Date:   Mon, 22 Apr 2019 20:34:45 +0000
From:   Saeed Mahameed <saeedm@...lanox.com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Vlad Buslov <vladbu@...lanox.com>
CC:     "jhs@...atatu.com" <jhs@...atatu.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>,
        "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>,
        "jiri@...nulli.us" <jiri@...nulli.us>
Subject: Re: [PATCH net-next] net: sched: flower: refactor reoffload for
 concurrent access

On Mon, 2019-04-22 at 10:21 +0300, Vlad Buslov wrote:
> Recent changes that introduced unlocked flower did not properly
> account for
> case when reoffload is initiated concurrently with filter updates. To
> fix
> the issue, extend flower with 'hw_filters' list that is used to store
> filters that don't have 'skip_hw' flag set. Filter is added to the
> list
> when it is inserted to hardware and only removed from it after being
> unoffloaded from all drivers that parent block is attached to. This
> ensures
> that concurrent reoffload can still access filter that is being
> deleted and
> prevents race condition when driver callback can be removed when
> filter is
> no longer accessible trough idr, but is still present in hardware.
> 
> Refactor fl_change() to respect new filter reference counter and to
> release
> filter reference with __fl_put() in case of error, instead of
> directly
> deallocating filter memory. This allows for concurrent access to
> filter
> from fl_reoffload() and protects it with reference counting. Refactor
> fl_reoffload() to iterate over hw_filters list instead of idr.
> Implement
> fl_get_next_hw_filter() helper function that is used to iterate over
> hw_filters list with reference counting and skips filters that are
> being
> concurrently deleted.
> 
> Fixes: 92149190067d ("net: sched: flower: set unlocked flag for
> flower proto ops")
> Signed-off-by: Vlad Buslov <vladbu@...lanox.com>
> Reported-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> ---
>  net/sched/cls_flower.c | 77 +++++++++++++++++++++++++++++++---------
> --
>  1 file changed, 57 insertions(+), 20 deletions(-)
> 
> diff --git a/net/sched/cls_flower.c b/net/sched/cls_flower.c
> index 4b5585358699..524b86560af3 100644
> --- a/net/sched/cls_flower.c
> +++ b/net/sched/cls_flower.c
> @@ -90,6 +90,7 @@ struct cls_fl_head {
>  	struct rhashtable ht;
>  	spinlock_t masks_lock; /* Protect masks list */
>  	struct list_head masks;
> +	struct list_head hw_filters;
>  	struct rcu_work rwork;
>  	struct idr handle_idr;
>  };
> @@ -102,6 +103,7 @@ struct cls_fl_filter {
>  	struct tcf_result res;
>  	struct fl_flow_key key;
>  	struct list_head list;
> +	struct list_head hw_list;
>  	u32 handle;
>  	u32 flags;
>  	u32 in_hw_count;
> @@ -315,6 +317,7 @@ static int fl_init(struct tcf_proto *tp)
>  
>  	spin_lock_init(&head->masks_lock);
>  	INIT_LIST_HEAD_RCU(&head->masks);
> +	INIT_LIST_HEAD(&head->hw_filters);
>  	rcu_assign_pointer(tp->root, head);
>  	idr_init(&head->handle_idr);
>  
> @@ -352,6 +355,16 @@ static bool fl_mask_put(struct cls_fl_head
> *head, struct fl_flow_mask *mask)
>  	return true;
>  }
>  
> +static struct cls_fl_head *fl_head_dereference(struct tcf_proto *tp)
> +{
> +	/* Flower classifier only changes root pointer during init and
> destroy.
> +	 * Users must obtain reference to tcf_proto instance before
> calling its
> +	 * API, so tp->root pointer is protected from concurrent call
> to
> +	 * fl_destroy() by reference counting.
> +	 */
> +	return rcu_dereference_raw(tp->root);
> +}
> +
>  static void __fl_destroy_filter(struct cls_fl_filter *f)
>  {
>  	tcf_exts_destroy(&f->exts);
> @@ -382,6 +395,8 @@ static void fl_hw_destroy_filter(struct tcf_proto
> *tp, struct cls_fl_filter *f,
>  
>  	tc_setup_cb_call(block, TC_SETUP_CLSFLOWER, &cls_flower,
