[v6,2/2] memory: Do not create circular reference with subregion

Commit Message

Akihiko Odaki Jan. 5, 2025, 8:56 a.m. UTC

memory_region_update_container_subregions() used to call
memory_region_ref(), which creates a reference to the owner of the
subregion, on behalf of the owner of the container. This results in a
circular reference if the subregion and container have the same owner.

memory_region_ref() creates a reference to the owner instead of the
memory region to match the lifetime of the owner and memory region. We
do not need such a hack if the subregion and container have the same
owner because the owner will be alive as long as the container is.
Therefore, create a reference to the subregion itself instead ot its
owner in such a case; the reference to the subregion is still necessary
to ensure that the subregion gets finalized after the container.

Signed-off-by: Akihiko Odaki <[e-mail]>
---
 system/memory.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

Patch

diff --git a/system/memory.c b/system/memory.c
index 78e17e0efa83..16b051249c5f 100644
--- a/system/memory.c
+++ b/system/memory.c
@@ -2644,7 +2644,9 @@  static void memory_region_update_container_subregions(MemoryRegion *subregion)
 
     memory_region_transaction_begin();
 
-    memory_region_ref(subregion);
+    object_ref(mr->owner == subregion->owner ?
+               OBJECT(subregion) : subregion->owner);
+
     QTAILQ_FOREACH(other, &mr->subregions, subregions_link) {
         if (subregion->priority >= other->priority) {
             QTAILQ_INSERT_BEFORE(other, subregion, subregions_link);
@@ -2702,7 +2704,9 @@  void memory_region_del_subregion(MemoryRegion *mr,
         assert(alias->mapped_via_alias >= 0);
     }
     QTAILQ_REMOVE(&mr->subregions, subregion, subregions_link);
-    memory_region_unref(subregion);
+    object_unref(mr->owner == subregion->owner ?
+                 OBJECT(subregion) : subregion->owner);
+
     memory_region_update_pending |= mr->enabled && subregion->enabled;
     memory_region_transaction_commit();
 }