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41
42#include "qplatformdefs.h"
43#include "qmutex.h"
44#include <qdebug.h>
45#include "qatomic.h"
46#include "qelapsedtimer.h"
47#include "qthread.h"
48#include "qmutex_p.h"
49
50#ifndef QT_LINUX_FUTEX
51#include "private/qfreelist_p.h"
52#endif
53
54QT_BEGIN_NAMESPACE
55
56/*
57 \class QBasicMutex
58 \inmodule QtCore
59 \brief QMutex POD
60 \internal
61
62 \ingroup thread
63
64 - Can be used as global static object.
65 - Always non-recursive
66 - Do not use tryLock with timeout > 0, else you can have a leak (see the ~QMutex destructor)
67*/
68
69/*!
70 \class QMutex
71 \inmodule QtCore
72 \brief The QMutex class provides access serialization between threads.
73
74 \threadsafe
75
76 \ingroup thread
77
78 The purpose of a QMutex is to protect an object, data structure or
79 section of code so that only one thread can access it at a time
80 (this is similar to the Java \c synchronized keyword). It is
81 usually best to use a mutex with a QMutexLocker since this makes
82 it easy to ensure that locking and unlocking are performed
83 consistently.
84
85 For example, say there is a method that prints a message to the
86 user on two lines:
87
88 \snippet code/src_corelib_thread_qmutex.cpp 0
89
90 If these two methods are called in succession, the following happens:
91
92 \snippet code/src_corelib_thread_qmutex.cpp 1
93
94 If these two methods are called simultaneously from two threads then the
95 following sequence could result:
96
97 \snippet code/src_corelib_thread_qmutex.cpp 2
98
99 If we add a mutex, we should get the result we want:
100
101 \snippet code/src_corelib_thread_qmutex.cpp 3
102
103 Then only one thread can modify \c number at any given time and
104 the result is correct. This is a trivial example, of course, but
105 applies to any other case where things need to happen in a
106 particular sequence.
107
108 When you call lock() in a thread, other threads that try to call
109 lock() in the same place will block until the thread that got the
110 lock calls unlock(). A non-blocking alternative to lock() is
111 tryLock().
112
113 QMutex is optimized to be fast in the non-contended case. It
114 will not allocate memory if there is no contention on that mutex.
115 It is constructed and destroyed with almost no overhead,
116 which means it is fine to have many mutexes as part of other classes.
117
118 \sa QRecursiveMutex, QMutexLocker, QReadWriteLock, QSemaphore, QWaitCondition
119*/
120
121/*!
122 \fn QMutex::QMutex()
123
124 Constructs a new mutex. The mutex is created in an unlocked state.
125*/
126
127/*! \fn QMutex::~QMutex()
128
129 Destroys the mutex.
130
131 \warning Destroying a locked mutex may result in undefined behavior.
132*/
133void QBasicMutex::destroyInternal(QMutexPrivate *d)
134{
135 if (!d)
136 return;
137#ifndef QT_LINUX_FUTEX
138 if (d != dummyLocked() && d->possiblyUnlocked.loadRelaxed() && tryLock()) {
139 unlock();
140 return;
141 }
142#endif
143 qWarning("QMutex: destroying locked mutex");
144}
145
146/*! \fn void QMutex::lock()
147
148 Locks the mutex. If another thread has locked the mutex then this
149 call will block until that thread has unlocked it.
150
151 Calling this function multiple times on the same mutex from the
152 same thread will cause a \e dead-lock.
153
154 \sa unlock()
155*/
156
157/*! \fn bool QMutex::tryLock(int timeout)
158
159 Attempts to lock the mutex. This function returns \c true if the lock
160 was obtained; otherwise it returns \c false. If another thread has
161 locked the mutex, this function will wait for at most \a timeout
162 milliseconds for the mutex to become available.
163
164 Note: Passing a negative number as the \a timeout is equivalent to
165 calling lock(), i.e. this function will wait forever until mutex
166 can be locked if \a timeout is negative.
167
168 If the lock was obtained, the mutex must be unlocked with unlock()
169 before another thread can successfully lock it.
170
171 Calling this function multiple times on the same mutex from the
172 same thread will cause a \e dead-lock.
