Fix rendering deadlock for software compositing

With software compositing render thread of Qt's scene graph and Viz's
thread may hit the same mutex inside output surface during frame update.

Fixes: QTBUG-97598
Change-Id: Ic02e161f7b16b22c9099b93f2b1a91e78347975f
Reviewed-by: Michal Klocek <michal.klocek@qt.io>
(cherry picked from commit 0c0d78a6abf68418ae81c202c5576206bd4e5f47)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/src/core/compositor/display_software_output_surface.cpp b/src/core/compositor/display_software_output_surface.cpp
index 98c104ddf..b811449c9 100644
--- a/src/core/compositor/display_software_output_surface.cpp
+++ b/src/core/compositor/display_software_output_surface.cpp
@@ -99,9 +99,12 @@ void DisplaySoftwareOutputSurface::Device::Resize(const gfx::Size &sizeInPixels,
 
 void DisplaySoftwareOutputSurface::Device::OnSwapBuffers(SwapBuffersCallback swap_ack_callback)
 {
-    QMutexLocker locker(&m_mutex);
-    m_taskRunner = base::ThreadTaskRunnerHandle::Get();
-    m_swapCompletionCallback = std::move(swap_ack_callback);
+    { // MEMO don't hold a lock together with an 'observer', as the call from Qt's scene graph may come at the same time
+        QMutexLocker locker(&m_mutex);
+        m_taskRunner = base::ThreadTaskRunnerHandle::Get();
+        m_swapCompletionCallback = std::move(swap_ack_callback);
+    }
+
     if (auto obs = observer())
         obs->readyToSwap();
 }