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Message-ID: <527D0303.8030806@redhat.com>
Date: Fri, 08 Nov 2013 16:28:03 +0100
From: Nikolay Aleksandrov <nikolay@...hat.com>
To: Ding Tianhong <dingtianhong@...wei.com>
CC: Jay Vosburgh <fubar@...ibm.com>,
Andy Gospodarek <andy@...yhouse.net>,
"David S. Miller" <davem@...emloft.net>,
Veaceslav Falico <vfalico@...hat.com>,
Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 2/10] bonding: rebuild the lock use for bond_mii_monitor()
On 11/08/2013 03:07 AM, Ding Tianhong wrote:
> The bond_mii_monitor() still use bond lock to protect bond slave list,
> it is no effect, I have 2 way to fix the problem, move the RTNL to the
> top of the function, or add RCU to protect the bond_has_slaves() and
> bond_miimon_inspect(), according to the Jay Vosburgh's opinion, 10 times
> one second is a truely big performance loss if use RTNL to protect the
> whole function, so I would take the advice and use RCU to protect the
> two functions, of course it need to add more modify, the
> bond_has_slave_rcu() is add for RCU use, and the bond_for_each_slave
> need to replace with bond_for_each_slave_rcu in bond_miimon_inspect.
>
> I move the peer notify before the queue_delayed_work(), and obviously
> it is no need to lock the RTNL twice if call bond_miimon_commit() and
> peer notify together, other path is no logic change, I think the
> performance is better than before.
>
> Suggested-by: Jay Vosburgh <fubar@...ibm.com>
> Suggested-by: Veaceslav Falico <vfalico@...hat.com>
> Signed-off-by: Ding Tianhong <dingtianhong@...wei.com>
> ---
> drivers/net/bonding/bond_main.c | 40 ++++++++++++++++++++--------------------
> drivers/net/bonding/bonding.h | 6 ++++++
> 2 files changed, 26 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
> index ba18719..def489d 100644
> --- a/drivers/net/bonding/bond_main.c
> +++ b/drivers/net/bonding/bond_main.c
> @@ -1913,7 +1913,7 @@ static int bond_miimon_inspect(struct bonding *bond)
>
> ignore_updelay = !bond->curr_active_slave ? true : false;
>
> - bond_for_each_slave(bond, slave, iter) {
> + bond_for_each_slave_rcu(bond, slave, iter) {
> slave->new_link = BOND_LINK_NOCHANGE;
>
> link_state = bond_check_dev_link(bond, slave->dev, 0);
> @@ -2111,47 +2111,47 @@ void bond_mii_monitor(struct work_struct *work)
> bool should_notify_peers = false;
> unsigned long delay;
>
> - read_lock(&bond->lock);
> -
> delay = msecs_to_jiffies(bond->params.miimon);
>
> - if (!bond_has_slaves(bond))
> + rcu_read_lock();
> +
> + if (!bond_has_slaves_rcu(bond)) {
> + rcu_read_unlock();
In fact the bond cannot disappear while this function is running, so this test
should be able to run outside the RCU region if I'm not missing something :-)
It'll be just as useful as running inside the region, at most a free run may
happen if there's one slave and it disappears.
> goto re_arm;
> + }
>
> should_notify_peers = bond_should_notify_peers(bond);
bond_should_notify_peers() is not RCU-safe, it uses curr_active_slave directly.
>
> if (bond_miimon_inspect(bond)) {
> - read_unlock(&bond->lock);
> + rcu_read_unlock();
>
> /* Race avoidance with bond_close cancel of workqueue */
> if (!rtnl_trylock()) {
> - read_lock(&bond->lock);
> delay = 1;
> - should_notify_peers = false;
> goto re_arm;
> }
>
> - read_lock(&bond->lock);
> -
> bond_miimon_commit(bond);
>
> - read_unlock(&bond->lock);
> + if (should_notify_peers)
> + call_netdevice_notifiers(NETDEV_NOTIFY_PEERS,
> + bond->dev);
> +
> rtnl_unlock(); /* might sleep, hold no other locks */
> - read_lock(&bond->lock);
> + } else {
> + rcu_read_unlock();
> + if (should_notify_peers) {
> + if (!rtnl_trylock())
> + goto re_arm;
> + call_netdevice_notifiers(NETDEV_NOTIFY_PEERS,
> + bond->dev);
> + rtnl_unlock();
> + }
> }
>
> re_arm:
> if (bond->params.miimon)
> queue_delayed_work(bond->wq, &bond->mii_work, delay);
> -
> - read_unlock(&bond->lock);
> -
> - if (should_notify_peers) {
> - if (!rtnl_trylock())
> - return;
> - call_netdevice_notifiers(NETDEV_NOTIFY_PEERS, bond->dev);
> - rtnl_unlock();
> - }
> }
>
> static bool bond_has_this_ip(struct bonding *bond, __be32 ip)
> diff --git a/drivers/net/bonding/bonding.h b/drivers/net/bonding/bonding.h
> index 046a605..deb9738 100644
> --- a/drivers/net/bonding/bonding.h
> +++ b/drivers/net/bonding/bonding.h
> @@ -81,6 +81,12 @@
>
> #define bond_has_slaves(bond) !list_empty(bond_slave_list(bond))
>
> +#define bond_has_slaves_rcu(bond) \
> + ({struct list_head *__ptr = (bond_slave_list(bond)); \
> + struct list_head *__next = ACCESS_ONCE(__ptr->next); \
> + __ptr != __next; \
> + })
> +
This is unnecessary, bond_has_slaves() should be enough. See bond_start_xmit()
and also the list_empty comment in include/linux/rculist.h for more information why.
My bond_has_slaves() comments apply to all the patches that use it.
> /* IMPORTANT: bond_first/last_slave can return NULL in case of an empty list */
> #define bond_first_slave(bond) \
> (bond_has_slaves(bond) ? \
>
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