Re: [PATCH 3/8] md: raid5: use refcount_t for reference counting instead atomic_t

[Matthew Wilcox]

On Wed, May 09, 2018 at 09:36:40PM +0200, Sebastian Andrzej Siewior wrote:
> refcount_t type and corresponding API should be used instead of atomic_t when
> the variable is used as a reference counter. This allows to avoid accidental
> refcounter overflows that might lead to use-after-free situations.
> 
> Most changes are 1:1 replacements except for
>       BUG_ON(atomic_inc_return(&sh->count) != 1);
> 
> which has been turned into
>         refcount_inc(&sh->count);
>         BUG_ON(refcount_read(&sh->count) != 1);

@@ -5387,7 +5387,8 @@ static struct stripe_head *__get_priority_stripe(struct
+r5conf *conf, int group)
                sh->group = NULL;
        }
        list_del_init(&sh->lru);
-       BUG_ON(atomic_inc_return(&sh->count) != 1);
+       refcount_inc(&sh->count);
+       BUG_ON(refcount_read(&sh->count) != 1);
        return sh;
 }


That's the only problematic usage.  And I think what it's really saying is:

        BUG_ON(refcount_read(&sh->count) != 0);
        refcount_set(&sh->count, 1);

With that, this looks like a reasonable use of refcount_t to me.