Subject: Re: [PATCH net-next v2 6/10] bonding: rebuild the lock use for bond_activebackup_arp_mon()

From: Ding Tianhong <[email-masked]>
Date: Sat, 09 Nov 2013 22:08:53 +0800

于 2013/11/9 0:01, Nikolay Aleksandrov 写道:
> On 11/08/2013 03:08 AM, Ding Tianhong wrote:
>> The bond_activebackup_arp_mon() use the bond lock for read to
>> protect the slave list, it is no effect, and the RTNL is only
>> called for bond_ab_arp_commit() and peer notify, for the performance
>> better, use RCU instead of the bond lock, because the bond slave
>> list need to called in RCU, add a new bond_first_slave_rcu()
>> to get the first slave in RCU protection.
>>
>> When bond_ab_arp_inspect() and should_notify_peers is true, the
>> RTNL will called twice, it is a loss of performance, so make the
>> two RTNL together to avoid performance loss.
>>
>> Suggested-by: Jay Vosburgh <[email-masked]>
>> Suggested-by: Veaceslav Falico <[email-masked]>
>> Signed-off-by: Ding Tianhong <[email-masked]>
>> ---
>>   drivers/net/bonding/bond_main.c | 35 +++++++++++++++++++----------------
>>   drivers/net/bonding/bonding.h   |  7 +++++++
>>   2 files changed, 26 insertions(+), 16 deletions(-)
>>
>> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
>> index 759dcd0..b48ca55 100644
>> --- a/drivers/net/bonding/bond_main.c
>> +++ b/drivers/net/bonding/bond_main.c
>> @@ -2524,7 +2524,7 @@ static int bond_ab_arp_inspect(struct bonding *bond)
>>   	struct slave *slave;
>>   	int commit = 0;
>>   
>> -	bond_for_each_slave(bond, slave, iter) {
>> +	bond_for_each_slave_rcu(bond, slave, iter) {
>>   		slave->new_link = BOND_LINK_NOCHANGE;
>>   		last_rx = slave_last_rx(bond, slave);
>>   
>> @@ -2586,7 +2586,7 @@ static int bond_ab_arp_inspect(struct bonding *bond)
>>    * Called to commit link state changes noted by inspection step of
>>    * active-backup mode ARP monitor.
>>    *
>> - * Called with RTNL and bond->lock for read.
>> + * Called with RTNL hold.
>>    */
>>   static void bond_ab_arp_commit(struct bonding *bond)
>>   {
>> @@ -2661,7 +2661,7 @@ do_failover:
>>   /*
>>    * Send ARP probes for active-backup mode ARP monitor.
>>    *
>> - * Called with bond->lock held for read.
>> + * Called with rcu_read_lock hold.
>>    */
>>   static void bond_ab_arp_probe(struct bonding *bond)
>>   {
>> @@ -2690,14 +2690,14 @@ static void bond_ab_arp_probe(struct bonding *bond)
>>   	 */
>>   
>>   	if (!bond->current_arp_slave) {
>> -		bond->current_arp_slave = bond_first_slave(bond);
>> +		bond->current_arp_slave = bond_first_slave_rcu(bond);
>>   		if (!bond->current_arp_slave)
>>   			return;
>>   	}
>>   
>>   	bond_set_slave_inactive_flags(bond->current_arp_slave);
>>   
>> -	bond_for_each_slave(bond, slave, iter) {
>> +	bond_for_each_slave_rcu(bond, slave, iter) {
>>   		if (!found && !before && IS_UP(slave->dev))
>>   			before = slave;
>>   
>> @@ -2745,43 +2745,46 @@ void bond_activebackup_arp_mon(struct work_struct *work)
>>   	bool should_notify_peers = false;
>>   	int delta_in_ticks;
>>   
>> -	read_lock(&bond->lock);
>> -
>>   	delta_in_ticks = msecs_to_jiffies(bond->params.arp_interval);
>>   
>> -	if (!bond_has_slaves(bond))
>> +	rcu_read_lock();
>> +
>> +	if (!bond_has_slaves_rcu(bond)) {
>> +		rcu_read_unlock();
>>   		goto re_arm;
>> +	}
>>   
>>   	should_notify_peers = bond_should_notify_peers(bond);
> Again, bond_should_notify_peers() is not RCU-safe.

yes.
>>   
>>   	if (bond_ab_arp_inspect(bond)) {
>> -		read_unlock(&bond->lock);
>> +		rcu_read_unlock();
>>   
>>   		/* Race avoidance with bond_close flush of workqueue */
>>   		if (!rtnl_trylock()) {
>> -			read_lock(&bond->lock);
>>   			delta_in_ticks = 1;
>>   			should_notify_peers = false;
>>   			goto re_arm;
>>   		}
>>   
>> -		read_lock(&bond->lock);
>> -
>>   		bond_ab_arp_commit(bond);
>>   
>> -		read_unlock(&bond->lock);
>> +		if (should_notify_peers) {
>> +			call_netdevice_notifiers(NETDEV_NOTIFY_PEERS,
>> +					bond->dev);
>> +			should_notify_peers = false;
>> +		}
>> +
>>   		rtnl_unlock();
>> -		read_lock(&bond->lock);
>> +		rcu_read_lock();
>>   	}
>>   
>>   	bond_ab_arp_probe(bond);
> Generally you might be safe in bond_ab_arp_probe() due to the synchronization
> done by netdev_rx_handler_unregister(), but this code may run after that (and
> after the unlinked slave) but before current_arp_slave is set to NULL and thus
> use it. Now I don't see a direct problem with that, only a complication that can
> bite us later. I vaguely remember that I re-worked the bond_ab_arp_probe() and
> the way current_arp_slave works when doing this transition in my patches.

maybe I miss the patch, pls send me the commit and I will check it again.
>> +	rcu_read_unlock();
>>   
>>   re_arm:
>>   	if (bond->params.arp_interval)
>>   		queue_delayed_work(bond->wq, &bond->arp_work, delta_in_ticks);
>>   
>> -	read_unlock(&bond->lock);
>> -
>>   	if (should_notify_peers) {
>>   		if (!rtnl_trylock())
>>   			return;
>> diff --git a/drivers/net/bonding/bonding.h b/drivers/net/bonding/bonding.h
>> index deb9738..90b745c 100644
>> --- a/drivers/net/bonding/bonding.h
>> +++ b/drivers/net/bonding/bonding.h
>> @@ -97,6 +97,13 @@
>>   		netdev_adjacent_get_private(bond_slave_list(bond)->prev) : \
>>   		NULL)
>>   
>> +#define bond_first_slave_rcu(bond) \
>> +	({struct list_head *__ptr = (bond_slave_list(bond)); \
>> +	 struct list_head *__next = ACCESS_ONCE(__ptr->next); \
>> +	 likely(__ptr != __next) ? \
>> +	 netdev_adjacent_get_private_rcu(__next) : NULL; \
>> +	 })
>> +
> Honestly, I don't like this, it sure can be re-written in a more
> straight-forward manner.

ok, I will re-write it and make it more comfortable.

Regards.
Ding

>>   #define bond_is_first_slave(bond, pos) (pos == bond_first_slave(bond))
>>   #define bond_is_last_slave(bond, pos) (pos == bond_last_slave(bond))
>>   
>>
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