linux-kernel - Re: [PATCH 3/8] md: raid5: use refcount_t for reference counting instead atomic

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Date:   Wed, 23 May 2018 06:21:19 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-mm@...ck.org,
        Shaohua Li <shli@...nel.org>, linux-raid@...r.kernel.org,
        Anna-Maria Gleixner <anna-maria@...utronix.de>
Subject: Re: [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.