173
174 \sa lock(), unlock()
175*/
176
177/*! \fn bool QMutex::tryLock()
178 \overload
179
180 Attempts to lock the mutex. This function returns \c true if the lock
181 was obtained; otherwise it returns \c false.
182
183 If the lock was obtained, the mutex must be unlocked with unlock()
184 before another thread can successfully lock it.
185
186 Calling this function multiple times on the same mutex from the
187 same thread will cause a \e dead-lock.
188
189 \sa lock(), unlock()
190*/
191
192/*! \fn bool QMutex::try_lock()
193 \since 5.8
194
195 Attempts to lock the mutex. This function returns \c true if the lock
196 was obtained; otherwise it returns \c false.
197
198 This function is provided for compatibility with the Standard Library
199 concept \c Lockable. It is equivalent to tryLock().
200*/
201
202/*! \fn template <class Rep, class Period> bool QMutex::try_lock_for(std::chrono::duration<Rep, Period> duration)
203 \since 5.8
204
205 Attempts to lock the mutex. This function returns \c true if the lock
206 was obtained; otherwise it returns \c false. If another thread has
207 locked the mutex, this function will wait for at least \a duration
208 for the mutex to become available.
209
210 Note: Passing a negative duration as the \a duration is equivalent to
211 calling try_lock(). This behavior differs from tryLock().
212
213 If the lock was obtained, the mutex must be unlocked with unlock()
214 before another thread can successfully lock it.
215
216 Calling this function multiple times on the same mutex from the
217 same thread will cause a \e dead-lock.
218
219 \sa lock(), unlock()
220*/
221
222/*! \fn template<class Clock, class Duration> bool QMutex::try_lock_until(std::chrono::time_point<Clock, Duration> timePoint)
223 \since 5.8
224
225 Attempts to lock the mutex. This function returns \c true if the lock
226 was obtained; otherwise it returns \c false. If another thread has
227 locked the mutex, this function will wait at least until \a timePoint
228 for the mutex to become available.
229
230 Note: Passing a \a timePoint which has already passed is equivalent
231 to calling try_lock(). This behavior differs from tryLock().
232
233 If the lock was obtained, the mutex must be unlocked with unlock()
234 before another thread can successfully lock it.
235
236 Calling this function multiple times on the same mutex from the
237 same thread will cause a \e dead-lock.
238
239 \sa lock(), unlock()
240*/
241
242/*! \fn void QMutex::unlock()
243
244 Unlocks the mutex. Attempting to unlock a mutex in a different
245 thread to the one that locked it results in an error. Unlocking a
246 mutex that is not locked results in undefined behavior.
247
248 \sa lock()
249*/
250
251/*!
252 \class QRecursiveMutex
253 \inmodule QtCore
254 \since 5.14
255 \brief The QRecursiveMutex class provides access serialization between threads.
256
257 \threadsafe
258
259 \ingroup thread
260
261 The QRecursiveMutex class is a mutex, like QMutex, with which it is
262 API-compatible. It differs from QMutex by accepting lock() calls from
263 the same thread any number of times. QMutex would deadlock in this situation.
264
265 QRecursiveMutex is much more expensive to construct and operate on, so
266 use a plain QMutex whenever you can. Sometimes, one public function,
267 however, calls another public function, and they both need to lock the
268 same mutex. In this case, you have two options:
269
270 \list
271 \li Factor the code that needs mutex protection into private functions,
272 which assume that the mutex is held when they are called, and lock a
273 plain QMutex in the public functions before you call the private
274 implementation ones.
275 \li Or use a recursive mutex, so it doesn't matter that the first public
276 function has already locked the mutex when the second one wishes to do so.
277 \endlist
278
279 \sa QMutex, QMutexLocker, QReadWriteLock, QSemaphore, QWaitCondition
280*/
281
282/*! \fn QRecursiveMutex::QRecursiveMutex()
283
284 Constructs a new recursive mutex. The mutex is created in an unlocked state.
285
286 \sa lock(), unlock()
287*/
288
289/*!
290 Destroys the mutex.
291
292 \warning Destroying a locked mutex may result in undefined behavior.
293*/
294QRecursiveMutex::~QRecursiveMutex()
295{
296}
297
298/*! \fn void QRecursiveMutex::lock()
299
300 Locks the mutex. If another thread has locked the mutex then this
301 call will block until that thread has unlocked it.
