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"
#include "Mutex.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() { //TODO const currently commented for the DoubleBuffer usage below
                        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(SynchronizedConfig* config);
                    virtual ~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(SynchronizedConfig* config) :
        parent(config), lock(0), lockCount(1) {
        parent->readers.insert(this);
    }

    template <class T>
    SynchronizedConfig<T>::Reader::~Reader() {
        parent->readers.erase(this);
    }
    
    
    // ----- Convenience classes on top of SynchronizedConfig ----


    /**
     * Base interface class for classes that implement synchronization of data
     * shared between multiple threads.
     */
    template<class T>
    class Synchronizer {
    public:
        /**
         * Signal intention to enter a synchronized code block. Depending
         * on the actual implementation, this call may block the calling
         * thread until it is safe to actually use the protected data. After
         * this call returns, it is safe for the calling thread to access and
         * modify the shared data. As soon as the thread is done with accessing
         * the shared data, it MUST call endSync().
         *
         * @return the shared protected data
         */
        virtual T& beginSync() = 0; //TODO: or call it lock() instead ?
            
        /**
         * Signal that the synchronized code block has been left. Depending
         * on the actual implementation, this call may block the calling
         * thread for a certain amount of time.
         */
        virtual void endSync() = 0; //TODO: or call it unlock() instead ?
    };

    /**
     * Wraps as a kind of pointer class some data object shared with other
     * threads, to protect / synchronize the shared data against
     * undeterministic concurrent access. It does so by locking the shared
     * data in the Sync constructor and unlocking the shared data in the Sync
     * destructor. Accordingly it can always be considered safe to access the
     * shared data during the whole life time of the Sync object. Due to
     * this design, a Sync object MUST only be accessed and destroyed
     * by exactly one and the same thread which created that same Sync object.
     */
    template<class T>
    class Sync {
    public:
        Sync(Synchronizer<T>* syncer) {
            this->syncer = syncer;
            this->data = &syncer->beginSync();
        }
        
        virtual ~Sync() {
            syncer->endSync();
        }
        
        Sync& operator =(const Sync& arg) {
            *this->data = *arg.data;
            return *this;
        }

        Sync& operator =(const T& arg) {
            *this->data = arg;
            return *this;
        }
        
        const T& operator *() const { return *data; }
        T&       operator *()       { return *data; }

        const T* operator ->() const { return data; }
        T*       operator ->()       { return data; }

    private:
        Synchronizer<T>* syncer; ///< Points to the object that shall be responsible to protect the shared data.
        T* data; ///< Points to the shared data that should be protected.
    };

    /**
     * BackBuffer object to be accessed by multiple non-real-time threads.
     *
     * Since a back buffer is designed for being accessed by non-real-time
     * threads, its methods involved may block the calling thread for a long
     * amount of time.
     */
    template<class T>
    class BackBuffer : public SynchronizedConfig<T>, public Synchronizer<T> {
    public:
        virtual T& beginSync() OVERRIDE {
            mutex.Lock();
            data = &SynchronizedConfig<T>::GetConfigForUpdate();
            return *data;
        }

        virtual void endSync() OVERRIDE {
            const T clone = *data;
            SynchronizedConfig<T>::SwitchConfig() = clone;
            mutex.Unlock();
        }

    private:
        T* data;
        Mutex mutex;
    };

    /**
     * FrontBuffer object to be accessed by exactly ONE real-time thread.
     * A front buffer is designed for real-time access. That is, its methods
     * involved are lock free, that is none of them block the calling thread
     * for a long time.
     *
     * If you need the front buffer's data to be accessed by multiple real-time
     * threads instead, then you need to create multiple instances of the
     * FrontBuffer object. They would point to the same data, but ensure
     * protection against concurrent access among those real-time threads.
     */
    template<class T>
    class FrontBuffer : public SynchronizedConfig<T>::Reader, public Synchronizer<T> {
    public:
        FrontBuffer(BackBuffer<T>& backBuffer) : SynchronizedConfig<T>::Reader::Reader(&backBuffer) {}
        virtual T& beginSync() OVERRIDE { return SynchronizedConfig<T>::Reader::Lock(); }
        virtual void endSync() OVERRIDE { SynchronizedConfig<T>::Reader::Unlock(); }
    };

