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

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Date:   Mon, 16 Jul 2018 14:30:38 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     tglx@...utronix.de, Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 3/6] bdi: Use refcount_t for reference counting instead
 atomic_t

On 2018-07-03 22:01:38 [+0200], To linux-kernel@...r.kernel.org 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.

Andrew, is it okay for you to collect this one (and 4/6 of this series,
both bdi)? The prerequisites are already in Linus' tree.

> Cc: linux-mm@...ck.org
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  include/linux/backing-dev-defs.h |  3 ++-
>  include/linux/backing-dev.h      |  4 ++--
>  mm/backing-dev.c                 | 12 ++++++------
>  3 files changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
> index 24251762c20c..9a6bc0951cfa 100644
> --- a/include/linux/backing-dev-defs.h
> +++ b/include/linux/backing-dev-defs.h
> @@ -12,6 +12,7 @@
>  #include <linux/timer.h>
>  #include <linux/workqueue.h>
>  #include <linux/kref.h>
> +#include <linux/refcount.h>
>  
>  struct page;
>  struct device;
> @@ -75,7 +76,7 @@ enum wb_reason {
>   */
>  struct bdi_writeback_congested {
>  	unsigned long state;		/* WB_[a]sync_congested flags */
> -	atomic_t refcnt;		/* nr of attached wb's and blkg */
> +	refcount_t refcnt;		/* nr of attached wb's and blkg */
>  
>  #ifdef CONFIG_CGROUP_WRITEBACK
>  	struct backing_dev_info *__bdi;	/* the associated bdi, set to NULL
> diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> index 72ca0f3d39f3..c28a47cbe355 100644
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -404,13 +404,13 @@ static inline bool inode_cgwb_enabled(struct inode *inode)
>  static inline struct bdi_writeback_congested *
>  wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
>  {
> -	atomic_inc(&bdi->wb_congested->refcnt);
> +	refcount_inc(&bdi->wb_congested->refcnt);
>  	return bdi->wb_congested;
>  }
>  
>  static inline void wb_congested_put(struct bdi_writeback_congested *congested)
>  {
> -	if (atomic_dec_and_test(&congested->refcnt))
> +	if (refcount_dec_and_test(&congested->refcnt))
>  		kfree(congested);
>  }
>  
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 2e5d3df0853d..55a233d75f39 100644
> --- 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;
> @@ -474,7 +474,7 @@ void wb_congested_put(struct bdi_writeback_congested *congested)
>  	unsigned long flags;
>  
>  	local_irq_save(flags);
> -	if (!atomic_dec_and_lock(&congested->refcnt, &cgwb_lock)) {
> +	if (!refcount_dec_and_lock(&congested->refcnt, &cgwb_lock)) {
>  		local_irq_restore(flags);
>  		return;
>  	}
> @@ -804,7 +804,7 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi)
>  	if (!bdi->wb_congested)
>  		return -ENOMEM;
>  
> -	atomic_set(&bdi->wb_congested->refcnt, 1);
> +	refcount_set(&bdi->wb_congested->refcnt, 1);
>  
>  	err = wb_init(&bdi->wb, bdi, 1, GFP_KERNEL);
>  	if (err) {
> -- 
> 2.18.0

Sebastian