src/common/SynchronizedConfig.h

/***************************************************************************
 *                                                                         *
 *   Copyright (C) 2006-2012 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.)
     *
     * Note that the non real time side isn't safe for concurrent
     * access, so if there are multiple non real time threads that
     * update the configuration data, a mutex has to be used.
     *
     * Implementation note: the memory order parameters and fences are
     * very carefully chosen to make the code fast but still safe for
     * memory access reordering made by the CPU.
     */
    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) 2006-2012 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	* Note that the non real time side isn't safe for concurrent
51	* access, so if there are multiple non real time threads that
52	* update the configuration data, a mutex has to be used.
53	*
54	* Implementation note: the memory order parameters and fences are
55	* very carefully chosen to make the code fast but still safe for
56	* memory access reordering made by the CPU.
57	*/
58	template<class T>
59	class SynchronizedConfig {
60	public:
61	SynchronizedConfig();
62
63	// methods for the real time thread
64
65	class Reader {
66	public:
67	/**
68	* Gets the configuration object for use by the
69	* real time thread. The object is safe to use
70	* (read only) until Unlock() is called.
71	*
72	* @returns a reference to the configuration
73	* object to be read by the real time
74	* thread
75	*/
76	const T& Lock() {
77	lock.store(lockCount += 2, memory_order_relaxed);
78	atomic_thread_fence(memory_order_seq_cst);
79	return parent.config[parent.indexAtomic.load(
80	memory_order_acquire)];
81	}
82
83	/**
84	* Unlock the configuration object. Unlock() must
85	* be called by the real time thread after it has
86	* finished reading the configuration object. If
87	* the non real time thread is waiting in
88	* SwitchConfig() it will be awaken when no real
89	* time threads are locked anymore.
90	*/
91	void Unlock() {
92	lock.store(0, memory_order_release);
93	}
94
95	Reader(SynchronizedConfig& config);
96	~Reader();
97	private:
98	friend class SynchronizedConfig;
99	SynchronizedConfig& parent;
100	int lockCount; // increased in every Lock(),
101	// lowest bit is always set.
102	atomic<int> lock; // equals lockCount when inside
103	// critical region, otherwise 0
104	Reader* next; // only used locally in SwitchConfig
105	int prevLock; // only used locally in SwitchConfig
106	};
107
108
109	// methods for the non real time thread
110
111	/**
112	* Gets the configuration object for use by the non real
113	* time thread. The object returned is not in use by the
114	* real time thread, so it can safely be updated. After
115	* the update is done, the non real time thread must call
116	* SwitchConfig() and the same update must be done again.
117	*
118	* @returns a reference to the configuration object to be
119	* updated by the non real time thread
120	*/
121	T& GetConfigForUpdate();
122
123	/**
124	* Atomically switch the newly updated configuration
125	* object with the one used by the real time thread, then
126	* wait for the real time thread to finish working with
127	* the old object before returning the old object.
128	* SwitchConfig() must be called by the non real time
129	* thread after an update has been done, and the object
130	* returned must be updated in the same way as the first.
131	*
132	* @returns a reference to the configuration object to be
133	* updated by the non real time thread
134	*/
135	T& SwitchConfig();
136
137	private:
138	atomic<int> indexAtomic;
139	int updateIndex;
140	T config[2];
141	std::set<Reader*> readers;
142	};
143
144	template<class T> SynchronizedConfig<T>::SynchronizedConfig() :
145	indexAtomic(0) {
146	updateIndex = 1;
147	}
148
149	template<class T> T& SynchronizedConfig<T>::GetConfigForUpdate() {
150	return config[updateIndex];
151	}
152
153	template<class T> T& SynchronizedConfig<T>::SwitchConfig() {
154	indexAtomic.store(updateIndex, memory_order_release);
155	atomic_thread_fence(memory_order_seq_cst);
156
157	// first put all locking readers in a linked list
158	Reader* lockingReaders = 0;
159	for (typename std::set<Reader*>::iterator iter = readers.begin() ;
160	iter != readers.end() ;
161	iter++) {
162	(iter)->prevLock = (iter)->lock.load(memory_order_acquire);
163	if ((*iter)->prevLock) {
164	(*iter)->next = lockingReaders;
165	lockingReaders = *iter;
166	}
167	}
168
169	// wait until there are no locking readers left
170	while (lockingReaders) {
171	usleep(50000);
172	Reader** prev = &lockingReaders;
173	for (Reader* p = lockingReaders ; p ; p = p->next) {
174	if (p->lock.load(memory_order_acquire) == p->prevLock) {
175	prev = &p->next;
176	} else {
177	*prev = p->next; // unlink
178	}
179	}
180	}
181
182	updateIndex ^= 1;
183	return config[updateIndex];
184	}
185
186
187	// ----- Reader ----
188
189	template <class T>
190	SynchronizedConfig<T>::Reader::Reader(SynchronizedConfig& config) :
191	parent(config), lock(0), lockCount(1) {
192	parent.readers.insert(this);
193	}
194
195	template <class T>
196	SynchronizedConfig<T>::Reader::~Reader() {
197	parent.readers.erase(this);
198	}
199
200	} // namespace LinuxSampler
201
202	#endif