Date:	Thu, 6 Sep 2007 08:46:12 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>, satyam@...radead.org,
	flo@...822.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, linux-wireless@...r.kernel.org,
	michal.k.k.piotrowski@...il.com,
	ipw3945-devel@...ts.sourceforge.net, yi.zhu@...el.com,
	flamingice@...rmilk.net
Subject: Re: BUG: scheduling while atomic: ifconfig/0x00000002/4170

On Thu, Sep 06, 2007 at 03:36:55PM +0200, Johannes Berg wrote:
> On Thu, 2007-09-06 at 20:36 +0800, Herbert Xu wrote:
> 
> > Yeah I think they're all under RTNL too.  So you don't need to
> > take the lock here at all since you should already have the RTNL.
> 
> Ok, this patch gets rid of the lock. I'd appreciate if you could give it
> a quick look for obvious RCU abuse as I haven't tested it. It also
> doesn't apply against net-2.6.24 because I based it after the patches I
> have queued with John for net-2.6.24.

Looks good to me from an RCU viewpoint.  I cannot claim familiarity with
this code.  I therefore especially like the indications of where RTNL
is held and not!!!

Some questions below based on a quick scan.  And a global question:
should the comments about RTNL being held be replaced by ASSERT_RTNL()?

						Thanx, Paul

> johannes
> 
> --- netdev-2.6.orig/net/mac80211/ieee80211.c	2007-09-06 15:35:23.324447686 +0200
> +++ netdev-2.6/net/mac80211/ieee80211.c	2007-09-06 15:35:23.394447686 +0200
> @@ -90,8 +90,9 @@ static struct dev_mc_list *ieee80211_get
> 
>  	/* start of iteration, both unassigned */
>  	if (!mcd->cur && !mcd->sdata) {
> -		mcd->sdata = list_entry(local->sub_if_list.next,
> -					struct ieee80211_sub_if_data, list);
> +		mcd->sdata = list_entry(
> +				rcu_dereference(local->interfaces.next),
> +				struct ieee80211_sub_if_data, list);
>  		mcd->cur = mcd->sdata->dev->mc_list;
>  	}
> 
> @@ -100,10 +101,11 @@ static struct dev_mc_list *ieee80211_get
> 
>  	while (!mcd->cur) {
>  		/* reached end of interface list? */
> -		if (mcd->sdata->list.next == &local->sub_if_list)
> +		if (rcu_dereference(mcd->sdata->list.next) ==
> +		    &local->interfaces)
>  			break;
>  		/* otherwise try next interface */
> -		mcd->sdata = list_entry(mcd->sdata->list.next,
> +		mcd->sdata = list_entry(rcu_dereference(mcd->sdata->list.next),
>  					struct ieee80211_sub_if_data, list);
>  		mcd->cur = mcd->sdata->dev->mc_list;
>  	}
> @@ -145,9 +147,10 @@ static void ieee80211_configure_filter(s
> 
>  	/*
>  	 * We can iterate through the device list for the multicast
> -	 * address list so need to lock it.
> +	 * address list so need to be in a RCU read-side section,
> +	 * the RTNL isn't held in this function.
>  	 */
> -	read_lock(&local->sub_if_lock);
> +	rcu_read_lock();
> 
>  	/* be a bit nasty */
>  	new_flags |= (1<<31);
> @@ -163,7 +166,7 @@ static void ieee80211_configure_filter(s
>  	WARN_ON(mcd.cur);
> 
>  	local->filter_flags = new_flags & ~(1<<31);
> -	read_unlock(&local->sub_if_lock);
> +	rcu_read_unlock();
> 
>  	netif_tx_unlock(local->mdev);
>  }
> @@ -176,14 +179,13 @@ static int ieee80211_master_open(struct 
>  	struct ieee80211_sub_if_data *sdata;
>  	int res = -EOPNOTSUPP;
> 
> -	read_lock(&local->sub_if_lock);
> -	list_for_each_entry(sdata, &local->sub_if_list, list) {
