| 1 | /* |
| 2 | * Copyright (C) 2011 - David Goulet <david.goulet@polymtl.ca> |
| 3 | * Mathieu Desnoyers <mathieu.desnoyers@efficios.com> |
| 4 | * 2013 - Jérémie Galarneau <jeremie.galarneau@efficios.com> |
| 5 | * |
| 6 | * This program is free software; you can redistribute it and/or modify |
| 7 | * it under the terms of the GNU General Public License, version 2 only, |
| 8 | * as published by the Free Software Foundation. |
| 9 | * |
| 10 | * This program is distributed in the hope that it will be useful, |
| 11 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 12 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| 13 | * GNU General Public License for more details. |
| 14 | * |
| 15 | * You should have received a copy of the GNU General Public License along |
| 16 | * with this program; if not, write to the Free Software Foundation, Inc., |
| 17 | * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. |
| 18 | */ |
| 19 | |
| 20 | #include <common/macros.h> |
| 21 | #include <urcu.h> |
| 22 | #include "lttng-sessiond.h" |
| 23 | |
| 24 | /* |
| 25 | * The initialization of the session daemon is done in multiple phases. |
| 26 | * |
| 27 | * While all threads are launched near-simultaneously, only some of them |
| 28 | * are needed to ensure the session daemon can start to respond to client |
| 29 | * requests. |
| 30 | * |
| 31 | * There are two important guarantees that we wish to offer with respect |
| 32 | * to the initialisation of the session daemon: |
| 33 | * - When the daemonize/background launcher process exits, the sessiond |
| 34 | * is fully able to respond to client requests, |
| 35 | * - Auto-loaded sessions are visible to clients. |
| 36 | * |
| 37 | * In order to achieve this, a number of support threads have to be launched |
| 38 | * to allow the "client" thread to function properly. Moreover, since the |
| 39 | * "load session" thread needs the client thread, we must provide a way |
| 40 | * for the "load session" thread to know that the "client" thread is up |
| 41 | * and running. |
| 42 | * |
| 43 | * Hence, the support threads decrement the lttng_sessiond_ready counter |
| 44 | * while the "client" threads waits for it to reach 0. Once the "client" thread |
| 45 | * unblocks, it posts the message_thread_ready semaphore which allows the |
| 46 | * "load session" thread to progress. |
| 47 | * |
| 48 | * This implies that the "load session" thread is the last to be initialized |
| 49 | * and will explicitly call sessiond_signal_parents(), which signals the parents |
| 50 | * that the session daemon is fully initialized. |
| 51 | * |
| 52 | * The four (4) support threads are: |
| 53 | * - agent_thread |
| 54 | * - notification_thread |
| 55 | * - rotation_thread |
| 56 | * - health_thread |
| 57 | */ |
| 58 | #define NR_LTTNG_SESSIOND_SUPPORT_THREADS 4 |
| 59 | int lttng_sessiond_ready = NR_LTTNG_SESSIOND_SUPPORT_THREADS; |
| 60 | |
| 61 | LTTNG_HIDDEN |
| 62 | void sessiond_notify_ready(void) |
| 63 | { |
| 64 | /* |
| 65 | * This memory barrier is paired with the one performed by |
| 66 | * the client thread after it has seen that 'lttng_sessiond_ready' is 0. |
| 67 | * |
| 68 | * The purpose of these memory barriers is to ensure that all |
| 69 | * initialization operations of the various threads that call this |
| 70 | * function to signal that they are ready are commited/published |
| 71 | * before the client thread can see the 'lttng_sessiond_ready' counter |
| 72 | * reach 0. |
| 73 | * |
| 74 | * Note that this could be a 'write' memory barrier, but a full barrier |
| 75 | * is used in case the code using this utility changes. The performance |
| 76 | * implications of this choice are minimal since this is a slow path. |
| 77 | */ |
| 78 | cmm_smp_mb(); |
| 79 | uatomic_sub(<tng_sessiond_ready, 1); |
| 80 | } |