Subject: Re: [PATCH v2 6/8] mac80211: Consolidate {mesh,mpp}_path_flush
 into one function
From: Johannes Berg <johannes@sipsolutions.net>
To: Javier Cardona <javier@cozybit.com>
Cc: "John W. Linville" <linville@tuxdriver.com>,
	Thomas Pedersen <thomas@cozybit.com>,
	devel@lists.open80211s.org, linux-wireless@vger.kernel.org,
	jlopex@gmail.com
In-Reply-To: <1314404294-4233-7-git-send-email-javier@cozybit.com> (sfid-20110827_021912_257192_62310EC9)
References: <1314236452-7226-1-git-send-email-thomas@cozybit.com>
	 <1314404294-4233-1-git-send-email-javier@cozybit.com>
	 <1314404294-4233-7-git-send-email-javier@cozybit.com>
	 (sfid-20110827_021912_257192_62310EC9)
Content-Type: text/plain; charset="UTF-8"
Date: Mon, 29 Aug 2011 15:49:05 +0200
Message-ID: <1314625745.8959.9.camel@jlt3.sipsolutions.net> (sfid-20110829_154914_919635_5B6C3A1F)
Mime-Version: 1.0
Sender: linux-wireless-owner@vger.kernel.org

On Fri, 2011-08-26 at 17:18 -0700, Javier Cardona wrote:
> Signed-off-by: Javier Cardona <javier@cozybit.com>
> 
> ---
> v2: - Fix extra space (checkpatch)
>     - Add lockdep check for RCU
>  net/mac80211/mesh_pathtbl.c |   65 +++++++++++++++++-------------------------
>  1 files changed, 26 insertions(+), 39 deletions(-)
> 
> diff --git a/net/mac80211/mesh_pathtbl.c b/net/mac80211/mesh_pathtbl.c
> index ea9e34a..3c03be9 100644
> --- a/net/mac80211/mesh_pathtbl.c
> +++ b/net/mac80211/mesh_pathtbl.c
> @@ -790,35 +790,6 @@ void mesh_plink_broken(struct sta_info *sta)
>  	rcu_read_unlock();
>  }
>  
> -/**
> - * mesh_path_flush_by_nexthop - Deletes mesh paths if their next hop matches
> - *
> - * @sta - mesh peer to match
> - *
> - * RCU notes: this function is called when a mesh plink transitions from
> - * PLINK_ESTAB to any other state, since PLINK_ESTAB state is the only one that
> - * allows path creation. This will happen before the sta can be freed (because
> - * sta_info_destroy() calls this) so any reader in a rcu read block will be
> - * protected against the plink disappearing.
> - */
> -void mesh_path_flush_by_nexthop(struct sta_info *sta)
> -{
> -	struct mesh_table *tbl;
> -	struct mesh_path *mpath;
> -	struct mpath_node *node;
> -	struct hlist_node *p;
> -	int i;
> -
> -	rcu_read_lock();
> -	tbl = rcu_dereference(mesh_paths);
> -	for_each_mesh_entry(tbl, p, node, i) {
> -		mpath = node->mpath;
> -		if (rcu_dereference(mpath->next_hop) == sta)
> -			mesh_path_del(mpath->dst, mpath->sdata);
> -	}
> -	rcu_read_unlock();
> -}
> -
>  static void mesh_path_node_reclaim(struct rcu_head *rp)
>  {
>  	struct mpath_node *node = container_of(rp, struct mpath_node, rcu);
> @@ -845,7 +816,18 @@ static void __mesh_path_del(struct mesh_table *tbl, struct mpath_node *node)
>  	atomic_dec(&tbl->entries);
>  }
>  
> -static void mesh_path_flush(struct ieee80211_sub_if_data *sdata)
> +/**
> + * mesh_path_flush_by_nexthop - Deletes mesh paths if their next hop matches
> + *
> + * @sta - mesh peer to match
> + *
> + * RCU notes: this function is called when a mesh plink transitions from
> + * PLINK_ESTAB to any other state, since PLINK_ESTAB state is the only one that
> + * allows path creation. This will happen before the sta can be freed (because
> + * sta_info_destroy() calls this) so any reader in a rcu read block will be
> + * protected against the plink disappearing.
> + */
> +void mesh_path_flush_by_nexthop(struct sta_info *sta)
>  {
>  	struct mesh_table *tbl;
>  	struct mesh_path *mpath;
> @@ -857,7 +839,7 @@ static void mesh_path_flush(struct ieee80211_sub_if_data *sdata)
>  	tbl = rcu_dereference(mesh_paths);
>  	for_each_mesh_entry(tbl, p, node, i) {
>  		mpath = node->mpath;
> -		if (mpath->sdata == sdata) {
> +		if (rcu_dereference(mpath->next_hop) == sta) {
>  			spin_lock_bh(&tbl->hashwlock[i]);
>  			__mesh_path_del(tbl, node);
>  			spin_unlock_bh(&tbl->hashwlock[i]);
> @@ -866,24 +848,23 @@ static void mesh_path_flush(struct ieee80211_sub_if_data *sdata)
>  	rcu_read_unlock();
>  }
>  
> -static void mpp_path_flush(struct ieee80211_sub_if_data *sdata)
> +static void table_flush_by_iface(struct mesh_table *tbl,
> +				 struct ieee80211_sub_if_data *sdata)
>  {
> -	struct mesh_table *tbl;
>  	struct mesh_path *mpath;
>  	struct mpath_node *node;
>  	struct hlist_node *p;
>  	int i;
>  
> -	read_lock_bh(&pathtbl_resize_lock);
> -	tbl = rcu_dereference_protected(mpp_paths,
> -					lockdep_is_held(pathtbl_resize_lock));
> +	WARN_ON(!rcu_read_lock_held());
>  	for_each_mesh_entry(tbl, p, node, i) {
>  		mpath = node->mpath;
> +		if (mpath->sdata != sdata)
> +			continue;
>  		spin_lock_bh(&tbl->hashwlock[i]);
>  		__mesh_path_del(tbl, node);
>  		spin_unlock_bh(&tbl->hashwlock[i]);
>  	}
> -	read_unlock_bh(&pathtbl_resize_lock);
>  }

So what protects against the table being grown at the same time? A copy
will be made, but here you'll be iterating the old table -- which won't
crash or anything but is semantically incorrect.

johannes