    /**
     * Synchronization / protection of data shared between multiple threads by
     * using a double buffer design. The FrontBuffer is meant to be accessed by
     * exactly one real-time thread, whereas the BackBuffer is meant to be
     * accessed by multiple non-real-time threads.
     *
     * This class is built on top of SynchronizedConfig as convenient API to
     * reduce the amount of code required to protect shared data.
     */
    template<class T>
    class DoubleBuffer {
    public:
        DoubleBuffer() : m_front(m_back) {}
        
        /**
         * Synchronized access of the shared data for EXACTLY one real-time
         * thread.
         *
         * The returned shared data is wrapped into a Sync object, which
         * ensures that the shared data is protected against concurrent access
         * during the life time of the returned Sync object.
         */
        inline
        Sync<T> front() { return Sync<T>(&m_front); }
        
        /**
         * Synchronized access of the shared data for multiple non-real-time
         * threads.
         *
         * The returned shared data is wrapped into a Sync object, which
         * ensures that the shared data is protected against concurrent access
         * during the life time of the returned Sync object.
         *
         * As soon as the returned Sync object is destroyed, the FrontBuffer
         * will automatically be exchanged by the hereby modified BackBuffer.
         */
        inline
        Sync<T> back() { return Sync<T>(&m_back); }

    private:
        BackBuffer<T> m_back; ///< Back buffer (non real-time thread(s) side).
        FrontBuffer<T> m_front; ///< Front buffer (real-time thread side).
    };

