Re: [Qemu-devel] [PATCH qemu v5 04/18] memory: Move AddressSpaceDispatch from AddressSpace to FlatView

[Alexey Kardashevskiy]

On 21/09/17 21:51, Paolo Bonzini wrote:
> On 21/09/2017 10:50, Alexey Kardashevskiy wrote:
>> * since FlatView::rcu is used now to dispose FV, call_rcu() in
>> address_space_update_topology() is replaced with direct call to
>> flatview_unref()
> 
> Hmm, this is not correct, as you could have
> 
> 
>    thread 1             thread 2             RCU thread
>   -------------------------------------------------------------
>    rcu_read_lock
>    read as->current_map
>                         set as->current_map
>                         flatview_unref
>                            '--> call_rcu
>    flatview_ref
>    rcu_read_unlock
>                                              flatview_destroy
> 
> I need to think a bit more about this (and possibly ask Paul...).
> 
> Paolo
> 

Nah, you're right, it should be like this:


diff --git a/memory.c b/memory.c
index 35b2fc5f7f..689bf53866 100644
--- a/memory.c
+++ b/memory.c
@@ -317,7 +317,7 @@ static void flatview_ref(FlatView *view)
 static void flatview_unref(FlatView *view)
 {
     if (atomic_fetch_dec(&view->ref) == 1) {
-        call_rcu(view, flatview_destroy, rcu);
+        flatview_destroy(view);
     }
 }

@@ -768,7 +768,7 @@ static FlatView *generate_memory_topology(MemoryRegion *mr)
         flatview_simplify(view);

         if (!view->nr) {
-            flatview_destroy(view);
+            flatview_unref(view);
             use_empty = true;
         }
     }
@@ -1026,7 +1026,7 @@ static void address_space_set_flatview(AddressSpace *as)
     /* Writes are protected by the BQL.  */
     atomic_rcu_set(&as->current_map, new_view);
     if (old_view) {
-        flatview_unref(old_view);
+        call_rcu(view, flatview_unref, rcu);
     }



-- 
Alexey