>  /**
> - * for_each_subsys - iterate all loaded cgroup subsystems
> + * for_each_subsys - iterate all enabled cgroup subsystems
>   * @ss: the iteration cursor
>   * @ssid: the index of @ss, CGROUP_SUBSYS_COUNT after reaching the end
> - *
> - * Iterates through all loaded subsystems.  Should be called under
> - * cgroup_mutex or cgroup_root_mutex.
>   */
>  #define for_each_subsys(ss, ssid)                                    \
> -     for (({ cgroup_assert_mutex_or_root_locked(); (ssid) = 0; });   \
> -          (ssid) < CGROUP_SUBSYS_COUNT; (ssid)++)                    \
> -             if (!((ss) = cgroup_subsys[(ssid)])) { }                \
> -             else
> -
> -/**
> - * for_each_builtin_subsys - iterate all built-in cgroup subsystems
> - * @ss: the iteration cursor
> - * @i: the index of @ss, CGROUP_BUILTIN_SUBSYS_COUNT after reaching the end
> - *
> - * Bulit-in subsystems are always present and iteration itself doesn't
> - * require any synchronization.
> - */
> -#define for_each_builtin_subsys(ss, i)                                       
> \
> -     for ((i) = 0; (i) < CGROUP_BUILTIN_SUBSYS_COUNT &&              \
> -          (((ss) = cgroup_subsys[i]) || true); (i)++)
> +     for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT &&                \
> +          (((ss) = cgroup_subsys[ssid]) || true); (ssid)++)

Now cgroup_subsys[i] won't be NULL, so we can drop "|| true".

>  
>  /* iterate across the active hierarchies */
>  #define for_each_active_root(root)                                   \
> @@ -975,50 +951,24 @@ static void cgroup_d_remove_dir(struct dentry *dentry)
>       remove_dir(dentry);
>  }
>  

...

> -     if (need_forkexit_callback) {
> -             /*
> -              * fork/exit callbacks are supported only for builtin
> -              * subsystems, and the builtin section of the subsys
> -              * array is immutable, so we don't need to lock the
> -              * subsys array here. On the other hand, modular section
> -              * of the array can be freed at module unload, so we
> -              * can't touch that.
> -              */
> -             for_each_builtin_subsys(ss, i)
> +     if (need_forkexit_callback)
> +             for_each_subsys(ss, i)
>                       if (ss->fork)
>                               ss->fork(child);
> -     }

This looks a bit ugly. How about leaving the parentheses for the
outmost if statement?

if (need_forkexit_callback) {
        for_each_subsys(ss, i)
                if (ss->fork)
                        ss->fork(child);
}


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