Ensure that we never increase the strong reference count up from zero.

Also add some thread stress tests to try and detect doing it wrong.

Reviewed-By: Bradley T. Hughes

1 files changed, 14 insertions, 4 deletions

diff --git a/src/corelib/tools/qsharedpointer_impl.h b/src/corelib/tools/qsharedpointer_impl.h
index b5daf5d..9fa8df4 100644
--- a/src/corelib/tools/qsharedpointer_impl.h
+++ b/src/corelib/tools/qsharedpointer_impl.h
@@ -364,12 +364,22 @@ namespace QtSharedPointer {
         inline void internalSet(Data *o, T *actual)
         {
             if (d == o) return;
-            if (o && !o->strongref)
-                o = 0;
             if (o) {
                 verifyReconstruction(actual);
-                o->weakref.ref();
-                o->strongref.ref();
+
+                // increase the strongref, but never up from zero
+                register int tmp = o->strongref;
+                while (tmp > 0) {
+                    // try to increment from "tmp" to "tmp + 1"
+                    if (o->strongref.testAndSetRelaxed(tmp, tmp + 1))
+                        break;   // succeeded
+                    tmp = o->strongref;  // failed, try again
+                }
+
+                if (tmp)
+                    o->weakref.ref();
+                else
+                    o = 0;
             }
             if (d && !deref())
                 delete d;