} // 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	#include "Mutex.h"
28
29	namespace LinuxSampler {
30
31	/**
32	* Thread-safe management of configuration data, where the data is
33	* updated by a single non real time thread and read by a number
34	* of real time threads.
35	*
36	* The synchronization is achieved by using two instances of the
37	* configuration data. The non real time thread gets access to the
38	* instance not currently in use by the real time threads by
39	* calling GetConfigForUpdate(). After the data is updated, the
40	* non real time thread must call SwitchConfig() and redo the
41	* update on the other instance. SwitchConfig() blocks until it is
42	* safe to modify the other instance.
43	*
44	* The real time threads need one Reader object each to access the
45	* configuration data. This object must be created outside the
46	* real time thread. The Lock() function returns a reference to
47	* the data to be read, and Unlock() must be called when finished
48	* reading the data. (Neither Lock nor Unlock will block the real
49	* time thread, or use any system calls.)
50	*
51	* Note that the non real time side isn't safe for concurrent
52	* access, so if there are multiple non real time threads that
53	* update the configuration data, a mutex has to be used.
54	*
55	* Implementation note: the memory order parameters and fences are
56	* very carefully chosen to make the code fast but still safe for
57	* memory access reordering made by the CPU.
58	*/
59	template<class T>
60	class SynchronizedConfig {
61	public:
62	SynchronizedConfig();
63
64	// methods for the real time thread
65
66	class Reader {
67	public:
68	/**
69	* Gets the configuration object for use by the
70	* real time thread. The object is safe to use
71	* (read only) until Unlock() is called.
72	*
73	* @returns a reference to the configuration
74	* object to be read by the real time
75	* thread
76	*/
77	/const/ T& Lock() { //TODO const currently commented for the DoubleBuffer usage below
78	lock.store(lockCount += 2, memory_order_relaxed);
79	atomic_thread_fence(memory_order_seq_cst);
80	return parent->config[parent->indexAtomic.load(
81	memory_order_acquire)];
82	}
83
84	/**
85	* Unlock the configuration object. Unlock() must
86	* be called by the real time thread after it has
87	* finished reading the configuration object. If
88	* the non real time thread is waiting in
89	* SwitchConfig() it will be awaken when no real
90	* time threads are locked anymore.
91	*/
92	void Unlock() {
93	lock.store(0, memory_order_release);
94	}
95
96	Reader(SynchronizedConfig& config);
97	Reader(SynchronizedConfig* config);
98	virtual ~Reader();
99	private:
100	friend class SynchronizedConfig;
101	SynchronizedConfig* parent;
102	int lockCount; // increased in every Lock(),
103	// lowest bit is always set.
104	atomic<int> lock; // equals lockCount when inside
105	// critical region, otherwise 0
106	Reader* next; // only used locally in SwitchConfig
107	int prevLock; // only used locally in SwitchConfig
108	};
109
110
111	// methods for the non real time thread
112
113	/**
114	* Gets the configuration object for use by the non real
115	* time thread. The object returned is not in use by the
116	* real time thread, so it can safely be updated. After
117	* the update is done, the non real time thread must call
118	* SwitchConfig() and the same update must be done again.
119	*
120	* @returns a reference to the configuration object to be
121	* updated by the non real time thread
122	*/
123	T& GetConfigForUpdate();
124
125	/**
126	* Atomically switch the newly updated configuration
127	* object with the one used by the real time thread, then
128	* wait for the real time thread to finish working with
129	* the old object before returning the old object.
130	* SwitchConfig() must be called by the non real time
131	* thread after an update has been done, and the object
132	* returned must be updated in the same way as the first.
133	*
134	* @returns a reference to the configuration object to be
135	* updated by the non real time thread
136	*/
137	T& SwitchConfig();
138
139	private:
140	atomic<int> indexAtomic;
141	int updateIndex;
142	T config[2];
143	std::set<Reader*> readers;
144	};
145
146	template<class T> SynchronizedConfig<T>::SynchronizedConfig() :
147	indexAtomic(0) {
148	updateIndex = 1;
149	}
150
151	template<class T> T& SynchronizedConfig<T>::GetConfigForUpdate() {
152	return config[updateIndex];
153	}
154
155	template<class T> T& SynchronizedConfig<T>::SwitchConfig() {
156	indexAtomic.store(updateIndex, memory_order_release);
157	atomic_thread_fence(memory_order_seq_cst);
158
159	// first put all locking readers in a linked list
160	Reader* lockingReaders = 0;
161	for (typename std::set<Reader*>::iterator iter = readers.begin() ;
162	iter != readers.end() ;
163	iter++) {
164	(iter)->prevLock = (iter)->lock.load(memory_order_acquire);
165	if ((*iter)->prevLock) {
166	(*iter)->next = lockingReaders;
167	lockingReaders = *iter;
168	}
169	}
170
171	// wait until there are no locking readers left
172	while (lockingReaders) {
173	usleep(50000);
174	Reader** prev = &lockingReaders;
175	for (Reader* p = lockingReaders ; p ; p = p->next) {
176	if (p->lock.load(memory_order_acquire) == p->prevLock) {
177	prev = &p->next;
178	} else {
179	*prev = p->next; // unlink
180	}
181	}
182	}
183
184	updateIndex ^= 1;
185	return config[updateIndex];
186	}
187
188
189	// ----- Reader ----
190
191	template <class T>
192	SynchronizedConfig<T>::Reader::Reader(SynchronizedConfig& config) :
193	parent(&config), lock(0), lockCount(1) {
194	parent->readers.insert(this);
195	}
196
197	template <class T>
198	SynchronizedConfig<T>::Reader::Reader(SynchronizedConfig* config) :
199	parent(config), lock(0), lockCount(1) {
200	parent->readers.insert(this);
201	}
202
203	template <class T>
204	SynchronizedConfig<T>::Reader::~Reader() {
