summaryrefslogtreecommitdiffstats
path: root/src/corelib/thread/qmutex.cpp
blob: 1009f7bf6c8e42460333c3377694f17f635bb6fb (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
/****************************************************************************
**
** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** This file is part of the QtCore module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** No Commercial Usage
** This file contains pre-release code and may not be distributed.
** You may use this file in accordance with the terms and conditions
** contained in the Technology Preview License Agreement accompanying
** this package.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file.  Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** In addition, as a special exception, Nokia gives you certain additional
** rights.  These rights are described in the Nokia Qt LGPL Exception
** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
**
** If you have questions regarding the use of this file, please contact
** Nokia at qt-info@nokia.com.
**
**
**
**
**
**
**
**
** $QT_END_LICENSE$
**
****************************************************************************/

#include "qplatformdefs.h"
#include "qmutex.h"
#include <qdebug.h>

#ifndef QT_NO_THREAD
#include "qatomic.h"
#include "qelapsedtimer.h"
#include "qthread.h"
#include "qmutex_p.h"

QT_BEGIN_NAMESPACE

/*!
    \class QMutex
    \brief The QMutex class provides access serialization between threads.

    \threadsafe

    \ingroup thread

    The purpose of a QMutex is to protect an object, data structure or
    section of code so that only one thread can access it at a time
    (this is similar to the Java \c synchronized keyword). It is
    usually best to use a mutex with a QMutexLocker since this makes
    it easy to ensure that locking and unlocking are performed
    consistently.

    For example, say there is a method that prints a message to the
    user on two lines:

    \snippet doc/src/snippets/code/src_corelib_thread_qmutex.cpp 0

    If these two methods are called in succession, the following happens:

    \snippet doc/src/snippets/code/src_corelib_thread_qmutex.cpp 1

    If these two methods are called simultaneously from two threads then the
    following sequence could result:

    \snippet doc/src/snippets/code/src_corelib_thread_qmutex.cpp 2

    If we add a mutex, we should get the result we want:

    \snippet doc/src/snippets/code/src_corelib_thread_qmutex.cpp 3

    Then only one thread can modify \c number at any given time and
    the result is correct. This is a trivial example, of course, but
    applies to any other case where things need to happen in a
    particular sequence.

    When you call lock() in a thread, other threads that try to call
    lock() in the same place will block until the thread that got the
    lock calls unlock(). A non-blocking alternative to lock() is
    tryLock().

    \sa QMutexLocker, QReadWriteLock, QSemaphore, QWaitCondition
*/

/*!
    \enum QMutex::RecursionMode

    \value Recursive  In this mode, a thread can lock the same mutex
                      multiple times and the mutex won't be unlocked
                      until a corresponding number of unlock() calls
                      have been made.

    \value NonRecursive  In this mode, a thread may only lock a mutex
                         once.

    \sa QMutex()
*/

/*!
    Constructs a new mutex. The mutex is created in an unlocked state.

    If \a mode is QMutex::Recursive, a thread can lock the same mutex
    multiple times and the mutex won't be unlocked until a
    corresponding number of unlock() calls have been made. The
    default is QMutex::NonRecursive.

    \sa lock(), unlock()
*/
QMutex::QMutex(RecursionMode mode)
    : d(new QMutexPrivate(mode))
{ }

/*!
    Destroys the mutex.

    \warning Destroying a locked mutex may result in undefined behavior.
*/
QMutex::~QMutex()
{ delete static_cast<QMutexPrivate *>(d); }

/*!
    Locks the mutex. If another thread has locked the mutex then this
    call will block until that thread has unlocked it.

    Calling this function multiple times on the same mutex from the
    same thread is allowed if this mutex is a
    \l{QMutex::Recursive}{recursive mutex}. If this mutex is a
    \l{QMutex::NonRecursive}{non-recursive mutex}, this function will
    \e dead-lock when the mutex is locked recursively.

