safe_cached.patch

diff --git a/lib/experimental.rakumod b/lib/experimental.rakumod
index 4f6694e5b..c441ad4ec 100644
--- a/lib/experimental.rakumod
+++ b/lib/experimental.rakumod
@@ -3,11 +3,16 @@
 package EXPORT::cached {
     multi sub trait_mod:<is>(Routine $r, :$cached!) {
         my %cache;
+        my $lock = Lock.new;
         $r.wrap(-> |c {
             my $key := c.gist;
-            %cache.EXISTS-KEY($key)
-              ?? %cache{$key}
-              !! (%cache{$key} := callsame);
+            if %cache.EXISTS-KEY($key) {
+                %cache{$key}
+            }
+            else {
+                my &target = nextcallee;
+                $lock.protect: { %cache{$key} := target(|c) }
+            }
         });
     }