> +	/* we hold the RTNL here so can safely walk the list */
> +	list_for_each_entry(sdata, &local->interfaces, list) {
>  		if (sdata->dev != dev && netif_running(sdata->dev)) {
>  			res = 0;
>  			break;
>  		}
>  	}
> -	read_unlock(&local->sub_if_lock);
>  	return res;
>  }
> 
> @@ -192,11 +194,10 @@ static int ieee80211_master_stop(struct 
>  	struct ieee80211_local *local = wdev_priv(dev->ieee80211_ptr);
>  	struct ieee80211_sub_if_data *sdata;
> 
> -	read_lock(&local->sub_if_lock);
> -	list_for_each_entry(sdata, &local->sub_if_list, list)
> +	/* we hold the RTNL here so can safely walk the list */
> +	list_for_each_entry(sdata, &local->interfaces, list)
>  		if (sdata->dev != dev && netif_running(sdata->dev))
>  			dev_close(sdata->dev);
> -	read_unlock(&local->sub_if_lock);
> 
>  	return 0;
>  }
> @@ -430,8 +431,8 @@ static int ieee80211_open(struct net_dev
> 
>  	sdata = IEEE80211_DEV_TO_SUB_IF(dev);
> 
> -	read_lock(&local->sub_if_lock);
> -	list_for_each_entry(nsdata, &local->sub_if_list, list) {
> +	/* we hold the RTNL here so can safely walk the list */
> +	list_for_each_entry(nsdata, &local->interfaces, list) {
>  		struct net_device *ndev = nsdata->dev;
> 
>  		if (ndev != dev && ndev != local->mdev && netif_running(ndev) &&
> @@ -440,10 +441,8 @@ static int ieee80211_open(struct net_dev
>  			 * check whether it may have the same address
>  			 */
>  			if (!identical_mac_addr_allowed(sdata->type,
> -							nsdata->type)) {
> -				read_unlock(&local->sub_if_lock);
> +							nsdata->type))
>  				return -ENOTUNIQ;
> -			}
> 
>  			/*
>  			 * can only add VLANs to enabled APs
> @@ -454,7 +453,6 @@ static int ieee80211_open(struct net_dev
>  				sdata->u.vlan.ap = nsdata;
>  		}
>  	}
> -	read_unlock(&local->sub_if_lock);
> 
>  	switch (sdata->type) {
>  	case IEEE80211_IF_TYPE_WDS:
> @@ -575,14 +573,13 @@ static int ieee80211_stop(struct net_dev
>  		sdata->u.sta.state = IEEE80211_DISABLED;
>  		del_timer_sync(&sdata->u.sta.timer);
>  		/*
> -		 * Holding the sub_if_lock for writing here blocks
> -		 * out the receive path and makes sure it's not
> -		 * currently processing a packet that may get
> -		 * added to the queue.
> +		 * When we get here, the interface is marked down.
> +		 * Call synchronize_rcu() to wait for the RX path
> +		 * should it be using the interface and enqueuing
> +		 * frames at this very time on another CPU.
>  		 */
> -		write_lock_bh(&local->sub_if_lock);
> +		synchronize_rcu();
>  		skb_queue_purge(&sdata->u.sta.skb_queue);
> -		write_unlock_bh(&local->sub_if_lock);
> 
>  		if (!local->ops->hw_scan &&
>  		    local->scan_dev == sdata->dev) {
> @@ -1134,9 +1131,9 @@ void ieee80211_tx_status(struct ieee8021
> 
>  	rthdr->data_retries = status->retry_count;
> 
> -	read_lock(&local->sub_if_lock);
> +	rcu_read_lock();
>  	monitors = local->monitors;
> -	list_for_each_entry(sdata, &local->sub_if_list, list) {
> +	list_for_each_entry_rcu(sdata, &local->interfaces, list) {
>  		/*
>  		 * Using the monitors counter is possibly racy, but
>  		 * if the value is wrong we simply either clone the skb
> @@ -1152,7 +1149,7 @@ void ieee80211_tx_status(struct ieee8021
>  				continue;
>  			monitors--;
>  			if (monitors)
> -				skb2 = skb_clone(skb, GFP_KERNEL);
