linux-kernel - Re: [PATCH 3/6] bdi: Use refcount_t for reference counting instead atomic

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Date:   Mon, 16 Jul 2018 15:57:16 -0700
From:   Andrew Morton <akpm@...ux-foundation.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@...nel.org>, linux-mm@...ck.org,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH 3/6] bdi: Use refcount_t for reference counting instead
 atomic_t

On Tue,  3 Jul 2018 22:01:38 +0200 Sebastian Andrzej Siewior <bigeasy@...utronix.de> 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.
> 
> ...
>
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -438,10 +438,10 @@ wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
>  	if (new_congested) {
>  		/* !found and storage for new one already allocated, insert */
>  		congested = new_congested;
> -		new_congested = NULL;
>  		rb_link_node(&congested->rb_node, parent, node);
>  		rb_insert_color(&congested->rb_node, &bdi->cgwb_congested_tree);
> -		goto found;
> +		spin_unlock_irqrestore(&cgwb_lock, flags);
> +		return congested;
>  	}
>  
>  	spin_unlock_irqrestore(&cgwb_lock, flags);
> @@ -451,13 +451,13 @@ wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
>  	if (!new_congested)
>  		return NULL;
>  
> -	atomic_set(&new_congested->refcnt, 0);
> +	refcount_set(&new_congested->refcnt, 1);
>  	new_congested->__bdi = bdi;
>  	new_congested->blkcg_id = blkcg_id;
>  	goto retry;
>  
>  found:
> -	atomic_inc(&congested->refcnt);
> +	refcount_inc(&congested->refcnt);
>  	spin_unlock_irqrestore(&cgwb_lock, flags);
>  	kfree(new_congested);
>  	return congested;
>
> ...
>

I'm not sure that the restructuring of wb_congested_get_create() was
desirable and it does make the patch harder to review.  But it looks
OK to me.