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Date:   Wed, 24 May 2017 22:38:25 -0700
From:   Mahesh Bandewar (महेश बंडेवार) 
        <maheshb@...gle.com>
To:     Nithin Nayak Sujir <nsujir@...tri.com>
Cc:     David Miller <davem@...emloft.net>,
        linux-netdev <netdev@...r.kernel.org>,
        Jay Vosburgh <jay.vosburgh@...onical.com>
Subject: Re: [PATCH] bonding: Don't update slave->link until ready to commit

On Wed, May 24, 2017 at 7:45 PM, Nithin Nayak Sujir <nsujir@...tri.com> wrote:
> In the loadbalance arp monitoring scheme, when a slave link change is
> detected, the slave->link is immediately updated and slave_state_changed
> is set. Later down the function, the rtnl_lock is acquired and the
> changes are committed, updating the bond link state.
>
> However, the acquisition of the rtnl_lock can fail. The next time the
> monitor runs, since slave->link is already updated, it determines that
> link is unchanged. This results in the bond link state permanently out
> of sync with the slave link.
>
> This patch modifies bond_loadbalance_arp_mon() to handle link changes
> identical to bond_ab_arp_{inspect/commit}(). The new link state is
> maintained in slave->new_link until we're ready to commit at which point
> it's copied into slave->link.
>
> NOTE: miimon_{inspect/commit}() has a more complex state machine
> requiring the use of the bond_{propose,commit}_link_state() functions
> which maintains the intermediate state in slave->link_new_state. The arp
> monitors don't require that.
>
> Testing: This bug is very easy to reproduce with the following steps.
> 1. In a loop, toggle a slave link of a bond slave interface.
> 2. In a separate loop, do ifconfig up/down of an unrelated interface to
> create contention for rtnl_lock.
> Within a few iterations, the bond link goes out of sync with the slave
> link.
>
> Signed-off-by: Nithin Nayak Sujir <nsujir@...tri.com>
Acked-by: Mahesh Bandewar <maheshb@...gle.com>
> Cc: Mahesh Bandewar <maheshb@...gle.com>
> Cc: Jay Vosburgh <jay.vosburgh@...onical.com>
> ---
>  drivers/net/bonding/bond_main.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
> index 7331331..2359478b 100644
> --- a/drivers/net/bonding/bond_main.c
> +++ b/drivers/net/bonding/bond_main.c
> @@ -2612,11 +2612,13 @@ static void bond_loadbalance_arp_mon(struct bonding *bond)
>         bond_for_each_slave_rcu(bond, slave, iter) {
>                 unsigned long trans_start = dev_trans_start(slave->dev);
>
> +               slave->new_link = BOND_LINK_NOCHANGE;
> +
>                 if (slave->link != BOND_LINK_UP) {
>                         if (bond_time_in_interval(bond, trans_start, 1) &&
>                             bond_time_in_interval(bond, slave->last_rx, 1)) {
>
> -                               slave->link  = BOND_LINK_UP;
> +                               slave->new_link = BOND_LINK_UP;
>                                 slave_state_changed = 1;
>
>                                 /* primary_slave has no meaning in round-robin
> @@ -2643,7 +2645,7 @@ static void bond_loadbalance_arp_mon(struct bonding *bond)
>                         if (!bond_time_in_interval(bond, trans_start, 2) ||
>                             !bond_time_in_interval(bond, slave->last_rx, 2)) {
>
> -                               slave->link  = BOND_LINK_DOWN;
> +                               slave->new_link = BOND_LINK_DOWN;
>                                 slave_state_changed = 1;
>
>                                 if (slave->link_failure_count < UINT_MAX)
> @@ -2674,6 +2676,11 @@ static void bond_loadbalance_arp_mon(struct bonding *bond)
>                 if (!rtnl_trylock())
>                         goto re_arm;
>
> +               bond_for_each_slave(bond, slave, iter) {
> +                       if (slave->new_link != BOND_LINK_NOCHANGE)
> +                               slave->link = slave->new_link;
> +               }
> +
>                 if (slave_state_changed) {
>                         bond_slave_state_change(bond);
>                         if (BOND_MODE(bond) == BOND_MODE_XOR)
> --
> 2.8.2
>

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