src/common/SynchronizedConfig.h

/***************************************************************************
 *                                                                         *
 *   Copyright (C) 2006-2009 Andreas Persson                               *
 *                                                                         *
 *   This program is free software; you can redistribute it and/or modify  *
 *   it under the terms of the GNU General Public License as published by  *
 *   the Free Software Foundation; either version 2 of the License, or     *
 *   (at your option) any later version.                                   *
 *                                                                         *
 *   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 St, Fifth Floor, Boston,                *
 *   MA  02110-1301  USA                                                   *
 ***************************************************************************/

#ifndef SYNCHRONIZEDCONFIG_H
#define SYNCHRONIZEDCONFIG_H

#include <set>
#include <unistd.h>
#include "lsatomic.h"

namespace LinuxSampler {

    /**
     * Thread safe management of configuration data, where the data is
     * updated by a single non real time thread and read by a number
     * of real time threads.
     *
     * The synchronization is achieved by using two instances of the
     * configuration data. The non real time thread gets access to the
     * instance not currently in use by the real time threads by
     * calling GetConfigForUpdate(). After the data is updated, the
     * non real time thread must call SwitchConfig() and redo the
     * update on the other instance. SwitchConfig() blocks until it is
     * safe to modify the other instance.
     *
     * The real time threads need one Reader object each to access the
     * configuration data. This object must be created outside the
     * real time thread. The Lock() function returns a reference to
     * the data to be read, and Unlock() must be called when finished
     * reading the data. (Neither Lock nor Unlock will block the real
     * time thread, or use any system calls.)
     */
    template<class T>
    class SynchronizedConfig {
        public:
            SynchronizedConfig();

            // methods for the real time thread

            class Reader {
                public:
                    /**
                     * Gets the configuration object for use by the
                     * real time thread. The object is safe to use
                     * (read only) until Unlock() is called.
                     *
                     * @returns a reference to the configuration
                     *          object to be read by the real time
                     *          thread
                     */
                    const T& Lock() {
                        lock.store(lockCount += 2, memory_order_relaxed);
                        atomic_thread_fence(memory_order_seq_cst);
                        return parent.config[parent.indexAtomic.load(
                                memory_order_acquire)];
                    }

                    /**
                     * Unlock the configuration object. Unlock() must
                     * be called by the real time thread after it has
                     * finished reading the configuration object. If
                     * the non real time thread is waiting in
                     * SwitchConfig() it will be awaken when no real
                     * time threads are locked anymore.
                     */
                    void Unlock() {
                        lock.store(0, memory_order_release);
                    }

                    Reader(SynchronizedConfig& config);
                    ~Reader();
                private:
                    friend class SynchronizedConfig;
                    SynchronizedConfig& parent;
                    int lockCount; // increased in every Lock(),
                                   // lowest bit is always set.
                    atomic<int> lock; // equals lockCount when inside
                                      // critical region, otherwise 0
                    Reader* next; // only used locally in SwitchConfig
                    int prevLock; // only used locally in SwitchConfig
            };


            // methods for the non real time thread

            /**
             * Gets the configuration object for use by the non real
             * time thread. The object returned is not in use by the
             * real time thread, so it can safely be updated. After
             * the update is done, the non real time thread must call
             * SwitchConfig() and the same update must be done again.
             *
             * @returns a reference to the configuration object to be
             *          updated by the non real time thread
             */
            T& GetConfigForUpdate();

