Fix: call_rcu: teardown default call_rcu worker on application exit

[urcu.git] / src / urcu-call-rcu-impl.h

diff --git a/src/urcu-call-rcu-impl.h b/src/urcu-call-rcu-impl.h
index cb010687829959f1750242dadb6ecfa99b9cd912..2c23a54f8bc003d46446c08b187a82958b4a4348 100644 (file)
--- a/src/urcu-call-rcu-impl.h
+++ b/src/urcu-call-rcu-impl.h
@@ -450,8 +450,7 @@ static void call_rcu_data_init(struct call_rcu_data **crdpp,
        cds_list_add(&crdp->list, &call_rcu_data_list);
        crdp->cpu_affinity = cpu_affinity;
        crdp->gp_count = 0;
-       cmm_smp_mb();  /* Structure initialized before pointer is planted. */
-       *crdpp = crdp;
+       rcu_set_pointer(crdpp, crdp);
 
        ret = sigfillset(&newmask);
        urcu_posix_assert(!ret);
@@ -575,22 +574,27 @@ int set_cpu_call_rcu_data(int cpu, struct call_rcu_data *crdp)
 
 /*
  * Return a pointer to the default call_rcu_data structure, creating
- * one if need be.  Because we never free call_rcu_data structures,
- * we don't need to be in an RCU read-side critical section.
+ * one if need be.
+ *
+ * The call to this function with intent to use the returned
+ * call_rcu_data should be protected by RCU read-side lock.
  */
 
 struct call_rcu_data *get_default_call_rcu_data(void)
 {
-       if (default_call_rcu_data != NULL)
-               return rcu_dereference(default_call_rcu_data);
+       struct call_rcu_data *crdp;
+
+       crdp = rcu_dereference(default_call_rcu_data);
+       if (crdp != NULL)
+               return crdp;
+
        call_rcu_lock(&call_rcu_mutex);
-       if (default_call_rcu_data != NULL) {
-               call_rcu_unlock(&call_rcu_mutex);
-               return default_call_rcu_data;
-       }
-       call_rcu_data_init(&default_call_rcu_data, 0, -1);
+       if (default_call_rcu_data == NULL)
+               call_rcu_data_init(&default_call_rcu_data, 0, -1);
+       crdp = default_call_rcu_data;
        call_rcu_unlock(&call_rcu_mutex);
-       return default_call_rcu_data;
+
+       return crdp;
 }
 
 /*
@@ -1065,3 +1069,58 @@ void urcu_unregister_rculfhash_atfork(struct urcu_atfork *atfork __attribute__((
 {
        urcu_die(EPERM);
 }
+
+/*
+ * Teardown the default call_rcu worker thread if there are no queued
+ * callbacks on process exit. This prevents leaking memory.
+ *
+ * Here is how an application can ensure graceful teardown of this
+ * worker thread:
+ *
+ * - An application queuing call_rcu callbacks should invoke
+ *   rcu_barrier() before it exits.
+ * - When chaining call_rcu callbacks, the number of calls to
+ *   rcu_barrier() on application exit must match at least the maximum
+ *   number of chained callbacks.
+ * - If an application chains callbacks endlessly, it would have to be
+ *   modified to stop chaining callbacks when it detects an application
+ *   exit (e.g. with a flag), and wait for quiescence with rcu_barrier()
+ *   after setting that flag.
+ * - The statements above apply to a library which queues call_rcu
+ *   callbacks, only it needs to invoke rcu_barrier in its library
+ *   destructor.
+ *
+ * Note that this function does not presume it is being called when the
+ * application is single-threaded even though this is invoked from a
+ * destructor: this function synchronizes against concurrent calls to
+ * get_default_call_rcu_data().
+ */
+static void urcu_call_rcu_exit(void)
+{
+       struct call_rcu_data *crdp;
+       bool teardown = true;
+
+       if (default_call_rcu_data == NULL)
+               return;
+       call_rcu_lock(&call_rcu_mutex);
+       /*
+        * If the application leaves callbacks in the default call_rcu
+        * worker queue, keep the default worker in place.
+        */
+       crdp = default_call_rcu_data;
+       if (!crdp) {
+               teardown = false;
+               goto unlock;
+       }
+       if (!cds_wfcq_empty(&crdp->cbs_head, &crdp->cbs_tail)) {
+               teardown = false;
+               goto unlock;
+       }
+       rcu_set_pointer(&default_call_rcu_data, NULL);
+unlock:
+       call_rcu_unlock(&call_rcu_mutex);
+       if (teardown) {
+               synchronize_rcu();
+               call_rcu_data_free(crdp);
+       }
+}