302
303 Calling this function multiple times on the same mutex from the
304 same thread is allowed.
305
306 \sa unlock()
307*/
308
309/*!
310 Attempts to lock the mutex. This function returns \c true if the lock
311 was obtained; otherwise it returns \c false. If another thread has
312 locked the mutex, this function will wait for at most \a timeout
313 milliseconds for the mutex to become available.
314
315 Note: Passing a negative number as the \a timeout is equivalent to
316 calling lock(), i.e. this function will wait forever until mutex
317 can be locked if \a timeout is negative.
318
319 If the lock was obtained, the mutex must be unlocked with unlock()
320 before another thread can successfully lock it.
321
322 Calling this function multiple times on the same mutex from the
323 same thread is allowed.
324
325 \sa lock(), unlock()
326*/
327bool QRecursiveMutex::tryLock(int timeout) QT_MUTEX_LOCK_NOEXCEPT
328{
329 Qt::HANDLE self = QThread::currentThreadId();
330 if (owner.loadRelaxed() == self) {
331 ++count;
332 Q_ASSERT_X(count != 0, "QMutex::lock", "Overflow in recursion counter");
333 return true;
334 }
335 bool success = true;
336 if (timeout == -1) {
337 mutex.lock();
338 } else {
339 success = mutex.tryLock(timeout);
340 }
341
342 if (success)
343 owner.storeRelaxed(self);
344 return success;
345}
346
347/*! \fn bool QRecursiveMutex::try_lock()
348 \since 5.8
349
350 Attempts to lock the mutex. This function returns \c true if the lock
351 was obtained; otherwise it returns \c false.
352
353 This function is provided for compatibility with the Standard Library
354 concept \c Lockable. It is equivalent to tryLock().
355*/
356
357/*! \fn template <class Rep, class Period> bool QRecursiveMutex::try_lock_for(std::chrono::duration<Rep, Period> duration)
358 \since 5.8
359
360 Attempts to lock the mutex. This function returns \c true if the lock
361 was obtained; otherwise it returns \c false. If another thread has
362 locked the mutex, this function will wait for at least \a duration
363 for the mutex to become available.
364
365 Note: Passing a negative duration as the \a duration is equivalent to
366 calling try_lock(). This behavior differs from tryLock().
367
368 If the lock was obtained, the mutex must be unlocked with unlock()
369 before another thread can successfully lock it.
370
371 Calling this function multiple times on the same mutex from the
372 same thread is allowed.
373
374 \sa lock(), unlock()
375*/
376
377/*! \fn template<class Clock, class Duration> bool QRecursiveMutex::try_lock_until(std::chrono::time_point<Clock, Duration> timePoint)
378 \since 5.8
379
380 Attempts to lock the mutex. This function returns \c true if the lock
381 was obtained; otherwise it returns \c false. If another thread has
382 locked the mutex, this function will wait at least until \a timePoint
383 for the mutex to become available.
384
385 Note: Passing a \a timePoint which has already passed is equivalent
386 to calling try_lock(). This behavior differs from tryLock().
387
388 If the lock was obtained, the mutex must be unlocked with unlock()
389 before another thread can successfully lock it.
390
391 Calling this function multiple times on the same mutex from the
392 same thread is allowed.
393
394 \sa lock(), unlock()
395*/
396
397/*!
398 Unlocks the mutex. Attempting to unlock a mutex in a different
399 thread to the one that locked it results in an error. Unlocking a
400 mutex that is not locked results in undefined behavior.
401
402 \sa lock()
403*/
404void QRecursiveMutex::unlock() noexcept
405{
406 Q_ASSERT(owner.loadRelaxed() == QThread::currentThreadId());
407
408 if (count > 0) {
409 count--;
410 } else {
411 owner.storeRelaxed(nullptr);
412 mutex.unlock();
413 }
414}
415
416
417/*!
418 \class QMutexLocker
419 \inmodule QtCore
420 \brief The QMutexLocker class is a convenience class that simplifies
421 locking and unlocking mutexes.
422
423 \threadsafe
424
425 \ingroup thread
426
427 Locking and unlocking a QMutex or QRecursiveMutex in complex functions and
428 statements or in exception handling code is error-prone and
429 difficult to debug. QMutexLocker can be used in such situations
430 to ensure that the state of the mutex is always well-defined.