            /**
             * Atomically switch the newly updated configuration
             * object with the one used by the real time thread, then
             * wait for the real time thread to finish working with
             * the old object before returning the old object.
             * SwitchConfig() must be called by the non real time
             * thread after an update has been done, and the object
             * returned must be updated in the same way as the first.
             *
             * @returns a reference to the configuration object to be
             *          updated by the non real time thread
             */
            T& SwitchConfig();

        private:
            atomic<int> indexAtomic;
            int updateIndex;
            T config[2];
            std::set<Reader*> readers;
    };

    template<class T> SynchronizedConfig<T>::SynchronizedConfig() :
        indexAtomic(0) {
        updateIndex = 1;
    }

    template<class T> T& SynchronizedConfig<T>::GetConfigForUpdate() {
        return config[updateIndex];
    }

    template<class T> T& SynchronizedConfig<T>::SwitchConfig() {
        indexAtomic.store(updateIndex, memory_order_release);
        atomic_thread_fence(memory_order_seq_cst);

        // first put all locking readers in a linked list
        Reader* lockingReaders = 0;
        for (typename std::set<Reader*>::iterator iter = readers.begin() ;
             iter != readers.end() ;
             iter++) {
            (*iter)->prevLock = (*iter)->lock.load(memory_order_acquire);
            if ((*iter)->prevLock) {
                (*iter)->next = lockingReaders;
                lockingReaders = *iter;
            }
        }

        // wait until there are no locking readers left
        while (lockingReaders) {
            usleep(50000);
            Reader** prev = &lockingReaders;
            for (Reader* p = lockingReaders ; p ; p = p->next) {
                if (p->lock.load(memory_order_acquire) == p->prevLock) {
                    prev = &p->next;
                } else {
                    *prev = p->next; // unlink
                }
            }
        }

        updateIndex ^= 1;
        return config[updateIndex];
    }


    // ----- Reader ----

    template <class T>
    SynchronizedConfig<T>::Reader::Reader(SynchronizedConfig& config) :
        parent(config), lock(0), lockCount(1) {
        parent.readers.insert(this);
    }

    template <class T>
    SynchronizedConfig<T>::Reader::~Reader() {
        parent.readers.erase(this);
    }

