ovs-thread: Do not always end quiescent state in ovs_thread_create().

A new thread must be started in a non quiescent state.  There is a call
to ovsrcu_quiesce_end() in ovsthread_wrapper(), to enforce this.

ovs_thread_create(), instead, is executed in the parent thread. It must
call ovsrcu_quiesce_end() on its first invocation, to put the main
thread in a non quiescent state.  On every other invocation, it doesn't
make sense to alter the calling thread state, so this commits wraps the
call to ovsrcu_quiesce_end() in an ovsthread_once construct.

This fixes a bug in ovs-rcu where the first call in the process to
ovsrcu_quiesce_start() will not be honored, because the calling thread
will need to create the 'urcu' thread (and creating a thread will
wrongly end its quiescent state).

ovsrcu_quiesce_start()
  ovs_rcu_quiesced()
    if (ovsthread_once_start(&once)) {
        ovs_thread_create("urcu") /*This will end the quiescent state*/
    }

This bug affects in particular ovs-vswitchd with DPDK.
In the DPDK case the first threads created are "vhost_thread" and
"dpdk_watchdog".  If dpdk_watchdog is the first to call
ovsrcu_quiesce_start() (via xsleep()), the call is not honored and
the RCU grace period lasts at least for DPDK_PORT_WATCHDOG_INTERVAL
(5s on current master).  If vhost_thread, on the other hand, is the
first to call ovsrcu_quiesce_start(), the call is not honored and the
RCU grace period lasts undefinitely, because no more calls to
ovsrcu_quiesce_start() are issued from vhost_thread.

For some reason (it's a race condition after all), on current master,
dpdk_watchdog will always be the first to call ovsrcu_quiesce_start(),
but with the upcoming DPDK database configuration changes, sometimes
vhost_thread will issue the first call to ovsrcu_quiesce_start().

Sample ovs-vswitchd.log:

2016-03-23T22:34:28.532Z|00004|ovs_rcu(urcu3)|WARN|blocked 8000 ms
waiting for vhost_thread2 to quiesce
2016-03-23T22:34:30.501Z|00118|ovs_rcu|WARN|blocked 8000 ms waiting for
vhost_thread2 to quiesce
2016-03-23T22:34:36.532Z|00005|ovs_rcu(urcu3)|WARN|blocked 16000 ms
waiting for vhost_thread2 to quiesce
2016-03-23T22:34:38.501Z|00119|ovs_rcu|WARN|blocked 16000 ms waiting for
vhost_thread2 to quiesce

The commit also adds a test for the ovs-rcu module to make sure that:
* A new thread is started in a non quiescent state.
* The first call to ovsrcu_quiesce_start() is honored.
* When a process becomes multithreaded the main thread is put in an
  active state

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ben Pfaff <blp@ovn.org>

lib/ovs-rcu.h

@@ -56,7 +56,8 @@
  *
  *         Brackets a time period during which the current thread is quiescent.
  *
- * A newly created thread is initially active, not quiescent.
+ * A newly created thread is initially active, not quiescent. When a process
+ * becomes multithreaded, the main thread becomes active, not quiescent.
  *
  * When a quiescient state has occurred in every thread, we say that a "grace
  * period" has occurred.  Following a grace period, all of the callbacks

lib/ovs-thread.c

@@ -364,13 +364,28 @@ set_min_stack_size(pthread_attr_t *attr, size_t min_stacksize)
 pthread_t
 ovs_thread_create(const char *name, void *(*start)(void *), void *arg)
 {
+    static struct ovsthread_once once = OVSTHREAD_ONCE_INITIALIZER;
     struct ovsthread_aux *aux;
     pthread_t thread;
     int error;
 
     forbid_forking("multiple threads exist");
     multithreaded = true;
-    ovsrcu_quiesce_end();
+
+    if (ovsthread_once_start(&once)) {
+        /* The first call to this function has to happen in the main thread.
+         * Before the process becomes multithreaded we make sure that the
+         * main thread is considered non quiescent.
+         *
+         * For other threads this is done in ovs_thread_wrapper(), but the
+         * main thread has no such wrapper.
+         *
+         * There's no reason to call ovsrcu_quiesce_end() in subsequent
+         * invocations of this function and it might introduce problems
+         * for other threads. */
+        ovsrcu_quiesce_end();
+        ovsthread_once_done(&once);
+    }
 
     aux = xmalloc(sizeof *aux);
     aux->start = start;

tests/automake.mk

@@ -308,6 +308,7 @@ tests_ovstest_SOURCES = \
 	tests/test-ovn.c \
 	tests/test-packets.c \
 	tests/test-random.c \
+	tests/test-rcu.c \
 	tests/test-reconnect.c \
 	tests/test-rstp.c \
 	tests/test-sflow.c \

tests/library.at

@@ -212,3 +212,7 @@ AT_CLEANUP
 AT_SETUP([test ofpbuf module])
 AT_CHECK([ovstest test-ofpbuf], [0], [])
 AT_CLEANUP
+
+AT_SETUP([test rcu])
+AT_CHECK([ovstest test-rcu-quiesce], [0], [])
+AT_CLEANUP

tests/test-rcu.c

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2016 Nicira, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <config.h>
+#undef NDEBUG
+#include "fatal-signal.h"
+#include "ovs-rcu.h"
+#include "ovs-thread.h"
+#include "ovstest.h"
+#include "util.h"
+
+static void *
+quiescer_main(void *aux OVS_UNUSED)
+{
+    /* A new thread must be not be quiescent */
+    ovs_assert(!ovsrcu_is_quiescent());
+    ovsrcu_quiesce_start();
+    /* After the above call it must be quiescent */
+    ovs_assert(ovsrcu_is_quiescent());
+
+    return NULL;
+}
+
+static void
+test_rcu_quiesce(int argc OVS_UNUSED, char *argv[] OVS_UNUSED)
+{
+    pthread_t quiescer;
+
+    fatal_signal_init();
+    quiescer = ovs_thread_create("quiescer", quiescer_main, NULL);
+
+    /* This is the main thread of the process. After spawning its first
+     * thread it must not be quiescent. */
+    ovs_assert(!ovsrcu_is_quiescent());
+
+    xpthread_join(quiescer, NULL);
+}
+
+OVSTEST_REGISTER("test-rcu-quiesce", test_rcu_quiesce);