Make QMutexPool actually be thread-safe (double-checked locking doesn't

work)

This code was apparently mostly unchanged since 2005. Update it to match
the atomics code.

Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>

2 files changed, 6 insertions, 13 deletions

diff --git a/src/corelib/thread/qmutexpool.cpp b/src/corelib/thread/qmutexpool.cpp
index 71ecab5..96a9940 100644
--- a/src/corelib/thread/qmutexpool.cpp
+++ b/src/corelib/thread/qmutexpool.cpp
@@ -96,9 +96,8 @@ Q_GLOBAL_STATIC_WITH_ARGS(QMutexPool, globalMutexPool, (true))
     QMutexPool is destructed.
 */
 QMutexPool::QMutexPool(bool recursive, int size)
-    : count(size), recurs(recursive)
+    : mutexes(size), count(size), recurs(recursive)
 {
-    mutexes = new QMutex*[count];
     for (int index = 0; index < count; ++index) {
         mutexes[index] = 0;
     }
@@ -110,13 +109,10 @@ QMutexPool::QMutexPool(bool recursive, int size)
 */
 QMutexPool::~QMutexPool()
 {
-    QMutexLocker locker(&mutex);
     for (int index = 0; index < count; ++index) {
         delete mutexes[index];
         mutexes[index] = 0;
     }
-    delete [] mutexes;
-    mutexes = 0;
 }
 
 /*!
@@ -138,12 +134,9 @@ QMutex *QMutexPool::get(const void *address)
 
     if (!mutexes[index]) {
         // mutex not created, create one
-
-        QMutexLocker locker(&mutex);
-        // we need to check once again that the mutex hasn't been created, since
-        // 2 threads could be trying to create a mutex at the same index...
-        if (!mutexes[index])
-            mutexes[index] = new QMutex(recurs ? QMutex::Recursive : QMutex::NonRecursive);
+        QMutex *newMutex = new QMutex(recurs ? QMutex::Recursive : QMutex::NonRecursive);
+        if (!mutexes[index].testAndSetOrdered(0, newMutex))
+            delete newMutex;
     }
 
     return mutexes[index];
diff --git a/src/corelib/thread/qmutexpool_p.h b/src/corelib/thread/qmutexpool_p.h
index 65a3b54..4b91e5b 100644
--- a/src/corelib/thread/qmutexpool_p.h
+++ b/src/corelib/thread/qmutexpool_p.h
@@ -54,6 +54,7 @@
 //
 
 #include "QtCore/qmutex.h"
+#include "QtCore/qvarlengtharray.h"
 
 #ifndef QT_NO_THREAD
 
@@ -70,8 +71,7 @@ public:
     static QMutex *globalInstanceGet(const void *address);
 
 private:
-    QMutex mutex;
-    QMutex **mutexes;
+    QVarLengthArray<QAtomicPointer<QMutex>, 128> mutexes;
     int count;
     bool recurs;
 };