1 |
/*************************************************************************** |
2 |
* * |
3 |
* Copyright (C) 2006-2016 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 |
* Get the data on update side for read-only access. |
127 |
*/ |
128 |
const T& GetUnsafeUpdateConfig() const { |
129 |
return config[updateIndex]; |
130 |
} |
131 |
|
132 |
/** |
133 |
* Atomically switch the newly updated configuration |
134 |
* object with the one used by the real time thread, then |
135 |
* wait for the real time thread to finish working with |
136 |
* the old object before returning the old object. |
137 |
* SwitchConfig() must be called by the non real time |
138 |
* thread after an update has been done, and the object |
139 |
* returned must be updated in the same way as the first. |
140 |
* |
141 |
* @returns a reference to the configuration object to be |
142 |
* updated by the non real time thread |
143 |
*/ |
144 |
T& SwitchConfig(); |
145 |
|
146 |
private: |
147 |
atomic<int> indexAtomic; |
148 |
int updateIndex; |
149 |
T config[2]; |
150 |
std::set<Reader*> readers; |
151 |
}; |
152 |
|
153 |
template<class T> SynchronizedConfig<T>::SynchronizedConfig() : |
154 |
indexAtomic(0) { |
155 |
updateIndex = 1; |
156 |
} |
157 |
|
158 |
template<class T> T& SynchronizedConfig<T>::GetConfigForUpdate() { |
159 |
return config[updateIndex]; |
160 |
} |
161 |
|
162 |
template<class T> T& SynchronizedConfig<T>::SwitchConfig() { |
163 |
indexAtomic.store(updateIndex, memory_order_release); |
164 |
atomic_thread_fence(memory_order_seq_cst); |
165 |
|
166 |
// first put all locking readers in a linked list |
167 |
Reader* lockingReaders = 0; |
168 |
for (typename std::set<Reader*>::iterator iter = readers.begin() ; |
169 |
iter != readers.end() ; |
170 |
iter++) { |
171 |
(*iter)->prevLock = (*iter)->lock.load(memory_order_acquire); |
172 |
if ((*iter)->prevLock) { |
173 |
(*iter)->next = lockingReaders; |
174 |
lockingReaders = *iter; |
175 |
} |
176 |
} |
177 |
|
178 |
// wait until there are no locking readers left |
179 |
while (lockingReaders) { |
180 |
usleep(50000); |
181 |
Reader** prev = &lockingReaders; |
182 |
for (Reader* p = lockingReaders ; p ; p = p->next) { |
183 |
if (p->lock.load(memory_order_acquire) == p->prevLock) { |
184 |
prev = &p->next; |
185 |
} else { |
186 |
*prev = p->next; // unlink |
187 |
} |
188 |
} |
189 |
} |
190 |
|
191 |
updateIndex ^= 1; |
192 |
return config[updateIndex]; |
193 |
} |
194 |
|
195 |
|
196 |
// ----- Reader ---- |
197 |
|
198 |
template <class T> |
199 |
SynchronizedConfig<T>::Reader::Reader(SynchronizedConfig& config) : |
200 |
parent(&config), lockCount(1), lock(0) { |
201 |
parent->readers.insert(this); |
202 |
} |
203 |
|
204 |
template <class T> |
205 |
SynchronizedConfig<T>::Reader::Reader(SynchronizedConfig* config) : |
206 |
parent(config), lockCount(1), lock(0) { |
207 |
parent->readers.insert(this); |
208 |
} |
209 |
|
210 |
template <class T> |
211 |
SynchronizedConfig<T>::Reader::~Reader() { |
212 |
parent->readers.erase(this); |
213 |
} |
214 |
|
215 |
|
216 |
// ----- Convenience classes on top of SynchronizedConfig ---- |
217 |
|
218 |
|
219 |
/** |
220 |
* Base interface class for classes that implement synchronization of data |
221 |
* shared between multiple threads. |
222 |
*/ |
223 |
template<class T> |
224 |
class Synchronizer { |
225 |
public: |
226 |
/** |
227 |
* Signal intention to enter a synchronized code block. Depending |
228 |
* on the actual implementation, this call may block the calling |
229 |
