fix bug in @atomic order

Thanks vtjnash!
#45 (comment)

Author: kalmarek

src/Perms/perm_images.jl

@@ -47,8 +47,8 @@ function Base.copy(p::Perm)
     if isdefined(p, :inv, :sequentially_consistent)
         inv_imgs = copy(@atomic(p.inv).images)
         q⁻¹ = typeof(p)(inv_imgs; check = false)
-        @atomic q.inv = q⁻¹
         @atomic q⁻¹.inv = q
+        @atomic q.inv = q⁻¹
     end
     return q
 end
@@ -59,11 +59,11 @@ function Base.inv(σ::Perm)
             @atomic σ.inv = σ
         else
             σ⁻¹ = typeof(σ)(invperm(σ.images); check = false)
-            # we don't want to end up with two copies of inverse σ floating around
-            if !isdefined(σ, :inv, :sequentially_consistent)
-                @atomic σ.inv = σ⁻¹
-                @atomic σ⁻¹.inv = σ
-            end
+            # this order is important:
+            # fuly initialize the "local" inverse first and only then
+            # update σ to make the local inverse visible globally
+            @atomic σ⁻¹.inv = σ
+            @atomic σ.inv = σ⁻¹
         end
     end
     return σ.inv