431
432 QMutexLocker should be created within a function where a
433 QMutex needs to be locked. The mutex is locked when QMutexLocker
434 is created. You can unlock and relock the mutex with \c unlock()
435 and \c relock(). If locked, the mutex will be unlocked when the
436 QMutexLocker is destroyed.
437
438 For example, this complex function locks a QMutex upon entering
439 the function and unlocks the mutex at all the exit points:
440
441 \snippet code/src_corelib_thread_qmutex.cpp 4
442
443 This example function will get more complicated as it is
444 developed, which increases the likelihood that errors will occur.
445
446 Using QMutexLocker greatly simplifies the code, and makes it more
447 readable:
448
449 \snippet code/src_corelib_thread_qmutex.cpp 5
450
451 Now, the mutex will always be unlocked when the QMutexLocker
452 object is destroyed (when the function returns since \c locker is
453 an auto variable).
454
455 The same principle applies to code that throws and catches
456 exceptions. An exception that is not caught in the function that
457 has locked the mutex has no way of unlocking the mutex before the
458 exception is passed up the stack to the calling function.
459
460 QMutexLocker also provides a \c mutex() member function that returns
461 the mutex on which the QMutexLocker is operating. This is useful
462 for code that needs access to the mutex, such as
463 QWaitCondition::wait(). For example:
464
465 \snippet code/src_corelib_thread_qmutex.cpp 6
466
467 \sa QReadLocker, QWriteLocker, QMutex
468*/
469
470/*!
471 \fn template <typename Mutex> QMutexLocker<Mutex>::QMutexLocker(Mutex *mutex) noexcept
472
473 Constructs a QMutexLocker and locks \a mutex. The mutex will be
474 unlocked when the QMutexLocker is destroyed. If \a mutex is \nullptr,
475 QMutexLocker does nothing.
476
477 \sa QMutex::lock()
478*/
479
480/*!
481 \fn template <typename Mutex> QMutexLocker<Mutex>::~QMutexLocker() noexcept
482
483 Destroys the QMutexLocker and unlocks the mutex that was locked
484 in the constructor.
485
486 \sa QMutex::unlock()
487*/
488
489/*!
490 \fn template <typename Mutex> void QMutexLocker<Mutex>::unlock() noexcept
491
492 Unlocks this mutex locker. You can use \c relock() to lock
493 it again. It does not need to be locked when destroyed.
494
495 \sa relock()
496*/
497
498/*!
499 \fn template <typename Mutex> void QMutexLocker<Mutex>::relock() noexcept
500
501 Relocks an unlocked mutex locker.
502
503 \sa unlock()
504*/
505
506/*!
507 \fn template <typename Mutex> QMutex *QMutexLocker<Mutex>::mutex() const
508
509 Returns the mutex on which the QMutexLocker is operating.
510
511*/
512
513#ifndef QT_LINUX_FUTEX //linux implementation is in qmutex_linux.cpp
514
515/*
516 For a rough introduction on how this works, refer to
517 http://woboq.com/blog/internals-of-qmutex-in-qt5.html
518 which explains a slightly simplified version of it.
519 The differences are that here we try to work with timeout (requires the
520 possiblyUnlocked flag) and that we only wake one thread when unlocking
521 (requires maintaining the waiters count)
522 We also support recursive mutexes which always have a valid d_ptr.
523
524 The waiters flag represents the number of threads that are waiting or about
525 to wait on the mutex. There are two tricks to keep in mind:
526 We don't want to increment waiters after we checked no threads are waiting
527 (waiters == 0). That's why we atomically set the BigNumber flag on waiters when
528 we check waiters. Similarly, if waiters is decremented right after we checked,
529 the mutex would be unlocked (d->wakeUp() has (or will) be called), but there is
530 no thread waiting. This is only happening if there was a timeout in tryLock at the
531 same time as the mutex is unlocked. So when there was a timeout, we set the
532 possiblyUnlocked flag.
533*/
534
535/*!
536 \internal helper for lock()
537 */
538void QBasicMutex::lockInternal() QT_MUTEX_LOCK_NOEXCEPT
539{
540 lockInternal(-1);
541}
542
543/*!
