Two threading issues:

- Drawing masks in image editor requires LOCK_DRAW_IMAGE around
  ED_space_image_get* functions since they'll acquire image buffer.

  Lock is needed because viewers will be modified directly in
  compositor (see commend in draw_image_main)

- Seems that was wrong order of invalidating render result and
  viewer image invalidation happened in Composite node, which
  could easily lead to thread lock.

2 files changed, 21 insertions, 5 deletions

diff --git a/source/blender/compositor/operations/COM_CompositorOperation.cpp b/source/blender/compositor/operations/COM_CompositorOperation.cpp
index 141d071dddc..e6a49082a71 100644
--- a/source/blender/compositor/operations/COM_CompositorOperation.cpp
+++ b/source/blender/compositor/operations/COM_CompositorOperation.cpp
@@ -91,14 +91,14 @@ void CompositorOperation::deinitExecution()
 			}
 		}
 
-		BLI_lock_thread(LOCK_DRAW_IMAGE);
-		BKE_image_signal(BKE_image_verify_viewer(IMA_TYPE_R_RESULT, "Render Result"), NULL, IMA_SIGNAL_FREE);
-		BLI_unlock_thread(LOCK_DRAW_IMAGE);
-
 		if (re) {
 			RE_ReleaseResult(re);
 			re = NULL;
 		}
+
+		BLI_lock_thread(LOCK_DRAW_IMAGE);
+		BKE_image_signal(BKE_image_verify_viewer(IMA_TYPE_R_RESULT, "Render Result"), NULL, IMA_SIGNAL_FREE);
+		BLI_unlock_thread(LOCK_DRAW_IMAGE);
 	}
 	else {
 		if (this->m_outputBuffer) {
diff --git a/source/blender/editors/space_image/space_image.c b/source/blender/editors/space_image/space_image.c
index 9492e29734d..39d256321bf 100644
--- a/source/blender/editors/space_image/space_image.c
+++ b/source/blender/editors/space_image/space_image.c
@@ -38,6 +38,7 @@
 
 #include "BLI_blenlib.h"
 #include "BLI_math.h"
+#include "BLI_threads.h"
 
 #include "BKE_colortools.h"
 #include "BKE_context.h"
@@ -692,10 +693,25 @@ static void image_main_area_draw(const bContext *C, ARegion *ar)
 	}
 
 	if (mask) {
-		int width, height;
+		Image *image = ED_space_image(sima);
+		int width, height, show_viewer;
 		float aspx, aspy;
+
+		show_viewer = (image && image->source == IMA_SRC_VIEWER);
+
+		if (show_viewer) {
+			/* ED_space_image_get will acquire image buffer which requires
+			 * lock here by the same reason why lock is needed in draw_image_main
+			 */
+			BLI_lock_thread(LOCK_DRAW_IMAGE);
+		}
+
 		ED_space_image_get_size(sima, &width, &height);
 		ED_space_image_get_aspect(sima, &aspx, &aspy);
+
+		if (show_viewer)
+			BLI_unlock_thread(LOCK_DRAW_IMAGE);
+
 		ED_mask_draw_region(mask, ar,
 		                    sima->mask_info.draw_flag, sima->mask_info.draw_type,
 		                    width, height,