} // namespace LinuxSampler

#endif
1	/***************************************************************************
2	* *
3	* Copyright (C) 2006-2009 Andreas Persson *
4	* *
5	* This program is free software; you can redistribute it and/or modify *
6	* it under the terms of the GNU General Public License as published by *
7	* the Free Software Foundation; either version 2 of the License, or *
8	* (at your option) any later version. *
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 *
16	* along with this program; if not, write to the Free Software *
17	* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, *
18	* MA 02110-1301 USA *
19	***************************************************************************/
20
21	#ifndef SYNCHRONIZEDCONFIG_H
22	#define SYNCHRONIZEDCONFIG_H
23
24	#include <set>
25	#include <unistd.h>
26	#include "lsatomic.h"
27
28	namespace LinuxSampler {
29
30	/**
31	* Thread safe management of configuration data, where the data is
32	* updated by a single non real time thread and read by a number
33	* of real time threads.
34	*
35	* The synchronization is achieved by using two instances of the
36	* configuration data. The non real time thread gets access to the
37	* instance not currently in use by the real time threads by
38	* calling GetConfigForUpdate(). After the data is updated, the
39	* non real time thread must call SwitchConfig() and redo the
40	* update on the other instance. SwitchConfig() blocks until it is
41	* safe to modify the other instance.
42	*
43	* The real time threads need one Reader object each to access the
44	* configuration data. This object must be created outside the
45	* real time thread. The Lock() function returns a reference to
46	* the data to be read, and Unlock() must be called when finished
47	* reading the data. (Neither Lock nor Unlock will block the real
48	* time thread, or use any system calls.)
49	*/
50	template<class T>
51	class SynchronizedConfig {
52	public:
53	SynchronizedConfig();
54
55	// methods for the real time thread
56
57	class Reader {
58	public:
59	/**
60	* Gets the configuration object for use by the
61	* real time thread. The object is safe to use
62	* (read only) until Unlock() is called.
63	*
64	* @returns a reference to the configuration
65	* object to be read by the real time
66	* thread
67	*/
68	const T& Lock() {
69	lock.store(lockCount += 2, memory_order_relaxed);
70	atomic_thread_fence(memory_order_seq_cst);
71	return parent.config[parent.indexAtomic.load(
72	memory_order_acquire)];
73	}
74
75	/**
76	* Unlock the configuration object. Unlock() must
77	* be called by the real time thread after it has
78	* finished reading the configuration object. If
79	* the non real time thread is waiting in
80	* SwitchConfig() it will be awaken when no real
81	* time threads are locked anymore.
82	*/
83	void Unlock() {
84	lock.store(0, memory_order_release);
85	}
86
87	Reader(SynchronizedConfig& config);
88	~Reader();
89	private:
90	friend class SynchronizedConfig;
91	SynchronizedConfig& parent;
92	int lockCount; // increased in every Lock(),
93	// lowest bit is always set.
94	atomic<int> lock; // equals lockCount when inside
95	// critical region, otherwise 0
96	Reader* next; // only used locally in SwitchConfig
97	int prevLock; // only used locally in SwitchConfig
98	};
99
100
101	// methods for the non real time thread
102
103	/**
104	* Gets the configuration object for use by the non real
105	* time thread. The object returned is not in use by the
106	* real time thread, so it can safely be updated. After
107	* the update is done, the non real time thread must call
108	* SwitchConfig() and the same update must be done again.
109	*
110	* @returns a reference to the configuration object to be
111	* updated by the non real time thread
112	*/
113	T& GetConfigForUpdate();
114
115	/**
116	* Atomically switch the newly updated configuration
117	* object with the one used by the real time thread, then
118	* wait for the real time thread to finish working with
119	* the old object before returning the old object.
120	* SwitchConfig() must be called by the non real time
121	* thread after an update has been done, and the object
122	* returned must be updated in the same way as the first.
123	*
124	* @returns a reference to the configuration object to be
125	* updated by the non real time thread
126	*/
127	T& SwitchConfig();
128
129	private:
130	atomic<int> indexAtomic;
131	int updateIndex;
132	T config[2];
133	std::set<Reader*> readers;
134	};
135
136	template<class T> SynchronizedConfig<T>::SynchronizedConfig() :
137	indexAtomic(0) {
138	updateIndex = 1;
139	}
140
141	template<class T> T& SynchronizedConfig<T>::GetConfigForUpdate() {
142	return config[updateIndex];
143	}
144
145	template<class T> T& SynchronizedConfig<T>::SwitchConfig() {
146	indexAtomic.store(updateIndex, memory_order_release);
147	atomic_thread_fence(memory_order_seq_cst);
148
149	// first put all locking readers in a linked list
150	Reader* lockingReaders = 0;
151	for (typename std::set<Reader*>::iterator iter = readers.begin() ;
152	iter != readers.end() ;
153	iter++) {
154	(iter)->prevLock = (iter)->lock.load(memory_order_acquire);
155	if ((*iter)->prevLock) {
156	(*iter)->next = lockingReaders;
157	lockingReaders = *iter;
158	}
159	}
160
161	// wait until there are no locking readers left
162	while (lockingReaders) {
163	usleep(50000);
164	Reader** prev = &lockingReaders;
165	for (Reader* p = lockingReaders ; p ; p = p->next) {
166	if (p->lock.load(memory_order_acquire) == p->prevLock) {
167	prev = &p->next;
168	} else {
169	*prev = p->next; // unlink
170	}
171	}
172	}
173
174	updateIndex ^= 1;
175	return config[updateIndex];
176	}
177
178
179	// ----- Reader ----
180
181	template <class T>
182	SynchronizedConfig<T>::Reader::Reader(SynchronizedConfig& config) :
183	parent(config), lock(0), lockCount(1) {
184	parent.readers.insert(this);
185	}
186
187	template <class T>
188	SynchronizedConfig<T>::Reader::~Reader() {
189	parent.readers.erase(this);
190	}
191
192	} // namespace LinuxSampler
193
194	#endif