544 \internal helper for lock(int)
545 */
546bool QBasicMutex::lockInternal(int timeout) QT_MUTEX_LOCK_NOEXCEPT
547{
548 while (!fastTryLock()) {
549 QMutexPrivate *copy = d_ptr.loadAcquire();
550 if (!copy) // if d is 0, the mutex is unlocked
551 continue;
552
553 if (copy == dummyLocked()) {
554 if (timeout == 0)
555 return false;
556 // The mutex is locked but does not have a QMutexPrivate yet.
557 // we need to allocate a QMutexPrivate
558 QMutexPrivate *newD = QMutexPrivate::allocate();
559 if (!d_ptr.testAndSetOrdered(dummyLocked(), newD)) {
560 //Either the mutex is already unlocked, or another thread already set it.
561 newD->deref();
562 continue;
563 }
564 copy = newD;
565 //the d->refCount is already 1 the deref will occurs when we unlock
566 }
567
568 QMutexPrivate *d = static_cast<QMutexPrivate *>(copy);
569 if (timeout == 0 && !d->possiblyUnlocked.loadRelaxed())
570 return false;
571
572 // At this point we have a pointer to a QMutexPrivate. But the other thread
573 // may unlock the mutex at any moment and release the QMutexPrivate to the pool.
574 // We will try to reference it to avoid unlock to release it to the pool to make
575 // sure it won't be released. But if the refcount is already 0 it has been released.
576 if (!d->ref())
577 continue; //that QMutexPrivate was already released
578
579 // We now hold a reference to the QMutexPrivate. It won't be released and re-used.
580 // But it is still possible that it was already re-used by another QMutex right before
581 // we did the ref(). So check if we still hold a pointer to the right mutex.
582 if (d != d_ptr.loadAcquire()) {
583 //Either the mutex is already unlocked, or relocked with another mutex
584 d->deref();
585 continue;
586 }
587
588 // In this part, we will try to increment the waiters count.
589 // We just need to take care of the case in which the old_waiters
590 // is set to the BigNumber magic value set in unlockInternal()
591 int old_waiters;
592 do {
593 old_waiters = d->waiters.loadRelaxed();
594 if (old_waiters == -QMutexPrivate::BigNumber) {
595 // we are unlocking, and the thread that unlocks is about to change d to 0
596 // we try to acquire the mutex by changing to dummyLocked()
597 if (d_ptr.testAndSetAcquire(d, dummyLocked())) {
598 // Mutex acquired
599 d->deref();
600 return true;
601 } else {
602 Q_ASSERT(d != d_ptr.loadRelaxed()); //else testAndSetAcquire should have succeeded
603 // Mutex is likely to bo 0, we should continue the outer-loop,
604 // set old_waiters to the magic value of BigNumber
605 old_waiters = QMutexPrivate::BigNumber;
606 break;
607 }
608 }
609 } while (!d->waiters.testAndSetRelaxed(old_waiters, old_waiters + 1));
610
611 if (d != d_ptr.loadAcquire()) {
612 // The mutex was unlocked before we incremented waiters.
613 if (old_waiters != QMutexPrivate::BigNumber) {
614 //we did not break the previous loop
615 Q_ASSERT(d->waiters.loadRelaxed() >= 1);
616 d->waiters.deref();
617 }
618 d->deref();
619 continue;
620 }
621
622 if (d->wait(timeout)) {
623 // reset the possiblyUnlocked flag if needed (and deref its corresponding reference)
624 if (d->possiblyUnlocked.loadRelaxed() && d->possiblyUnlocked.testAndSetRelaxed(true, false))
625 d->deref();
626 d->derefWaiters(1);
627 //we got the lock. (do not deref)
628 Q_ASSERT(d == d_ptr.loadRelaxed());
629 return true;
630 } else {
631 Q_ASSERT(timeout >= 0);
632 //timeout
633 d->derefWaiters(1);
634 //There may be a race in which the mutex is unlocked right after we timed out,
635 // and before we deref the waiters, so maybe the mutex is actually unlocked.
636 // Set the possiblyUnlocked flag to indicate this possibility.
637 if (!d->possiblyUnlocked.testAndSetRelaxed(false, true)) {
638 // We keep a reference when possiblyUnlocked is true.
