[Qemu-devel] [PATCH 2/6] test-rcu-list: avoid torn accesses to n_reclaims and n_nodes_removed

[Emilio G. Cota]

Signed-off-by: Emilio G. Cota <c...@braap.org>
---
 tests/test-rcu-list.c | 67 +++++++++++++++++++++++++++++++++++++------
 1 file changed, 59 insertions(+), 8 deletions(-)

diff --git a/tests/test-rcu-list.c b/tests/test-rcu-list.c
index 192bfbf02e..2606b7c19d 100644
--- a/tests/test-rcu-list.c
+++ b/tests/test-rcu-list.c
@@ -25,6 +25,23 @@
 #include "qemu/rcu.h"
 #include "qemu/thread.h"
 #include "qemu/rcu_queue.h"
+#include "qemu/seqlock.h"
+
+/*
+ * Abstraction to avoid torn accesses when there is a single thread updating
+ * the count.
+ *
+ * If CONFIG_ATOMIC64 is defined, we simply use atomic accesses. Otherwise, we
+ * use a seqlock without a lock, since only one thread can update the count.
+ */
+struct Count {
+    long long val;
+#ifndef CONFIG_ATOMIC64
+    QemuSeqLock sequence;
+#endif
+};
+
+typedef struct Count Count;
 
 /*
  * Test variables.
@@ -33,8 +50,8 @@
 static QemuMutex counts_mutex;
 static long long n_reads = 0LL;
 static long long n_updates = 0LL;
-static long long n_reclaims = 0LL;
-static long long n_nodes_removed = 0LL;
+static Count n_reclaims;
+static Count n_nodes_removed;
 static long long n_nodes = 0LL;
 static int g_test_in_charge = 0;
 
@@ -60,6 +77,38 @@ static int select_random_el(int max)
     return (rand() % max);
 }
 
+static inline long long count_read(Count *c)
+{
+#ifdef CONFIG_ATOMIC64
+    /* use __nocheck because sizeof(void *) might be < sizeof(long long) */
+    return atomic_read__nocheck(&c->val);
+#else
+    unsigned int version;
+    long long val;
+
+    do {
+        version = seqlock_read_begin(&c->sequence);
+        val = c->val;
+    } while (seqlock_read_retry(&c->sequence, version));
+    return val;
+#endif
+}
+
+static inline void count_add(Count *c, long long val)
+{
+#ifdef CONFIG_ATOMIC64
+    atomic_set__nocheck(&c->val, c->val + val);
+#else
+    seqlock_write_begin(&c->sequence);
+    c->val += val;
+    seqlock_write_end(&c->sequence);
+#endif
+}
+
+static inline void count_inc(Count *c)
+{
+    count_add(c, 1);
+}
 
 static void create_thread(void *(*func)(void *))
 {
@@ -104,7 +153,7 @@ static void reclaim_list_el(struct rcu_head *prcu)
     struct list_element *el = container_of(prcu, struct list_element, rcu);
     g_free(el);
     /* Accessed only from call_rcu thread.  */
-    n_reclaims++;
+    count_inc(&n_reclaims);
 }
 
 #if TEST_LIST_TYPE == 1
@@ -232,7 +281,7 @@ static void *rcu_q_updater(void *arg)
     qemu_mutex_lock(&counts_mutex);
     n_nodes += n_nodes_local;
     n_updates += n_updates_local;
-    n_nodes_removed += n_removed_local;
+    count_add(&n_nodes_removed, n_removed_local);
     qemu_mutex_unlock(&counts_mutex);
     return NULL;
 }
@@ -286,19 +335,21 @@ static void rcu_qtest(const char *test, int duration, int 
nreaders)
         n_removed_local++;
     }
     qemu_mutex_lock(&counts_mutex);
-    n_nodes_removed += n_removed_local;
+    count_add(&n_nodes_removed, n_removed_local);
     qemu_mutex_unlock(&counts_mutex);
     synchronize_rcu();
-    while (n_nodes_removed > n_reclaims) {
+    while (count_read(&n_nodes_removed) > count_read(&n_reclaims)) {
         g_usleep(100);
         synchronize_rcu();
     }
     if (g_test_in_charge) {
-        g_assert_cmpint(n_nodes_removed, ==, n_reclaims);
+        g_assert_cmpint(count_read(&n_nodes_removed), ==,
+                        count_read(&n_reclaims));
     } else {
         printf("%s: %d readers; 1 updater; nodes read: "  \
                "%lld, nodes removed: %lld; nodes reclaimed: %lld\n",
-               test, nthreadsrunning - 1, n_reads, n_nodes_removed, 
n_reclaims);
+               test, nthreadsrunning - 1, n_reads,
+               count_read(&n_nodes_removed), count_read(&n_reclaims));
         exit(0);
     }
 }
-- 
2.17.1