    \sa unlock()
*/
void QMutex::lock()
{
    QMutexPrivate *d = static_cast<QMutexPrivate *>(this->d);
    Qt::HANDLE self;

    if (d->recursive) {
        self = QThread::currentThreadId();
        if (d->owner == self) {
            ++d->count;
            Q_ASSERT_X(d->count != 0, "QMutex::lock", "Overflow in recursion counter");
            return;
        }

        bool isLocked = d->contenders.testAndSetAcquire(0, 1);
        if (!isLocked) {
            // didn't get the lock, wait for it
            isLocked = d->wait();
            Q_ASSERT_X(isLocked, "QMutex::lock",
                       "Internal error, infinite wait has timed out.");
        }

        d->owner = self;
        ++d->count;
        Q_ASSERT_X(d->count != 0, "QMutex::lock", "Overflow in recursion counter");
        return;
    }

    bool isLocked = d->contenders.testAndSetAcquire(0, 1);
    if (!isLocked) {
        lockInternal();
    }
}

/*!
    Attempts to lock the mutex. If the lock was obtained, this function
    returns true. If another thread has locked the mutex, this
    function returns false immediately.

    If the lock was obtained, the mutex must be unlocked with unlock()
    before another thread can successfully lock it.

    Calling this function multiple times on the same mutex from the
    same thread is allowed if this mutex is a
    \l{QMutex::Recursive}{recursive mutex}. If this mutex is a
    \l{QMutex::NonRecursive}{non-recursive mutex}, this function will
    \e always return false when attempting to lock the mutex
    recursively.

    \sa lock(), unlock()
*/
bool QMutex::tryLock()
{
    QMutexPrivate *d = static_cast<QMutexPrivate *>(this->d);
    Qt::HANDLE self;

    if (d->recursive) {
        self = QThread::currentThreadId();
        if (d->owner == self) {
            ++d->count;
            Q_ASSERT_X(d->count != 0, "QMutex::tryLock", "Overflow in recursion counter");
            return true;
        }

        bool isLocked = d->contenders.testAndSetAcquire(0, 1);
        if (!isLocked) {
            // some other thread has the mutex locked, or we tried to
            // recursively lock an non-recursive mutex
            return isLocked;
        }

        d->owner = self;
        ++d->count;
        Q_ASSERT_X(d->count != 0, "QMutex::tryLock", "Overflow in recursion counter");
        return isLocked;
    }

    return d->contenders.testAndSetAcquire(0, 1);
}

/*! \overload

    Attempts to lock the mutex. This function returns true if the lock
    was obtained; otherwise it returns false. If another thread has
    locked the mutex, this function will wait for at most \a timeout
    milliseconds for the mutex to become available.

    Note: Passing a negative number as the \a timeout is equivalent to
    calling lock(), i.e. this function will wait forever until mutex
    can be locked if \a timeout is negative.

    If the lock was obtained, the mutex must be unlocked with unlock()
    before another thread can successfully lock it.

    Calling this function multiple times on the same mutex from the
    same thread is allowed if this mutex is a
    \l{QMutex::Recursive}{recursive mutex}. If this mutex is a
    \l{QMutex::NonRecursive}{non-recursive mutex}, this function will
    \e always return false when attempting to lock the mutex
    recursively.

    \sa lock(), unlock()
*/
bool QMutex::tryLock(int timeout)
{
    QMutexPrivate *d = static_cast<QMutexPrivate *>(this->d);
    Qt::HANDLE self;

    if (d->recursive) {
        self = QThread::currentThreadId();
        if (d->owner == self) {
            ++d->count;
            Q_ASSERT_X(d->count != 0, "QMutex::tryLock", "Overflow in recursion counter");
            return true;
        }

        bool isLocked = d->contenders.testAndSetAcquire(0, 1);
        if (!isLocked) {
            // didn't get the lock, wait for it
            isLocked = d->wait(timeout);
            if (!isLocked)
                return false;
        }

        d->owner = self;
        ++d->count;
        Q_ASSERT_X(d->count != 0, "QMutex::tryLock", "Overflow in recursion counter");
        return true;
    }

    return (d->contenders.testAndSetAcquire(0, 1)
            // didn't get the lock, wait for it
            || d->wait(timeout));
}


/*!
    Unlocks the mutex. Attempting to unlock a mutex in a different
    thread to the one that locked it results in an error. Unlocking a
    mutex that is not locked results in undefined behavior.