> +				skb2 = skb_clone(skb, GFP_ATOMIC);
>  			else
>  				skb2 = NULL;
>  			skb->dev = sdata->dev;
> @@ -1167,7 +1164,7 @@ void ieee80211_tx_status(struct ieee8021
>  		}
>  	}
>   out:
> -	read_unlock(&local->sub_if_lock);
> +	rcu_read_unlock();
>  	if (skb)
>  		dev_kfree_skb(skb);
>  }
> @@ -1255,8 +1252,7 @@ struct ieee80211_hw *ieee80211_alloc_hw(
> 
>  	INIT_LIST_HEAD(&local->modes_list);
> 
> -	rwlock_init(&local->sub_if_lock);
> -	INIT_LIST_HEAD(&local->sub_if_list);
> +	INIT_LIST_HEAD(&local->interfaces);
> 
>  	INIT_DELAYED_WORK(&local->scan_work, ieee80211_sta_scan_work);
>  	ieee80211_rx_bss_list_init(mdev);
> @@ -1275,7 +1271,8 @@ struct ieee80211_hw *ieee80211_alloc_hw(
>  	sdata->u.ap.force_unicast_rateidx = -1;
>  	sdata->u.ap.max_ratectrl_rateidx = -1;
>  	ieee80211_if_sdata_init(sdata);
> -	list_add_tail(&sdata->list, &local->sub_if_list);
> +	/* no RCU needed since we're still during init phase */
> +	list_add_tail(&sdata->list, &local->interfaces);
> 
>  	tasklet_init(&local->tx_pending_tasklet, ieee80211_tx_pending,
>  		     (unsigned long)local);
> @@ -1434,7 +1431,6 @@ void ieee80211_unregister_hw(struct ieee
>  {
>  	struct ieee80211_local *local = hw_to_local(hw);
>  	struct ieee80211_sub_if_data *sdata, *tmp;
> -	struct list_head tmp_list;
>  	int i;
> 
>  	tasklet_kill(&local->tx_pending_tasklet);
> @@ -1448,11 +1444,12 @@ void ieee80211_unregister_hw(struct ieee
>  	if (local->apdev)
>  		ieee80211_if_del_mgmt(local);
> 
> -	write_lock_bh(&local->sub_if_lock);
> -	list_replace_init(&local->sub_if_list, &tmp_list);
> -	write_unlock_bh(&local->sub_if_lock);
> -
> -	list_for_each_entry_safe(sdata, tmp, &tmp_list, list)
> +	/*
> +	 * At this point, interface list manipulations are fine
> +	 * because the driver cannot be handing us frames any
> +	 * more and the tasklet is killed.
> +	 */
> +	list_for_each_entry_safe(sdata, tmp, &local->interfaces, list)
>  		__ieee80211_if_del(local, sdata);
> 
>  	rtnl_unlock();
> --- netdev-2.6.orig/net/mac80211/ieee80211_i.h	2007-09-06 15:35:23.334447686 +0200
> +++ netdev-2.6/net/mac80211/ieee80211_i.h	2007-09-06 15:35:23.404447686 +0200
> @@ -481,9 +481,8 @@ struct ieee80211_local {
>  	ieee80211_rx_handler *rx_handlers;
>  	ieee80211_tx_handler *tx_handlers;
> 
> -	rwlock_t sub_if_lock; /* Protects sub_if_list. Cannot be taken under
> -			       * sta_bss_lock or sta_lock. */
> -	struct list_head sub_if_list;
> +	struct list_head interfaces;
> +
>  	int sta_scanning;
>  	int scan_channel_idx;
>  	enum { SCAN_SET_CHANNEL, SCAN_SEND_PROBE } scan_state;
> --- netdev-2.6.orig/net/mac80211/ieee80211_iface.c	2007-09-06 15:35:23.334447686 +0200
> +++ netdev-2.6/net/mac80211/ieee80211_iface.c	2007-09-06 15:35:23.404447686 +0200
> @@ -79,16 +79,15 @@ int ieee80211_if_add(struct net_device *
>  	ieee80211_debugfs_add_netdev(sdata);
>  	ieee80211_if_set_type(ndev, type);
> 
> -	write_lock_bh(&local->sub_if_lock);
> +	/* we're under RTNL so all this is fine */
>  	if (unlikely(local->reg_state == IEEE80211_DEV_UNREGISTERED)) {
> -		write_unlock_bh(&local->sub_if_lock);
>  		__ieee80211_if_del(local, sdata);
>  		return -ENODEV;
>  	}
> -	list_add(&sdata->list, &local->sub_if_list);
> +	list_add_tail_rcu(&sdata->list, &local->interfaces);

