Clean-up: move global sessiond symbols out of main.o

[lttng-tools.git] / src / bin / lttng-sessiond / ready.c

diff --git a/src/bin/lttng-sessiond/ready.c b/src/bin/lttng-sessiond/ready.c
new file mode 100644 (file)
index 0000000..3a268b0
--- /dev/null
+++ b/src/bin/lttng-sessiond/ready.c
@@ -0,0 +1,80 @@
+/*
+ * Copyright (C) 2011 - David Goulet <david.goulet@polymtl.ca>
+ *                      Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *               2013 - Jérémie Galarneau <jeremie.galarneau@efficios.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License, version 2 only,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include <common/macros.h>
+#include <urcu.h>
+#include "lttng-sessiond.h"
+
+/*
+ * The initialization of the session daemon is done in multiple phases.
+ *
+ * While all threads are launched near-simultaneously, only some of them
+ * are needed to ensure the session daemon can start to respond to client
+ * requests.
+ *
+ * There are two important guarantees that we wish to offer with respect
+ * to the initialisation of the session daemon:
+ *   - When the daemonize/background launcher process exits, the sessiond
+ *     is fully able to respond to client requests,
+ *   - Auto-loaded sessions are visible to clients.
+ *
+ * In order to achieve this, a number of support threads have to be launched
+ * to allow the "client" thread to function properly. Moreover, since the
+ * "load session" thread needs the client thread, we must provide a way
+ * for the "load session" thread to know that the "client" thread is up
+ * and running.
+ *
+ * Hence, the support threads decrement the lttng_sessiond_ready counter
+ * while the "client" threads waits for it to reach 0. Once the "client" thread
+ * unblocks, it posts the message_thread_ready semaphore which allows the
+ * "load session" thread to progress.
+ *
+ * This implies that the "load session" thread is the last to be initialized
+ * and will explicitly call sessiond_signal_parents(), which signals the parents
+ * that the session daemon is fully initialized.
+ *
+ * The four (4) support threads are:
+ *  - agent_thread
+ *  - notification_thread
+ *  - rotation_thread
+ *  - health_thread
+ */
+#define NR_LTTNG_SESSIOND_SUPPORT_THREADS 4
+int lttng_sessiond_ready = NR_LTTNG_SESSIOND_SUPPORT_THREADS;
+
+LTTNG_HIDDEN
+void sessiond_notify_ready(void)
+{
+       /*
+        * This memory barrier is paired with the one performed by
+        * the client thread after it has seen that 'lttng_sessiond_ready' is 0.
+        *
+        * The purpose of these memory barriers is to ensure that all
+        * initialization operations of the various threads that call this
+        * function to signal that they are ready are commited/published
+        * before the client thread can see the 'lttng_sessiond_ready' counter
+        * reach 0.
+        *
+        * Note that this could be a 'write' memory barrier, but a full barrier
+        * is used in case the code using this utility changes. The performance
+        * implications of this choice are minimal since this is a slow path.
+        */
+       cmm_smp_mb();
+       uatomic_sub(&lttng_sessiond_ready, 1);
+}