* thread until it is safe to actually use the protected data. After |
230 |
* this call returns, it is safe for the calling thread to access and |
231 |
* modify the shared data. As soon as the thread is done with accessing |
232 |
* the shared data, it MUST call endSync(). |
233 |
* |
234 |
* @return the shared protected data |
235 |
*/ |
236 |
virtual void beginSync() = 0; //TODO: or call it lock() instead ? |
237 |
|
238 |
/** |
239 |
* Retrieve reference to critical, shared data. This method shall be |
240 |
* called between a beginSync() and endSync() call pair, to be sure |
241 |
* that shared data can be accessed safely. |
242 |
*/ |
243 |
virtual T& syncedData() = 0; |
244 |
|
245 |
/** |
246 |
* Signal that the synchronized code block has been left. Depending |
247 |
* on the actual implementation, this call may block the calling |
248 |
* thread for a certain amount of time. |
249 |
*/ |
250 |
virtual void endSync() = 0; //TODO: or call it unlock() instead ? |
251 |
}; |
252 |
|
253 |
/** |
254 |
* Wraps as a kind of pointer class some data object shared with other |
255 |
* threads, to protect / synchronize the shared data against |
256 |
* undeterministic concurrent access. It does so by locking the shared |
257 |
* data in the Sync constructor and unlocking the shared data in the Sync |
258 |
* destructor. Accordingly it can always be considered safe to access the |
259 |
* shared data during the whole life time of the Sync object. Due to |
260 |
* this design, a Sync object MUST only be accessed and destroyed |
261 |
* by exactly one and the same thread which created that same Sync object. |
262 |
*/ |
263 |
template<class T> |
264 |
class Sync { |
265 |
public: |
266 |
Sync(Synchronizer<T>* syncer) { |
267 |
this->syncer = syncer; |
268 |
syncer->beginSync(); |
269 |
} |
270 |
|
271 |
virtual ~Sync() { |
272 |
syncer->endSync(); |
273 |
} |
274 |
|
275 |
/*Sync& operator =(const Sync& arg) { |
276 |
*this->data = *arg.data; |
277 |
return *this; |
278 |
}*/ |
279 |
|
280 |
/*Sync& operator =(const T& arg) { |
281 |
*this->data = arg; |
282 |
return *this; |
283 |
}*/ |
284 |
|
285 |
const T& operator *() const { return syncer->syncedData(); } |
286 |
T& operator *() { return syncer->syncedData(); } |
287 |
|
288 |
const T* operator ->() const { return &syncer->syncedData(); } |
289 |
T* operator ->() { return &syncer->syncedData(); } |
290 |
|
291 |
private: |
292 |
Synchronizer<T>* syncer; ///< Points to the object that shall be responsible to protect the shared data. |
293 |
}; |
294 |
|
295 |
/** |
296 |
* BackBuffer object to be accessed by multiple non-real-time threads. |
297 |
* |
298 |
* Since a back buffer is designed for being accessed by non-real-time |
299 |
* threads, its methods involved may block the calling thread for a long |
300 |
* amount of time. |
301 |
*/ |
302 |
template<class T> |
303 |
class BackBuffer : public SynchronizedConfig<T>, public Synchronizer<T> { |
304 |
public: |
305 |
virtual void beginSync() OVERRIDE { |
306 |
mutex.Lock(); |
307 |
} |
308 |
|
309 |
virtual T& syncedData() OVERRIDE { |
310 |
return SynchronizedConfig<T>::GetConfigForUpdate(); |
311 |
} |
312 |
|
313 |
virtual void endSync() OVERRIDE { |
314 |
const T clone = SynchronizedConfig<T>::GetConfigForUpdate(); |
315 |
SynchronizedConfig<T>::SwitchConfig() = clone; |
316 |
mutex.Unlock(); |
317 |
} |
318 |
|
319 |
const T& unsafeData() const { |
320 |
return SynchronizedConfig<T>::GetUnsafeUpdateConfig(); |
321 |
} |
322 |
|
323 |
private: |
324 |
Mutex mutex; |
325 |
}; |
326 |
|
327 |
/** |
328 |