639 // but if possiblyUnlocked was already true, we don't need to keep the reference.
640 d->deref();
641 }
642 return false;
643 }
644 }
645 Q_ASSERT(d_ptr.loadRelaxed() != 0);
646 return true;
647}
648
649/*!
650 \internal
651*/
652void QBasicMutex::unlockInternal() noexcept
653{
654 QMutexPrivate *copy = d_ptr.loadAcquire();
655 Q_ASSERT(copy); //we must be locked
656 Q_ASSERT(copy != dummyLocked()); // testAndSetRelease(dummyLocked(), 0) failed
657
658 QMutexPrivate *d = reinterpret_cast<QMutexPrivate *>(copy);
659
660 // If no one is waiting for the lock anymore, we should reset d to 0x0.
661 // Using fetchAndAdd, we atomically check that waiters was equal to 0, and add a flag
662 // to the waiters variable (BigNumber). That way, we avoid the race in which waiters is
663 // incremented right after we checked, because we won't increment waiters if is
664 // equal to -BigNumber
665 if (d->waiters.fetchAndAddRelease(-QMutexPrivate::BigNumber) == 0) {
666 //there is no one waiting on this mutex anymore, set the mutex as unlocked (d = 0)
667 if (d_ptr.testAndSetRelease(d, 0)) {
668 // reset the possiblyUnlocked flag if needed (and deref its corresponding reference)
669 if (d->possiblyUnlocked.loadRelaxed() && d->possiblyUnlocked.testAndSetRelaxed(true, false))
670 d->deref();
671 }
672 d->derefWaiters(0);
673 } else {
674 d->derefWaiters(0);
675 //there are thread waiting, transfer the lock.
676 d->wakeUp();
677 }
678 d->deref();
679}
680
681//The freelist management
682namespace {
683struct FreeListConstants : QFreeListDefaultConstants {
684 enum { BlockCount = 4, MaxIndex=0xffff };
685 static const int Sizes[BlockCount];
686};
687const int FreeListConstants::Sizes[FreeListConstants::BlockCount] = {
688 16,
689 128,
690 1024,
691 FreeListConstants::MaxIndex - (16 + 128 + 1024)
692};
693
694typedef QFreeList<QMutexPrivate, FreeListConstants> FreeList;
695// We cannot use Q_GLOBAL_STATIC because it uses QMutex
696static FreeList freeList_;
697FreeList *freelist()
698{
699 return &freeList_;
700}
701}
702
703QMutexPrivate *QMutexPrivate::allocate()
704{
705 int i = freelist()->next();
706 QMutexPrivate *d = &(*freelist())[i];
707 d->id = i;
708 Q_ASSERT(d->refCount.loadRelaxed() == 0);
709 Q_ASSERT(!d->possiblyUnlocked.loadRelaxed());
710 Q_ASSERT(d->waiters.loadRelaxed() == 0);
711 d->refCount.storeRelaxed(1);
712 return d;
713}
714
715void QMutexPrivate::release()
716{
717 Q_ASSERT(refCount.loadRelaxed() == 0);
718 Q_ASSERT(!possiblyUnlocked.loadRelaxed());
719 Q_ASSERT(waiters.loadRelaxed() == 0);
720 freelist()->release(id);
721}
722
723// atomically subtract "value" to the waiters, and remove the QMutexPrivate::BigNumber flag
724void QMutexPrivate::derefWaiters(int value) noexcept
725{
726 int old_waiters;
727 int new_waiters;
728 do {
729 old_waiters = waiters.loadRelaxed();
730 new_waiters = old_waiters;
731 if (new_waiters < 0) {
732 new_waiters += QMutexPrivate::BigNumber;
733 }
734 new_waiters -= value;
735 } while (!waiters.testAndSetRelaxed(old_waiters, new_waiters));
736}
737#endif
738
739QT_END_NAMESPACE
740
741#ifdef QT_LINUX_FUTEX
742# include "qmutex_linux.cpp"
743#elif defined(Q_OS_MAC)
744# include "qmutex_mac.cpp"
745#elif defined(Q_OS_WIN)
746# include "qmutex_win.cpp"
747#else
748# include "qmutex_unix.cpp"
749#endif
750