QMutex: Make freelist() return a real global static

Since Qt 5.6.0, some applications such as Kate (built with clang, libc++
and libcxxrt) on FreeBSD occasionally crash with the following error
message on exit:

    QMutex::lock(): sem_wait failure: Invalid argument
    [or pthread_cond_wait in the 5.6 branch]

Investigation by Gleb Popov, Thiago Macieira and Olivier Goffart has
shown that this is caused by the fact that QDBusConnectionManager is a
Q_GLOBAL_STATIC (so it will be destroyed with all the other
Q_GLOBAL_STATICs in the reverse order of construction). In the
Q_COMPILER_THREADSAFE_STATICS case, freelist() also returns a
function-level static that is constructed on first use, so it may be
destroyed earlier than the QDBusConnectionManager object, making it
impossible to lock a contended mutex.

We now make freelist() return a global static, so that it is always
destroyed after QDBusConnectionManager and other function-static
variables.

Change-Id: I210fa7c18dbdf2345863da49141b9a85cffdef52
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

1 files changed, 2 insertions, 25 deletions

diff --git a/src/corelib/thread/qmutex.cpp b/src/corelib/thread/qmutex.cpp
index fa3bb080ae..e9709e34c5 100644
--- a/src/corelib/thread/qmutex.cpp
+++ b/src/corelib/thread/qmutex.cpp
@@ -571,34 +571,11 @@ const int FreeListConstants::Sizes[FreeListConstants::BlockCount] = {
 
 typedef QFreeList<QMutexPrivate, FreeListConstants> FreeList;
 // We cannot use Q_GLOBAL_STATIC because it uses QMutex
-#if defined(Q_COMPILER_THREADSAFE_STATICS)
+static FreeList freeList_;
 FreeList *freelist()
 {
-    static FreeList list;
-    return &list;
+    return &freeList_;
 }
-#else
-static QBasicAtomicPointer<FreeList> freeListPtr;
-
-FreeList *freelist()
-{
-    FreeList *local = freeListPtr.loadAcquire();
-    if (!local) {
-        local = new FreeList;
-        if (!freeListPtr.testAndSetRelease(0, local)) {
-            delete local;
-            local = freeListPtr.loadAcquire();
-        }
-    }
-    return local;
-}
-
-static void qFreeListDeleter()
-{
-    delete freeListPtr.load();
-}
-Q_DESTRUCTOR_FUNCTION(qFreeListDeleter)
-#endif
 }
 
 QMutexPrivate *QMutexPrivate::allocate()