> false);
>  	spin_lock(&tp->lock);
> +	if (!list_empty(&f->hw_list))
> +		list_del_init(&f->hw_list);
>  	tcf_block_offload_dec(block, &f->flags);
>  	spin_unlock(&tp->lock);
>  
> @@ -393,6 +408,7 @@ static int fl_hw_replace_filter(struct tcf_proto
> *tp,
>  				struct cls_fl_filter *f, bool
> rtnl_held,
>  				struct netlink_ext_ack *extack)
>  {
> +	struct cls_fl_head *head = fl_head_dereference(tp);
>  	struct tc_cls_flower_offload cls_flower = {};
>  	struct tcf_block *block = tp->chain->block;
>  	bool skip_sw = tc_skip_sw(f->flags);
> @@ -444,6 +460,9 @@ static int fl_hw_replace_filter(struct tcf_proto
> *tp,
>  		goto errout;
>  	}
>  
> +	spin_lock(&tp->lock);
> +	list_add(&f->hw_list, &head->hw_filters);
> +	spin_unlock(&tp->lock);
>  errout:
>  	if (!rtnl_held)
>  		rtnl_unlock();
> @@ -475,23 +494,11 @@ static void fl_hw_update_stats(struct tcf_proto
> *tp, struct cls_fl_filter *f,
>  		rtnl_unlock();
>  }
>  
> -static struct cls_fl_head *fl_head_dereference(struct tcf_proto *tp)
> -{
> -	/* Flower classifier only changes root pointer during init and
> destroy.
> -	 * Users must obtain reference to tcf_proto instance before
> calling its
> -	 * API, so tp->root pointer is protected from concurrent call
> to
> -	 * fl_destroy() by reference counting.
> -	 */
> -	return rcu_dereference_raw(tp->root);
> -}
> -
>  static void __fl_put(struct cls_fl_filter *f)
>  {
>  	if (!refcount_dec_and_test(&f->refcnt))
>  		return;
>  
> -	WARN_ON(!f->deleted);
> -
>  	if (tcf_exts_get_net(&f->exts))
>  		tcf_queue_work(&f->rwork, fl_destroy_filter_work);
>  	else
> @@ -1522,6 +1529,7 @@ static int fl_change(struct net *net, struct
> sk_buff *in_skb,
>  		err = -ENOBUFS;
>  		goto errout_tb;
>  	}
> +	INIT_LIST_HEAD(&fnew->hw_list);
>  	refcount_set(&fnew->refcnt, 1);
>  
>  	err = tcf_exts_init(&fnew->exts, net, TCA_FLOWER_ACT, 0);
> @@ -1569,7 +1577,6 @@ static int fl_change(struct net *net, struct
> sk_buff *in_skb,
>  		goto errout_hw;
>  	}
>  
> -	refcount_inc(&fnew->refcnt);
>  	if (fold) {
>  		/* Fold filter was deleted concurrently. Retry lookup.
> */
>  		if (fold->deleted) {
> @@ -1591,6 +1598,7 @@ static int fl_change(struct net *net, struct
> sk_buff *in_skb,
>  			in_ht = true;
>  		}
>  
> +		refcount_inc(&fnew->refcnt);
>  		rhashtable_remove_fast(&fold->mask->ht,
>  				       &fold->ht_node,
>  				       fold->mask->filter_ht_params);
> @@ -1631,6 +1639,7 @@ static int fl_change(struct net *net, struct
> sk_buff *in_skb,
>  		if (err)
>  			goto errout_hw;
>  
> +		refcount_inc(&fnew->refcnt);
>  		fnew->handle = handle;
>  		list_add_tail_rcu(&fnew->list, &fnew->mask->filters);
>  		spin_unlock(&tp->lock);
> @@ -1642,19 +1651,20 @@ static int fl_change(struct net *net, struct
> sk_buff *in_skb,
>  	kfree(mask);
>  	return 0;
>  
> +errout_ht:
> +	spin_lock(&tp->lock);
>  errout_hw:
> +	fnew->deleted = true;
>  	spin_unlock(&tp->lock);
>  	if (!tc_skip_hw(fnew->flags))
>  		fl_hw_destroy_filter(tp, fnew, rtnl_held, NULL);
> -errout_ht:
>  	if (in_ht)
>  		rhashtable_remove_fast(&fnew->mask->ht, &fnew->ht_node,
>  				       fnew->mask->filter_ht_params);
>  errout_mask:
>  	fl_mask_put(head, fnew->mask);
>  errout:
> -	tcf_exts_get_net(&fnew->exts);
> -	tcf_queue_work(&fnew->rwork, fl_destroy_filter_work);
> +	__fl_put(fnew);
>  errout_tb:
>  	kfree(tb);
>  errout_mask_alloc:
> @@ -1699,16 +1709,44 @@ static void fl_walk(struct tcf_proto *tp,
> struct tcf_walker *arg,
>  	}
>  }
>  
> +static struct cls_fl_filter *
> +fl_get_next_hw_filter(struct tcf_proto *tp, struct cls_fl_filter *f,
> bool add)
> +{
> +	struct cls_fl_head *head = fl_head_dereference(tp);
> +
> +	spin_lock(&tp->lock);
> +	if (!f) {
> +		if (list_empty(&head->hw_filters)) {
> +			spin_unlock(&tp->lock);
> +			return NULL;
> +		}

