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

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