    \sa lock()
*/
void QMutex::unlock()
{
    QMutexPrivate *d = static_cast<QMutexPrivate *>(this->d);
    if (d->recursive) {
        if (!--d->count) {
            d->owner = 0;
            if (!d->contenders.testAndSetRelease(1, 0))
                d->wakeUp();
        }
    } else {
        if (!d->contenders.testAndSetRelease(1, 0))
            d->wakeUp();
    }
}

/*!
    \fn bool QMutex::locked()

    Returns true if the mutex is locked by another thread; otherwise
    returns false.

    It is generally a bad idea to use this function, because code
    that uses it has a race condition. Use tryLock() and unlock()
    instead.

    \oldcode
        bool isLocked = mutex.locked();
    \newcode
        bool isLocked = true;
        if (mutex.tryLock()) {
            mutex.unlock();
            isLocked = false;
        }
    \endcode
*/

/*!
    \class QMutexLocker
    \brief The QMutexLocker class is a convenience class that simplifies
    locking and unlocking mutexes.

    \threadsafe

    \ingroup thread

    Locking and unlocking a QMutex in complex functions and
    statements or in exception handling code is error-prone and
    difficult to debug. QMutexLocker can be used in such situations
    to ensure that the state of the mutex is always well-defined.

    QMutexLocker should be created within a function where a
    QMutex needs to be locked. The mutex is locked when QMutexLocker
    is created. You can unlock and relock the mutex with \c unlock()
    and \c relock(). If locked, the mutex will be unlocked when the
    QMutexLocker is destroyed.

    For example, this complex function locks a QMutex upon entering
    the function and unlocks the mutex at all the exit points:

    \snippet doc/src/snippets/code/src_corelib_thread_qmutex.cpp 4

    This example function will get more complicated as it is
    developed, which increases the likelihood that errors will occur.

    Using QMutexLocker greatly simplifies the code, and makes it more
    readable:

    \snippet doc/src/snippets/code/src_corelib_thread_qmutex.cpp 5

    Now, the mutex will always be unlocked when the QMutexLocker
    object is destroyed (when the function returns since \c locker is
    an auto variable).

    The same principle applies to code that throws and catches
    exceptions. An exception that is not caught in the function that
    has locked the mutex has no way of unlocking the mutex before the
    exception is passed up the stack to the calling function.

    QMutexLocker also provides a \c mutex() member function that returns
    the mutex on which the QMutexLocker is operating. This is useful
    for code that needs access to the mutex, such as
    QWaitCondition::wait(). For example:

    \snippet doc/src/snippets/code/src_corelib_thread_qmutex.cpp 6

    \sa QReadLocker, QWriteLocker, QMutex
*/

/*!
    \fn QMutexLocker::QMutexLocker(QMutex *mutex)

    Constructs a QMutexLocker and locks \a mutex. The mutex will be
    unlocked when the QMutexLocker is destroyed. If \a mutex is zero,
    QMutexLocker does nothing.

    \sa QMutex::lock()
*/

/*!
    \fn QMutexLocker::~QMutexLocker()

    Destroys the QMutexLocker and unlocks the mutex that was locked
    in the constructor.

    \sa QMutex::unlock()
*/

/*!
    \fn QMutex *QMutexLocker::mutex() const

    Returns a pointer to the mutex that was locked in the
    constructor.
*/

/*!
    \fn void QMutexLocker::unlock()

    Unlocks this mutex locker. You can use \c relock() to lock
    it again. It does not need to be locked when destroyed.

