Less aggressive locking for bindings add/reomve.

All binding operations aquire special locks for destination and source.
This is done to make sure bindings cannot be added or removed while
being cleaned up.

This means that adding and removing bindings for the same records may
conflict.

To avoid conflict the locks aquired during add/remove are now read locks.

This should improve performance in case of concurrent bindings creation
for the same resource.

Author: hairyhum

src/rabbit_binding.erl

@@ -154,8 +154,8 @@ add(Binding, InnerFun, ActingUser) ->
       fun (Src, Dst, B) ->
               case rabbit_exchange:validate_binding(Src, B) of
                   ok ->
-                      lock_resource(Src),
-                      lock_resource(Dst),
+                      lock_resource(Src, read),
+                      lock_resource(Dst, read),
                       %% this argument is used to check queue exclusivity;
                       %% in general, we want to fail on that in preference to
                       %% anything else
@@ -174,8 +174,6 @@ add(Binding, InnerFun, ActingUser) ->
       end, fun not_found_or_absent_errs/1).
 
 add(Src, Dst, B, ActingUser) ->
-    lock_resource(Src),
-    lock_resource(Dst),
     [SrcDurable, DstDurable] = [durable(E) || E <- [Src, Dst]],
     ok = sync_route(#route{binding = B}, SrcDurable, DstDurable,
                     fun mnesia:write/3),
@@ -198,8 +196,8 @@ remove(Binding, InnerFun, ActingUser) ->
     binding_action(
       Binding,
       fun (Src, Dst, B) ->
-              lock_resource(Src),
-              lock_resource(Dst),
+              lock_resource(Src, read),
+              lock_resource(Dst, read),
               case mnesia:read(rabbit_route, B, write) of
                   [] -> case mnesia:read(rabbit_durable_route, B, write) of
                             [] -> rabbit_misc:const(ok);
@@ -216,8 +214,6 @@ remove(Binding, InnerFun, ActingUser) ->
       end, fun absent_errs_only/1).
 
 remove(Src, Dst, B, ActingUser) ->
-    lock_resource(Src),
-    lock_resource(Dst),
     ok = sync_route(#route{binding = B}, durable(Src), durable(Dst),
                     fun delete/3),
     Deletions = maybe_auto_delete(
@@ -536,9 +532,11 @@ remove_for_destination(DstName, OnlyDurable, Fun) ->
 
 %% Instead of locking entire table on remove operations we can lock the
 %% affected resource only.
-lock_resource(Name) ->
+lock_resource(Name) -> lock_resource(Name, write).
+
+lock_resource(Name, LockKind) ->
     mnesia:lock({global, Name, mnesia:table_info(rabbit_route, where_to_write)},
-                write).
+                LockKind).
 
 %% Requires that its input binding list is sorted in exchange-name
 %% order, so that the grouping of bindings (for passing to