Subject: Re: [PATCH 2/2] bonding: fix bonding_masters race condition in bond unloading

From: Veaceslav Falico <[email-masked]>
Date: Mon, 8 Apr 2013 12:08:09 +0200

On Mon, Apr 08, 2013 at 11:15:23AM +0200, Nikolay Aleksandrov wrote:
<snip>
>> @@ -4823,9 +4818,9 @@ int bond_create(struct net *net, const char *name)
>>
>>      netif_carrier_off(bond_dev);
>>
>> -    rtnl_unlock();
>>      if (res < 0)
>>          bond_destructor(bond_dev);
>> +
>>      return res;
>>  }
>>
>bond_destructor calls free_netdev, which is usually called without rtnl
>after unregister_netdevice is called under rtnl.
>(net/core/dev.c - free_netdev comments)

It shouldn't be called under rtnl_lock() mainly because of sysfs(), however
with this patch we've added rtnl_trylock() to it and should be safe.

It's already used under rtnl_lock() in several places already, so I think
it's safe.

>> @@ -4879,7 +4874,9 @@ static int __init bonding_init(void)
>>      bond_create_debugfs();
>>
>>      for (i = 0; i < max_bonds; i++) {
>> +        rtnl_lock();
>>          res = bond_create(&init_net, NULL);
>> +        rtnl_unlock();
>>          if (res)
>>              goto err;
>>      }
>> @@ -4901,8 +4898,10 @@ static void __exit bonding_exit(void)
>>
>>      bond_destroy_debugfs();
>>
>> +    rtnl_lock();
>> +    __rtnl_link_unregister(&bond_link_ops);
>>      unregister_pernet_subsys(&bond_net_ops);
>> -    rtnl_link_unregister(&bond_link_ops);
>> +    rtnl_unlock();
>>
>The usual way is to obtain net_mutex and then rtnl,
>this reverses it.

Good point, we might easily deadlock here. I'll dig and come back if I'll
find a way to avoid it...

>>  #ifdef CONFIG_NET_POLL_CONTROLLER
>>      /*
>> diff --git a/drivers/net/bonding/bond_sysfs.c
>> b/drivers/net/bonding/bond_sysfs.c
>> index ea7a388..cd1d60f 100644
>> --- a/drivers/net/bonding/bond_sysfs.c
>> +++ b/drivers/net/bonding/bond_sysfs.c
>> @@ -59,7 +59,8 @@ static ssize_t bonding_show_bonds(struct class *cls,
>>      int res = 0;
>>      struct bonding *bond;
>>
>> -    rtnl_lock();
>> +    if (!rtnl_trylock())
>> +        return restart_syscall();
>>
>>      list_for_each_entry(bond, &bn->dev_list, bond_list) {
>>          if (res > (PAGE_SIZE - IFNAMSIZ)) {
>> @@ -107,6 +108,9 @@ static ssize_t bonding_store_bonds(struct class *cls,
>>      char *ifname;
>>      int rv, res = count;
>>
>> +    if (!rtnl_trylock())
>> +        return restart_syscall();
>> +
>>      sscanf(buffer, "%16s", command); /* IFNAMSIZ*/
>>      ifname = command + 1;
>>      if ((strlen(command) <= 1) ||
>> @@ -126,7 +130,6 @@ static ssize_t bonding_store_bonds(struct class *cls,
>>      } else if (command[0] == '-') {
>>          struct net_device *bond_dev;
>>
>> -        rtnl_lock();
>>          bond_dev = bond_get_by_name(bn, ifname);
>>          if (bond_dev) {
>>              pr_info("%s is being deleted...\n", ifname);
>> @@ -135,10 +138,11 @@ static ssize_t bonding_store_bonds(struct class
>> *cls,
>>              pr_err("unable to delete non-existent %s\n", ifname);
>>              res = -ENODEV;
>>          }
>> -        rtnl_unlock();
>>      } else
>>          goto err_no_cmd;
>>
>> +    rtnl_unlock();
>> +
>>      /* Always return either count or an error.  If you return 0, you'll
>>       * get called forever, which is bad.
>>       */
>> @@ -146,6 +150,7 @@ static ssize_t bonding_store_bonds(struct class *cls,
>>
>>  err_no_cmd:
>>      pr_err("no command found in bonding_masters. Use +ifname or
>> -ifname.\n");
>> +    rtnl_unlock();
>>      return -EPERM;
>>  }
>>
>