Re: [PATCH] mac80211: add interface list lock

[Ivo van Doorn <ivdoorn@gmail.com>]

On Thursday 22 January 2009, Johannes Berg wrote:
> Using only the RTNL has a number of problems, most notably that
> ieee80211_iterate_active_interfaces() and other interface list
> traversals cannot be done from the internal workqueue because it
> needs to be flushed under the RTNL.
> 
> This patch introduces a new mutex that protects the interface list
> against modifications. A more detailed explanation is part of the
> code change.
> 
> Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
> ---
> Need this for the scan stuff I'm doing next, but this has the side
> effect of allowing you to call ieee80211_iterate_active_interfaces()
> from mac80211's workqueue, which might be interesting. Ivo, that's
> the reason you got a copy of this.

Thanks  :)

When this is merged, I'll see which rt2x00 work structures can be moved
back to the mac80211 workqueue again.

But it probably also mean the kerneldoc for the workqueue field of ieee80211_hw
needs to be changed as well. It now says:

 * @workqueue: single threaded workqueue available for driver use,
 *	allocated by mac80211 on registration and flushed when an
 *	interface is removed.
 *	NOTICE: All work performed on this workqueue should NEVER
 *	acquire the RTNL lock (i.e. Don't use the function
 *	ieee80211_iterate_active_interfaces())


>  net/mac80211/ieee80211_i.h |    2 ++
>  net/mac80211/iface.c       |   31 +++++++++++++++++++++++++++++++
>  net/mac80211/main.c        |    3 +++
>  net/mac80211/util.c        |    4 ++--
>  4 files changed, 38 insertions(+), 2 deletions(-)
> 
> --- wireless-testing.orig/net/mac80211/ieee80211_i.h	2009-01-21 12:48:12.000000000 +0100
> +++ wireless-testing/net/mac80211/ieee80211_i.h	2009-01-21 13:57:49.000000000 +0100
> @@ -643,7 +643,9 @@ struct ieee80211_local {
>  	struct crypto_blkcipher *wep_rx_tfm;
>  	u32 wep_iv;
>  
> +	/* see iface.c */
>  	struct list_head interfaces;
> +	struct mutex iflist_mtx;
>  
>  	/*
>  	 * Key lock, protects sdata's key_list and sta_info's
> --- wireless-testing.orig/net/mac80211/iface.c	2009-01-21 12:48:12.000000000 +0100
> +++ wireless-testing/net/mac80211/iface.c	2009-01-21 13:57:49.000000000 +0100
> @@ -21,6 +21,23 @@
>  #include "mesh.h"
>  #include "led.h"
>  
> +/**
> + * DOC: Interface list locking
> + *
> + * The interface list in each struct ieee80211_local is protected
> + * three-fold:
> + *
> + * (1) modifications may only be done under the RTNL
> + * (2) modifications and readers are protected against each other by
> + *     the iflist_mtx.
> + * (3) modifications are done in an RCU manner so atomic readers
> + *     can traverse the list in RCU-safe blocks.
> + *
> + * As a consequence, reads (traversals) of the list can be protected
> + * by either the RTNL, the iflist_mtx or RCU.
> + */
> +
> +
>  static int ieee80211_change_mtu(struct net_device *dev, int new_mtu)
>  {
>  	int meshhdrlen;
> @@ -800,7 +817,9 @@ int ieee80211_if_add(struct ieee80211_lo
>  					    params->mesh_id_len,
>  					    params->mesh_id);
>  
> +	mutex_lock(&local->iflist_mtx);
>  	list_add_tail_rcu(&sdata->list, &local->interfaces);
> +	mutex_unlock(&local->iflist_mtx);
>  
>  	if (new_dev)
>  		*new_dev = ndev;
> @@ -816,7 +835,10 @@ void ieee80211_if_remove(struct ieee8021
>  {
>  	ASSERT_RTNL();
>  
> +	mutex_lock(&sdata->local->iflist_mtx);
>  	list_del_rcu(&sdata->list);
> +	mutex_unlock(&sdata->local->iflist_mtx);
> +
>  	synchronize_rcu();
>  	unregister_netdevice(sdata->dev);
>  }
> @@ -832,7 +854,16 @@ void ieee80211_remove_interfaces(struct 
>  	ASSERT_RTNL();
>  
>  	list_for_each_entry_safe(sdata, tmp, &local->interfaces, list) {
> +		/*
> +		 * we cannot hold the iflist_mtx across unregister_netdevice,
> +		 * but we only need to hold it for list modifications to lock
> +		 * out readers since we're under the RTNL here as all other
> +		 * writers.
> +		 */
> +		mutex_lock(&local->iflist_mtx);
>  		list_del(&sdata->list);
> +		mutex_unlock(&local->iflist_mtx);
> +
>  		unregister_netdevice(sdata->dev);
>  	}
>  }
> --- wireless-testing.orig/net/mac80211/main.c	2009-01-21 12:48:21.000000000 +0100
> +++ wireless-testing/net/mac80211/main.c	2009-01-21 13:57:50.000000000 +0100
> @@ -718,6 +718,7 @@ struct ieee80211_hw *ieee80211_alloc_hw(
>  	local->hw.conf.radio_enabled = true;
>  
>  	INIT_LIST_HEAD(&local->interfaces);
> +	mutex_init(&local->iflist_mtx);
>  
>  	spin_lock_init(&local->key_lock);
>  
> @@ -968,6 +969,8 @@ void ieee80211_free_hw(struct ieee80211_
>  {
>  	struct ieee80211_local *local = hw_to_local(hw);
>  
> +	mutex_destroy(&local->iflist_mtx);
> +
>  	wiphy_free(local->hw.wiphy);
>  }
>  EXPORT_SYMBOL(ieee80211_free_hw);
> --- wireless-testing.orig/net/mac80211/util.c	2009-01-21 12:48:12.000000000 +0100
> +++ wireless-testing/net/mac80211/util.c	2009-01-21 12:48:21.000000000 +0100
> @@ -459,7 +459,7 @@ void ieee80211_iterate_active_interfaces
>  	struct ieee80211_local *local = hw_to_local(hw);
>  	struct ieee80211_sub_if_data *sdata;
>  
> -	rtnl_lock();
> +	mutex_lock(&local->iflist_mtx);
>  
>  	list_for_each_entry(sdata, &local->interfaces, list) {
>  		switch (sdata->vif.type) {
> @@ -480,7 +480,7 @@ void ieee80211_iterate_active_interfaces
>  				 &sdata->vif);
>  	}
>  
> -	rtnl_unlock();
> +	mutex_unlock(&local->iflist_mtx);
>  }
>  EXPORT_SYMBOL_GPL(ieee80211_iterate_active_interfaces);
>  
> 
> 
>