Shouldn't this be a pre-condition to the whole function ? i mean
regardless of whether 'f' is NULL or not ?

> +
> +		f = list_first_entry(&head->hw_filters, struct
> cls_fl_filter,
> +				     hw_list);
> +	} else {
> +		f = list_next_entry(f, hw_list);
> +	}
> +

Maybe if you use  list_for_each_entry_continue below, might simplify
the above logic. it is weird that you need to figure out next entry
then call list_for_each_from, list 'continue' variation is made for
such use cases.

> +	list_for_each_entry_from(f, &head->hw_filters, hw_list) {
> +		if (!(add && f->deleted) && refcount_inc_not_zero(&f-
> >refcnt)) {
> +			spin_unlock(&tp->lock);
> +			return f;
> +		}
> +	}
> +
> +	spin_unlock(&tp->lock);
> +	return NULL;
> +}
> +
>  static int fl_reoffload(struct tcf_proto *tp, bool add,
> tc_setup_cb_t *cb,
>  			void *cb_priv, struct netlink_ext_ack *extack)
>  {
>  	struct tc_cls_flower_offload cls_flower = {};
>  	struct tcf_block *block = tp->chain->block;
> -	unsigned long handle = 0;
> -	struct cls_fl_filter *f;
> +	struct cls_fl_filter *f = NULL;
>  	int err;
>  
> -	while ((f = fl_get_next_filter(tp, &handle))) {
> +	while ((f = fl_get_next_hw_filter(tp, f, add))) {
>  		if (tc_skip_hw(f->flags))
>  			goto next_flow;

this can never be true as it is already a pre-condition for
fl_hw_replace_filter which actually adds  to the hw_filters list, i
think it needs to be removed. if it is, then i think it should be part
of fl_get_next_hw_filter and not the caller responsibility.

>  
> @@ -1757,7 +1795,6 @@ static int fl_reoffload(struct tcf_proto *tp,
> bool add, tc_setup_cb_t *cb,
>  					  add);
>  		spin_unlock(&tp->lock);
>  next_flow:
> -		handle++;
>  		__fl_put(f);
>  	}
>  

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