The _rcu is required because this list isn't protected by RTNL?

> +
>  	if (new_dev)
>  		*new_dev = ndev;
> -	write_unlock_bh(&local->sub_if_lock);
> 
>  	return 0;
> 
> @@ -226,22 +225,22 @@ void ieee80211_if_reinit(struct net_devi
>  		/* Remove all virtual interfaces that use this BSS
>  		 * as their sdata->bss */
>  		struct ieee80211_sub_if_data *tsdata, *n;
> -		LIST_HEAD(tmp_list);
> 
> -		write_lock_bh(&local->sub_if_lock);

This code is also protected by RTNL?

> -		list_for_each_entry_safe(tsdata, n, &local->sub_if_list, list) {
> +		list_for_each_entry_safe(tsdata, n, &local->interfaces, list) {
>  			if (tsdata != sdata && tsdata->bss == &sdata->u.ap) {
>  				printk(KERN_DEBUG "%s: removing virtual "
>  				       "interface %s because its BSS interface"
>  				       " is being removed\n",
>  				       sdata->dev->name, tsdata->dev->name);
> -				list_move_tail(&tsdata->list, &tmp_list);
> +				list_del_rcu(&tsdata->list);
> +				/*
> +				 * We have lots of time and can afford
> +				 * to sync for each interface
> +				 */
> +				synchronize_rcu();
> +				__ieee80211_if_del(local, tsdata);
>  			}
>  		}
> -		write_unlock_bh(&local->sub_if_lock);
> -
> -		list_for_each_entry_safe(tsdata, n, &tmp_list, list)
> -			__ieee80211_if_del(local, tsdata);
> 
>  		kfree(sdata->u.ap.beacon_head);
>  		kfree(sdata->u.ap.beacon_tail);
> @@ -318,18 +317,16 @@ int ieee80211_if_remove(struct net_devic
> 
>  	ASSERT_RTNL();

I -like- this!!!  ;-)