* FrontBuffer object to be accessed by exactly ONE real-time thread. |
329 |
* A front buffer is designed for real-time access. That is, its methods |
330 |
* involved are lock free, that is none of them block the calling thread |
331 |
* for a long time. |
332 |
* |
333 |
* If you need the front buffer's data to be accessed by multiple real-time |
334 |
* threads instead, then you need to create multiple instances of the |
335 |
* FrontBuffer object. They would point to the same data, but ensure |
336 |
* protection against concurrent access among those real-time threads. |
337 |
*/ |
338 |
template<class T> |
339 |
class FrontBuffer : public SynchronizedConfig<T>::Reader, public Synchronizer<T> { |
340 |
public: |
341 |
FrontBuffer(BackBuffer<T>& backBuffer) : SynchronizedConfig<T>::Reader::Reader(&backBuffer) {} |
342 |
virtual void beginSync() OVERRIDE { data = &SynchronizedConfig<T>::Reader::Lock(); } |
343 |
virtual T& syncedData() OVERRIDE { return *data; } |
344 |
virtual void endSync() OVERRIDE { SynchronizedConfig<T>::Reader::Unlock(); } |
345 |
private: |
346 |
T* data; |
347 |
}; |
348 |
|
349 |
/** |
350 |
* Synchronization / protection of data shared between multiple threads by |
351 |
* using a double buffer design. The FrontBuffer is meant to be accessed by |
352 |
* exactly one real-time thread, whereas the BackBuffer is meant to be |
353 |
* accessed by multiple non-real-time threads. |
354 |
* |
355 |
* This class is built on top of SynchronizedConfig as convenient API to |
356 |
* reduce the amount of code required to protect shared data. |
357 |
*/ |
358 |
template<class T> |
359 |
class DoubleBuffer { |
360 |
public: |
361 |
DoubleBuffer() : m_front(m_back) {} |
362 |
|
363 |
/** |
364 |
* Synchronized access of the shared data for EXACTLY one real-time |
365 |
* thread. |
366 |
* |
367 |
* The returned shared data is wrapped into a Sync object, which |
368 |
* ensures that the shared data is protected against concurrent access |
369 |
* during the life time of the returned Sync object. |
370 |
*/ |
371 |
inline |
372 |
Sync<T> front() { return Sync<T>(&m_front); } |
373 |
|
374 |
/** |
375 |
* Synchronized access of the shared data for multiple non-real-time |
376 |
* threads. |
377 |
* |
378 |
* The returned shared data is wrapped into a Sync object, which |
379 |
* ensures that the shared data is protected against concurrent access |
380 |
* during the life time of the returned Sync object. |
381 |
* |
382 |
* As soon as the returned Sync object is destroyed, the FrontBuffer |
383 |
* will automatically be exchanged by the hereby modified BackBuffer. |
384 |
*/ |
385 |
inline |
386 |
Sync<T> back() { return Sync<T>(&m_back); } |
387 |
|
388 |
/** |
389 |
* Get the backbuffer data <b>unprotected</b>, that is <b>without</b> |
390 |
* locking or any means of synchronizations. |
391 |
* |
392 |
* Due to its nature this must only be called for read access and |
393 |
* you have to make sure by yourself, that the data/member you |
394 |
* access is really safe for concurrent read access (i.e. SGI's |
395 |
* implementation of std::vector::size() would be safe). |
396 |
* |
397 |
* Only use this when you are absolutely sure what you are doing! |
398 |
*/ |
399 |
const T& unsafeBack() const { return m_back.unsafeData(); } |
400 |
|
401 |
private: |
402 |
BackBuffer<T> m_back; ///< Back buffer (non real-time thread(s) side). |
403 |
FrontBuffer<T> m_front; ///< Front buffer (real-time thread side). |
404 |
}; |
405 |
|
406 |
} // namespace LinuxSampler |
407 |
|
408 |
#endif |