block: Avoid processing BDS twice in bdrv_set_aio_context_ignore()

Some graphs may contain an indirect reference to the first BDS in the
chain that can be reached while walking it bottom->up from one its
children.

Doubling-processing of a BDS is especially problematic for the
aio_notifiers, as they might attempt to work on both the old and the
new AIO contexts.

To avoid this problem, add every child and parent to the ignore list
before actually processing them.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Sergio Lopez <slp@redhat.com>
Message-Id: <20210201125032.44713-2-slp@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 27 insertions, 7 deletions

diff --git a/block.c b/block.c
index 91a66d4f3e..5c428e1595 100644
--- a/block.c
+++ b/block.c
@@ -6439,7 +6439,10 @@ void bdrv_set_aio_context_ignore(BlockDriverState *bs,
                                  AioContext *new_context, GSList **ignore)
 {
     AioContext *old_context = bdrv_get_aio_context(bs);
-    BdrvChild *child;
+    GSList *children_to_process = NULL;
+    GSList *parents_to_process = NULL;
+    GSList *entry;
+    BdrvChild *child, *parent;
 
     g_assert(qemu_get_current_aio_context() == qemu_get_aio_context());
 
@@ -6454,16 +6457,33 @@ void bdrv_set_aio_context_ignore(BlockDriverState *bs,
             continue;
         }
         *ignore = g_slist_prepend(*ignore, child);
-        bdrv_set_aio_context_ignore(child->bs, new_context, ignore);
+        children_to_process = g_slist_prepend(children_to_process, child);
     }
-    QLIST_FOREACH(child, &bs->parents, next_parent) {
-        if (g_slist_find(*ignore, child)) {
+
+    QLIST_FOREACH(parent, &bs->parents, next_parent) {
+        if (g_slist_find(*ignore, parent)) {
             continue;
         }
-        assert(child->klass->set_aio_ctx);
-        *ignore = g_slist_prepend(*ignore, child);
-        child->klass->set_aio_ctx(child, new_context, ignore);
+        *ignore = g_slist_prepend(*ignore, parent);
+        parents_to_process = g_slist_prepend(parents_to_process, parent);
+    }
+
+    for (entry = children_to_process;
+         entry != NULL;
+         entry = g_slist_next(entry)) {
+        child = entry->data;
+        bdrv_set_aio_context_ignore(child->bs, new_context, ignore);
+    }
+    g_slist_free(children_to_process);
+
+    for (entry = parents_to_process;
+         entry != NULL;
+         entry = g_slist_next(entry)) {
+        parent = entry->data;
+        assert(parent->klass->set_aio_ctx);
+        parent->klass->set_aio_ctx(parent, new_context, ignore);
     }
+    g_slist_free(parents_to_process);
 
     bdrv_detach_aio_context(bs);