netdev - Question about force_primary in bonding driver

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Date:   Wed, 6 Jun 2018 03:18:23 -0700
From:   Xiangning Yu <yuxiangning@...il.com>
To:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Question about force_primary in bonding driver

Hi netdev folks,

While playing with bonding active-standby mode, we found a possible
timing issue that the primary might not initialized in bond_enslave():

        if (bond_uses_primary(bond) && bond->params.primary[0]) {
                /* if there is a primary slave, remember it */
                if (strcmp(bond->params.primary, new_slave->dev->name) == 0) {
                        rcu_assign_pointer(bond->primary_slave, new_slave);
                        bond->force_primary = true;
                }
        }

Hence bond->force_primary won't be set and bond_select_active_slave()
ends up not selecting the primary slave as specified in bond params.

We applied the following patch to make sure force_primary is set when
primary is copied, and it seems to fix the issue. Could you please
provide some comments/thoughts on this?

Thank you very much!

- Xiangning

diff --git a/drivers/net/bonding/bond_options.c
b/drivers/net/bonding/bond_options.c
index 58c705f..b594bae 100644
--- a/drivers/net/bonding/bond_options.c
+++ b/drivers/net/bonding/bond_options.c
@@ -1142,6 +1142,7 @@ static int bond_option_primary_set(struct bonding *bond,
                                   slave->dev->name);
                        rcu_assign_pointer(bond->primary_slave, slave);
                        strcpy(bond->params.primary, slave->dev->name);
+                       bond->force_primary = true;
                        bond_select_active_slave(bond);
                        goto out;
                }