tv_nsec = 30000L,
+    };
 
     if (rcu_init())
         abort();
 
     thread_barrier(&test_barrier, N_READERS + 1);
+    nanosleep(&ts, NULL);
 
     rcu_read_lock();
 
     tmp = rcu_dereference(dut);
 
-    if (tmp->count != atomic_load_explicit(&prev_count, memory_order_acquire)) {
-        old_prev_count = atomic_exchange_explicit(&prev_count, tmp->count,
-                                                  memory_order_release);
-        if (tmp->count != old_prev_count)
-            atomic_fetch_add_explicit(&gp_idx, 1, memory_order_release);
-        if (atomic_load_explicit(&gp_idx, memory_order_acquire) >
-            N_UPDATE_RUN) {
-            fprintf(stderr, "grace period index (%u) is over bound (%u).\n",
-                    atomic_load_explicit(&gp_idx, memory_order_acquire),
-                    N_UPDATE_RUN);
-            abort();
-        }
+    old_prev_count = atomic_load_explicit(&prev_count, memory_order_acquire);
+    if (old_prev_count < tmp->count) {
+        atomic_compare_exchange_strong(&prev_count, &old_prev_count,
+                                       tmp->count);
+    } else if (tmp->count < old_prev_count) {
+        fprintf(stderr,
+                "old count (%u) should not be larger than new one (%u).\n",
+                old_prev_count, tmp->count);
+        abort();
     }
 
-    atomic_fetch_add_explicit(
-        &grace_periods[atomic_load_explicit(&gp_idx, memory_order_acquire)], 1,
-        memory_order_relaxed);
+    local_gp_idx = atomic_load_explicit(&gp_idx, memory_order_acquire);
+    if (local_gp_idx > N_UPDATE_RUN) {
+        fprintf(stderr, "grace period index (%u) is over bound (%u).\n",
+                local_gp_idx, N_UPDATE_RUN);
+        abort();
+    }
+    atomic_fetch_add_explicit(&grace_periods[local_gp_idx], 1,
+                              memory_order_relaxed);
 
     rcu_read_unlock();
 
@@ -103,6 +112,7 @@ static void *updater_func(void *argv)
         newval->count = i;
         oldp = rcu_assign_pointer(dut, newval);
         synchronize_rcu();
+        atomic_fetch_add_explicit(&gp_idx, 1, memory_order_release);
         free(oldp);
     }
 
@@ -140,8 +150,8 @@ int main(int argc, char *argv[])
 
     if (total != N_READERS)
         fprintf(stderr,
-                "The Sum of records in the array of grace period(s) (%u) is "
-                "not the same with number of reader(s) (%u)\n",
+                "The sum of records in the array of grace period(s) (%u)\n"
+                "is not the same with number of reader(s) (%u)\n",
                 total, N_READERS);
 
     return 0;

thread-rcu/rcu.h

@@ -168,7 +168,7 @@ struct rcu_data {
     } while (0)
 #define rcu_next(np) \
     ((struct rcu_node *) (READ_ONCE((np)->__next_rcu_nesting) & ~0x3))
-#define rcu_next_mask(nrn) ((struct rcu_node *) ((uintptr_t)(nrn) & ~0x3))
+#define rcu_next_mask(nrn) ((struct rcu_node *) ((uintptr_t) (nrn) & ~0x3))
 
 static struct rcu_data rcu_data = {
     .nr_thread = 0,