205	parent->readers.erase(this);
206	}
207
208
209	// ----- Convenience classes on top of SynchronizedConfig ----
210
211
212	/**
213	* Base interface class for classes that implement synchronization of data
214	* shared between multiple threads.
215	*/
216	template<class T>
217	class Synchronizer {
218	public:
219	/**
220	* Signal intention to enter a synchronized code block. Depending
221	* on the actual implementation, this call may block the calling
222	* thread until it is safe to actually use the protected data. After
223	* this call returns, it is safe for the calling thread to access and
224	* modify the shared data. As soon as the thread is done with accessing
225	* the shared data, it MUST call endSync().
226	*
227	* @return the shared protected data
228	*/
229	virtual T& beginSync() = 0; //TODO: or call it lock() instead ?
230
231	/**
232	* Signal that the synchronized code block has been left. Depending
233	* on the actual implementation, this call may block the calling
234	* thread for a certain amount of time.
235	*/
236	virtual void endSync() = 0; //TODO: or call it unlock() instead ?
237	};
238
239	/**
240	* Wraps as a kind of pointer class some data object shared with other
241	* threads, to protect / synchronize the shared data against
242	* undeterministic concurrent access. It does so by locking the shared
243	* data in the Sync constructor and unlocking the shared data in the Sync
244	* destructor. Accordingly it can always be considered safe to access the
245	* shared data during the whole life time of the Sync object. Due to
246	* this design, a Sync object MUST only be accessed and destroyed
247	* by exactly one and the same thread which created that same Sync object.
248	*/
249	template<class T>
250	class Sync {
251	public:
252	Sync(Synchronizer<T>* syncer) {
253	this->syncer = syncer;
254	this->data = &syncer->beginSync();
255	}
256
257	virtual ~Sync() {
258	syncer->endSync();
259	}
260
261	Sync& operator =(const Sync& arg) {
262	this->data = arg.data;
263	return *this;
264	}
265
266	Sync& operator =(const T& arg) {
267	*this->data = arg;
268	return *this;
269	}
270
271	const T& operator () const { return data; }
272	T& operator () { return data; }
273
274	const T* operator ->() const { return data; }
275	T* operator ->() { return data; }
276
277	private:
278	Synchronizer<T>* syncer; ///< Points to the object that shall be responsible to protect the shared data.
279	T* data; ///< Points to the shared data that should be protected.
280	};
281
282	/**
283	* BackBuffer object to be accessed by multiple non-real-time threads.
284	*
285	* Since a back buffer is designed for being accessed by non-real-time
286	* threads, its methods involved may block the calling thread for a long
287	* amount of time.
288	*/
289	template<class T>
290	class BackBuffer : public SynchronizedConfig<T>, public Synchronizer<T> {
291	public:
292	virtual T& beginSync() OVERRIDE {
293	mutex.Lock();
294	data = &SynchronizedConfig<T>::GetConfigForUpdate();
295	return *data;
296	}
297
298	virtual void endSync() OVERRIDE {
299	const T clone = *data;
300	SynchronizedConfig<T>::SwitchConfig() = clone;
301	mutex.Unlock();
302	}
303
304	private:
305	T* data;
306	Mutex mutex;
307	};
308
309	/**
310	* FrontBuffer object to be accessed by exactly ONE real-time thread.
311	* A front buffer is designed for real-time access. That is, its methods
312	* involved are lock free, that is none of them block the calling thread
313	* for a long time.
314	*
315	* If you need the front buffer's data to be accessed by multiple real-time
316	* threads instead, then you need to create multiple instances of the
317	* FrontBuffer object. They would point to the same data, but ensure
318	* protection against concurrent access among those real-time threads.
319	*/
320	template<class T>
321	class FrontBuffer : public SynchronizedConfig<T>::Reader, public Synchronizer<T> {
322	public:
323	FrontBuffer(BackBuffer<T>& backBuffer) : SynchronizedConfig<T>::Reader::Reader(&backBuffer) {}
324	virtual T& beginSync() OVERRIDE { return SynchronizedConfig<T>::Reader::Lock(); }
325	virtual void endSync() OVERRIDE { SynchronizedConfig<T>::Reader::Unlock(); }
326	};
327
328	/**
329	* Synchronization / protection of data shared between multiple threads by
330	* using a double buffer design. The FrontBuffer is meant to be accessed by
331	* exactly one real-time thread, whereas the BackBuffer is meant to be
332	* accessed by multiple non-real-time threads.
333	*
334	* This class is built on top of SynchronizedConfig as convenient API to
335	* reduce the amount of code required to protect shared data.
336	*/
337	template<class T>
338	class DoubleBuffer {
339	public:
340	DoubleBuffer() : m_front(m_back) {}
341
342	/**
343	* Synchronized access of the shared data for EXACTLY one real-time
344	* thread.
345	*
346	* The returned shared data is wrapped into a Sync object, which
347	* ensures that the shared data is protected against concurrent access
348	* during the life time of the returned Sync object.
349	*/
350	inline
351	Sync<T> front() { return Sync<T>(&m_front); }
352
353	/**
354	* Synchronized access of the shared data for multiple non-real-time
355	* threads.
356	*
357	* The returned shared data is wrapped into a Sync object, which
358	* ensures that the shared data is protected against concurrent access
359	* during the life time of the returned Sync object.
360	*
361	* As soon as the returned Sync object is destroyed, the FrontBuffer
362	* will automatically be exchanged by the hereby modified BackBuffer.
363	*/
364	inline
365	Sync<T> back() { return Sync<T>(&m_back); }
366
367	private:
368	BackBuffer<T> m_back; ///< Back buffer (non real-time thread(s) side).
369	FrontBuffer<T> m_front; ///< Front buffer (real-time thread side).
370	};
371
372	} // namespace LinuxSampler
373
374	#endif