From fddeec60cba806b88c5eb0e4758c3951457518a2 Mon Sep 17 00:00:00 2001 From: Giuseppe D'Angelo Date: Thu, 15 Dec 2022 01:06:43 +0100 Subject: [PATCH] Wait conditions example: fix an incorrect condition variable usage MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 3a449bbb69c9a3c3a5bc6a052f2de98ab79be7e9 amended the code to remove acquiring a lock when waking up a condition variable. It is fine to not have a lock associated when waking a condition variable; what I misunderstood was the scope of the lock, which (and this underlines the importance of commenting _what exactly_ a lock protects, for each and ever lock) protected both the buffer as well as the counter of the buffer. This made my reasoning flawed: it is necessary to keep the lock while notifying, otherwise the counterpart could verify the condition isn't satisfied and wait (e.g. see numUsedBytes==0), missing the wake from the other thread (which could arrive between the check and the wait). Amends the previous commit. Change-Id: If7db2d045331f1b33b976fb6bf6aa9117c41678f Pick-to: 5.15 6.2 6.4 6.5 Reviewed-by: Morten Johan Sørvig --- .../threads/doc/src/waitconditions.qdoc | 7 ++---- .../threads/waitconditions/waitconditions.cpp | 24 +++++++++++-------- 2 files changed, 16 insertions(+), 15 deletions(-) diff --git a/examples/corelib/threads/doc/src/waitconditions.qdoc b/examples/corelib/threads/doc/src/waitconditions.qdoc index 7715bd8ab63..ae9e767c138 100644 --- a/examples/corelib/threads/doc/src/waitconditions.qdoc +++ b/examples/corelib/threads/doc/src/waitconditions.qdoc @@ -75,16 +75,13 @@ that the condition \c bufferNotEmpty is true, since \c numUsedBytes is necessarily greater than 0. - The QWaitCondition::wait() function accepts a + We guard all accesses to the \c numUsedBytes variable with a + mutex. In addition, the QWaitCondition::wait() function accepts a mutex as its argument. This mutex is unlocked before the thread is put to sleep and locked when the thread wakes up. Furthermore, the transition from the locked state to the wait state is atomic, to prevent race conditions from occurring. - Accesses to the \c numUsedBytes variable do not need mutex - protection, as that variable is a QAtomicInt; atomic variables - do not participate in data races. - \section1 Consumer Class Let's turn to the \c Consumer class: diff --git a/examples/corelib/threads/waitconditions/waitconditions.cpp b/examples/corelib/threads/waitconditions/waitconditions.cpp index 36f02f78968..bdc24acd8c2 100644 --- a/examples/corelib/threads/waitconditions/waitconditions.cpp +++ b/examples/corelib/threads/waitconditions/waitconditions.cpp @@ -2,7 +2,6 @@ // Copyright (C) 2022 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Giuseppe D'Angelo // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause -#include #include #include #include @@ -18,13 +17,12 @@ constexpr int DataSize = 100000; constexpr int BufferSize = 8192; -QMutex mutex; // protects the buffer +QMutex mutex; // protects the buffer and the counter char buffer[BufferSize]; +int numUsedBytes; QWaitCondition bufferNotEmpty; QWaitCondition bufferNotFull; - -QAtomicInt numUsedBytes; //! [0] //! [1] @@ -43,14 +41,17 @@ private: for (int i = 0; i < DataSize; ++i) { { const QMutexLocker locker(&mutex); - while (numUsedBytes.loadAcquire() == BufferSize) + while (numUsedBytes == BufferSize) bufferNotFull.wait(&mutex); } buffer[i % BufferSize] = "ACGT"[QRandomGenerator::global()->bounded(4)]; - numUsedBytes.fetchAndAddRelease(1); - bufferNotEmpty.wakeAll(); + { + const QMutexLocker locker(&mutex); + ++numUsedBytes; + bufferNotEmpty.wakeAll(); + } } } }; @@ -72,14 +73,17 @@ private: for (int i = 0; i < DataSize; ++i) { { const QMutexLocker locker(&mutex); - while (numUsedBytes.loadAcquire() == 0) + while (numUsedBytes == 0) bufferNotEmpty.wait(&mutex); } fprintf(stderr, "%c", buffer[i % BufferSize]); - numUsedBytes.fetchAndAddRelease(-1); - bufferNotFull.wakeAll(); + { + const QMutexLocker locker(&mutex); + --numUsedBytes; + bufferNotFull.wakeAll(); + } } fprintf(stderr, "\n"); }