QLibrary: fix deadlock caused by fix to QTBUG-39642

Commit ae6f73e8566fa76470937aca737141183929a5ec inserted a mutex around
the entire load_sys(). We had reasoed that deadlocks would only occur if
the object creation in instance() recursed into its own instance(),
which was already a bug. But we had forgotten that dlopen()/
LoadLibrary() executes initialization code from the module being loaded,
which could cause a recursion back into the same QPluginLoader or
QLibrary object. This recursion is benign because the module *is* loaded
and dlopen()/LoadLibrary() returns the same handle.

[ChangeLog][QtCore][QLibrary and QPluginLoader] Fixed a deadlock that
would happen if the plugin or library being loaded has load-time
initialization code (C++ global variables) that recursed back into the
same QLibrary or QPluginLoader object.

PS: QLibraryPrivate::loadPlugin() updates pluginState outside a mutex
lock, so pluginState should be made an atomic variable. Once that is
done, we'll only need locking the mutex to update errorString (no
locking before loading).

Fixes: QTBUG-83207
Task-number: QTBUG-39642
Change-Id: Ibdc95e9af7bd456a94ecfffd160209304e5ab2eb
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: David Faure <david.faure@kdab.com>

1 files changed, 0 insertions, 2 deletions

diff --git a/src/corelib/plugin/qlibrary.cpp b/src/corelib/plugin/qlibrary.cpp
index ddb053c26f..be9d92b204 100644
--- a/src/corelib/plugin/qlibrary.cpp
+++ b/src/corelib/plugin/qlibrary.cpp
@@ -576,9 +576,7 @@ bool QLibraryPrivate::load()
 
     Q_TRACE(QLibraryPrivate_load_entry, fileName);
 
-    mutex.lock();
     bool ret = load_sys();
-    mutex.unlock();
     if (qt_debug_component()) {
         if (ret) {
             qDebug() << "loaded library" << fileName;