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  1. Qt
  2. QTBUG-84463

Q_PLUGIN_INSTANCE macro is not threadsafe

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Details

    • Bug
    • Resolution: Unresolved
    • P2: Important
    • None
    • 5.12.2, 5.15.0 RC2
    • Core: Plugins
    • None
    • All

    Description

      The Q_PLUGIN_INSTANCE is not thread safe - while the static block ensures that instance_ is set to null once, it is possible that multiple threads could perform the check that if (!_instance) and then create more than one actual object. (While this is unlikely in practice, our thread sanitizer picked this up)

      By throwing the whole initialization check into a lambda and calling it we don't have this problem

      --- a/src/corelib/plugin/qplugin.h
+++ b/src/corelib/plugin/qplugin.h
@@ -129,11 +129,10 @@
 
 #define Q_PLUGIN_INSTANCE(IMPLEMENTATION) \
         { \
-            static QT_PREPEND_NAMESPACE(QPointer)<QT_PREPEND_NAMESPACE(QObject)> _instance; \
-            if (!_instance) {    \
+            static QT_PREPEND_NAMESPACE(QPointer)<QT_PREPEND_NAMESPACE(QObject)> _instance = [] { \
                 QT_PLUGIN_RESOURCE_INIT \
-                _instance = new IMPLEMENTATION; \
-            } \
+                return new IMPLEMENTATION; \
+            }(); \
             return _instance; \
         }

       

      Attachments

        For Gerrit Dashboard: QTBUG-84463
        # Subject Branch Project Status CR V
        301944,1 QPluginLoader: Fix thread-safety in the instance-creation function dev qt/qtbase Status: ABANDONED +2 0

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            thiago Thiago Macieira
            cwgthornton Chris Thornton
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            Dates

              Created:
              Updated:

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