    \sa relock()
*/

/*!
    \fn void QMutexLocker::relock()

    Relocks an unlocked mutex locker.

    \sa unlock()
*/

/*!
    \fn QMutex::QMutex(bool recursive)

    Use the constructor that takes a RecursionMode parameter instead.
*/

/*!
    \internal helper for lockInline()
 */
void QMutex::lockInternal()
{
    QMutexPrivate *d = static_cast<QMutexPrivate *>(this->d);

    if (QThread::idealThreadCount() == 1) {
        // don't spin on single cpu machines
        bool isLocked = d->wait();
        Q_ASSERT_X(isLocked, "QMutex::lock",
                   "Internal error, infinite wait has timed out.");
        Q_UNUSED(isLocked);
        return;
    }

    QElapsedTimer elapsedTimer;
    elapsedTimer.start();
    do {
        qint64 spinTime = elapsedTimer.nsecsElapsed();
        if (spinTime > d->maximumSpinTime) {
            // didn't get the lock, wait for it, since we're not going to gain anything by spinning more
            elapsedTimer.start();
            bool isLocked = d->wait();
            Q_ASSERT_X(isLocked, "QMutex::lock",
                       "Internal error, infinite wait has timed out.");
            Q_UNUSED(isLocked);

            qint64 maximumSpinTime = d->maximumSpinTime;
            qint64 averageWaitTime = d->averageWaitTime;
            qint64 actualWaitTime = elapsedTimer.nsecsElapsed();
            if (actualWaitTime < (QMutexPrivate::MaximumSpinTimeThreshold * 3 / 2)) {
                // measure the wait times
                averageWaitTime = d->averageWaitTime = qMin((averageWaitTime + actualWaitTime) / 2, qint64(QMutexPrivate::MaximumSpinTimeThreshold));
            }

            // adjust the spin count when spinning does not benefit contention performance
            if ((spinTime + actualWaitTime) - qint64(QMutexPrivate::MaximumSpinTimeThreshold) >= qint64(QMutexPrivate::MaximumSpinTimeThreshold)) {
                // long waits, stop spinning
                d->maximumSpinTime = 0;
            } else {
                // allow spinning if wait times decrease, but never spin more than the average wait time (otherwise we may perform worse)
                d->maximumSpinTime = qBound(qint64(averageWaitTime * 3 / 2), maximumSpinTime / 2, qint64(QMutexPrivate::MaximumSpinTimeThreshold));
            }
            return;
        }
        // be a good citizen... yielding lets something else run if there is something to run, but may also relieve memory pressure if not
        QThread::yieldCurrentThread();
    } while (d->contenders != 0 || !d->contenders.testAndSetAcquire(0, 1));

    // spinning is working, do not change the spin time (unless we are using much less time than allowed to spin)
    qint64 maximumSpinTime = d->maximumSpinTime;
    qint64 spinTime = elapsedTimer.nsecsElapsed();
    if (spinTime < maximumSpinTime / 2) {
        // we are using much less time than we need, adjust the limit
        d->maximumSpinTime = qBound(qint64(d->averageWaitTime * 3 / 2), maximumSpinTime / 2, qint64(QMutexPrivate::MaximumSpinTimeThreshold));
    }
}

/*!
    \internal
*/
void QMutex::unlockInternal()
{
    static_cast<QMutexPrivate *>(d)->wakeUp();
}

/*!
   \fn QMutex::lockInline()
   \internal
   inline version of QMutex::lock()
*/

/*!
   \fn QMutex::unlockInline()
   \internal
   inline version of QMutex::unlock()
*/

/*!
   \fn QMutex::tryLockInline()
   \internal
   inline version of QMutex::tryLock()
*/


QT_END_NAMESPACE

#endif // QT_NO_THREAD