Subject: Re: [PATCH v2 net-next 2/2] bonding: restructure locking of bond_ab_arp_probe()

From: Veaceslav Falico <[email-masked]>
Date: Thu, 23 Jan 2014 22:56:15 +0100

On Thu, Jan 23, 2014 at 11:25:38AM -0800, Jay Vosburgh wrote:
>Veaceslav Falico <[email-masked]> wrote:
>
>>Currently we're calling it from under RCU context, however we're using some
>>functions that require rtnl to be held.
>>
>>Fix this by restructuring the locking - don't call it under any locks,
>>aquire rcu_read_lock() if we're sending _only_ (i.e. we have the active
>>slave present), and use rtnl locking otherwise - if we need to modify
>>(in)active flags of a slave.
>>
>>CC: Jay Vosburgh <[email-masked]>
>>CC: Andy Gospodarek <[email-masked]>
>>Signed-off-by: Veaceslav Falico <[email-masked]>
>>---
>> drivers/net/bonding/bond_main.c | 38 ++++++++++++++++++++++++--------------
>> 1 file changed, 24 insertions(+), 14 deletions(-)
>>
>>diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
>>index 22d8b69..f879e9e 100644
>>--- a/drivers/net/bonding/bond_main.c
>>+++ b/drivers/net/bonding/bond_main.c
>>@@ -2605,11 +2605,14 @@ do_failover:
>> static void bond_ab_arp_probe(struct bonding *bond)
>> {
>> 	struct slave *slave, *before = NULL, *new_slave = NULL,
>>-		     *curr_arp_slave = rcu_dereference(bond->current_arp_slave),
>>-		     *curr_active_slave = rcu_dereference(bond->curr_active_slave);
>>+		     *curr_arp_slave, *curr_active_slave;
>> 	struct list_head *iter;
>> 	bool found = false;
>>
>>+	rcu_read_lock();
>>+	curr_arp_slave = rcu_dereference(bond->current_arp_slave);
>>+	curr_active_slave = rcu_dereference(bond->curr_active_slave);
>>+
>> 	if (curr_arp_slave && curr_active_slave)
>> 		pr_info("PROBE: c_arp %s && cas %s BAD\n",
>> 			curr_arp_slave->dev->name,
>>@@ -2617,23 +2620,31 @@ static void bond_ab_arp_probe(struct bonding *bond)
>>
>> 	if (curr_active_slave) {
>> 		bond_arp_send_all(bond, curr_active_slave);
>>+		rcu_read_unlock();
>> 		return;
>> 	}
>>+	rcu_read_unlock();
>>
>> 	/* if we don't have a curr_active_slave, search for the next available
>> 	 * backup slave from the current_arp_slave and make it the candidate
>> 	 * for becoming the curr_active_slave
>> 	 */
>>
>>+	rtnl_lock();
>
>	I don't believe we can unconditionally acquire RTNL here, as it
>may deadlock with bond_close's cancel_delayed_work_sync() calls (which
>occur under RTNL).
>
>	I think I'd make this a rtnl_trylock, and if that fails, have
>the bond_ab_arp_probe function return non-zero as an indication for
>activebackup_arp_mon to change delta_in_ticks to 1.  Changing the delta
>is just an optimization; without it, things will still work, but it will
>take longer to run the curr_arp_slave probe cycle.

True, missed this race. Will re-send once I'll fix it.

Thank you!

>
>	-J
>
>>+	/* curr_arp_slave might have gone away */
>>+	curr_arp_slave = rcu_dereference(bond->current_arp_slave);
>>+
>> 	if (!curr_arp_slave) {
>>-		curr_arp_slave = bond_first_slave_rcu(bond);
>>-		if (!curr_arp_slave)
>>+		curr_arp_slave = bond_first_slave(bond);
>>+		if (!curr_arp_slave) {
>>+			rtnl_unlock();
>> 			return;
>>+		}
>> 	}
>>
>> 	bond_set_slave_inactive_flags(curr_arp_slave);
>>
>>-	bond_for_each_slave_rcu(bond, slave, iter) {
>>+	bond_for_each_slave(bond, slave, iter) {
>> 		if (!found && !before && IS_UP(slave->dev))
>> 			before = slave;
>>
>>@@ -2663,21 +2674,24 @@ static void bond_ab_arp_probe(struct bonding *bond)
>> 	if (!new_slave && before)
>> 		new_slave = before;
>>
>>-	if (!new_slave)
>>+	if (!new_slave) {
>>+		rtnl_unlock();
>> 		return;
>>+	}
>>
>> 	new_slave->link = BOND_LINK_BACK;
>> 	bond_set_slave_active_flags(new_slave);
>> 	bond_arp_send_all(bond, new_slave);
>> 	new_slave->jiffies = jiffies;
>> 	rcu_assign_pointer(bond->current_arp_slave, new_slave);
>>+	rtnl_unlock();
>> }
>>
>> static void bond_activebackup_arp_mon(struct work_struct *work)
>> {
>> 	struct bonding *bond = container_of(work, struct bonding,
>> 					    arp_work.work);
>>-	bool should_notify_peers = false;
>>+	bool should_notify_peers = false, should_commit = false;
>> 	int delta_in_ticks;
>>
>> 	delta_in_ticks = msecs_to_jiffies(bond->params.arp_interval);
>>@@ -2686,12 +2700,11 @@ static void bond_activebackup_arp_mon(struct work_struct *work)
>> 		goto re_arm;
>>
>> 	rcu_read_lock();
>>-
>> 	should_notify_peers = bond_should_notify_peers(bond);
>>+	should_commit = bond_ab_arp_inspect(bond);
>>+	rcu_read_unlock();
>>
>>-	if (bond_ab_arp_inspect(bond)) {
>>-		rcu_read_unlock();
>>-
>>+	if (should_commit) {
>> 		/* Race avoidance with bond_close flush of workqueue */
>> 		if (!rtnl_trylock()) {
>> 			delta_in_ticks = 1;
>>@@ -2700,13 +2713,10 @@ static void bond_activebackup_arp_mon(struct work_struct *work)
>> 		}
>>
>> 		bond_ab_arp_commit(bond);
>>-
>> 		rtnl_unlock();
>>-		rcu_read_lock();
>> 	}
>>
>> 	bond_ab_arp_probe(bond);
>>-	rcu_read_unlock();
>>
>> re_arm:
>> 	if (bond->params.arp_interval)
>>--
>>1.8.4
>>
>
>---
>	-Jay Vosburgh, IBM Linux Technology Center, [email-masked]
>