> -	write_lock_bh(&local->sub_if_lock);
> -	list_for_each_entry_safe(sdata, n, &local->sub_if_list, list) {
> +	list_for_each_entry_safe(sdata, n, &local->interfaces, list) {
>  		if ((sdata->type == id || id == -1) &&
>  		    strcmp(name, sdata->dev->name) == 0 &&
>  		    sdata->dev != local->mdev) {
> -			list_del(&sdata->list);
> -			write_unlock_bh(&local->sub_if_lock);
> +			list_del_rcu(&sdata->list);
> +			synchronize_rcu();
>  			__ieee80211_if_del(local, sdata);
>  			return 0;
>  		}
>  	}
> -	write_unlock_bh(&local->sub_if_lock);
>  	return -ENODEV;
>  }
> 
> --- netdev-2.6.orig/net/mac80211/ieee80211_sta.c	2007-09-06 15:35:23.224447686 +0200
> +++ netdev-2.6/net/mac80211/ieee80211_sta.c	2007-09-06 15:35:23.414447686 +0200
> @@ -2659,8 +2659,8 @@ void ieee80211_scan_completed(struct iee
>  	memset(&wrqu, 0, sizeof(wrqu));
>  	wireless_send_event(dev, SIOCGIWSCAN, &wrqu, NULL);
> 
> -	read_lock(&local->sub_if_lock);
> -	list_for_each_entry(sdata, &local->sub_if_list, list) {
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(sdata, &local->interfaces, list) {
> 
>  		/* No need to wake the master device. */
>  		if (sdata->dev == local->mdev)
> @@ -2674,7 +2674,7 @@ void ieee80211_scan_completed(struct iee
> 
>  		netif_wake_queue(sdata->dev);
>  	}
> -	read_unlock(&local->sub_if_lock);
> +	rcu_read_unlock();
> 
>  	sdata = IEEE80211_DEV_TO_SUB_IF(dev);
>  	if (sdata->type == IEEE80211_IF_TYPE_IBSS) {
> @@ -2811,8 +2811,8 @@ static int ieee80211_sta_start_scan(stru
> 
>  	local->sta_scanning = 1;
> 
> -	read_lock(&local->sub_if_lock);
> -	list_for_each_entry(sdata, &local->sub_if_list, list) {
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(sdata, &local->interfaces, list) {
> 
>  		/* Don't stop the master interface, otherwise we can't transmit
>  		 * probes! */
> @@ -2824,7 +2824,7 @@ static int ieee80211_sta_start_scan(stru
>  		    (sdata->u.sta.flags & IEEE80211_STA_ASSOCIATED))
>  			ieee80211_send_nullfunc(local, sdata, 1);
>  	}
> -	read_unlock(&local->sub_if_lock);
> +	rcu_read_unlock();
> 
>  	if (ssid) {
>  		local->scan_ssid_len = ssid_len;
> --- netdev-2.6.orig/net/mac80211/rx.c	2007-09-06 15:35:23.214447686 +0200
> +++ netdev-2.6/net/mac80211/rx.c	2007-09-06 15:35:23.414447686 +0200
> @@ -1385,8 +1385,9 @@ void __ieee80211_rx(struct ieee80211_hw 
>  	}
> 
>  	/*
> -	 * key references are protected using RCU and this requires that
> -	 * we are in a read-site RCU section during receive processing
> +	 * key references and virtual interfaces are protected using RCU
> +	 * and this requires that we are in a read-side RCU section during
> +	 * receive processing
>  	 */
>  	rcu_read_lock();
> 
> @@ -1441,8 +1442,7 @@ void __ieee80211_rx(struct ieee80211_hw 
> 
>  	bssid = ieee80211_get_bssid(hdr, skb->len - radiotap_len);
> 
> -	read_lock(&local->sub_if_lock);
> -	list_for_each_entry(sdata, &local->sub_if_list, list) {
> +	list_for_each_entry_rcu(sdata, &local->interfaces, list) {
>  		rx.flags |= IEEE80211_TXRXD_RXRA_MATCH;
> 
>  		if (!netif_running(sdata->dev))
> @@ -1494,7 +1494,6 @@ void __ieee80211_rx(struct ieee80211_hw 
>  					     &rx, sta);
>  	} else
>  		dev_kfree_skb(skb);
> -	read_unlock(&local->sub_if_lock);
> 
>   end:
>  	rcu_read_unlock();
> --- netdev-2.6.orig/net/mac80211/tx.c	2007-09-06 15:35:23.074447686 +0200
> +++ netdev-2.6/net/mac80211/tx.c	2007-09-06 15:35:23.424447686 +0200
> @@ -291,8 +291,12 @@ static void purge_old_ps_buffers(struct 
>  	struct ieee80211_sub_if_data *sdata;
>  	struct sta_info *sta;
> 
> -	read_lock(&local->sub_if_lock);
> -	list_for_each_entry(sdata, &local->sub_if_list, list) {
> +	/*
> +	 * virtual interfaces are protected by RCU
> +	 */
> +	rcu_read_lock();
> +
> +	list_for_each_entry_rcu(sdata, &local->interfaces, list) {
>  		struct ieee80211_if_ap *ap;
>  		if (sdata->dev == local->mdev ||
>  		    sdata->type != IEEE80211_IF_TYPE_AP)
> @@ -305,7 +309,7 @@ static void purge_old_ps_buffers(struct 
>  		}
>  		total += skb_queue_len(&ap->ps_bc_buf);
>  	}
> -	read_unlock(&local->sub_if_lock);
> +	rcu_read_unlock();
> 
>  	read_lock_bh(&local->sta_lock);
>  	list_for_each_entry(sta, &local->sta_list, list) {
> 
> 
> -
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