netdev - Re: [Patch net-next 1/3] net: introduce helper dev_change_tx_queue

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Message-ID: <CAM_iQpWwNnJUqy1tzR57yNc8UGJwecN-QiGgT7tMGMaOuE9zAg@mail.gmail.com>
Date:   Sun, 21 Jan 2018 12:30:20 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [Patch net-next 1/3] net: introduce helper dev_change_tx_queue_len()

On Sat, Jan 20, 2018 at 8:52 PM, John Fastabend
<john.fastabend@...il.com> wrote:
> On 01/19/2018 03:09 PM, Cong Wang wrote:
>> This patch promotes the local change_tx_queue_len() to a core
>> helper function, dev_change_tx_queue_len(), so that rtnetlink
>> and net-sysfs could share the code. This also prepares for the
>> following patch.
>>
>> Note, the -EFAULT in the original code doesn't make sense,
>> we should propagate the errno from notifiers.
>>
>> Cc: John Fastabend <john.fastabend@...il.com>
>> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
>> ---
>
>
> [...]
>
>>  static ssize_t tx_queue_len_store(struct device *dev,
>>                                 struct device_attribute *attr,
>>                                 const char *buf, size_t len)
>> @@ -355,7 +332,7 @@ static ssize_t tx_queue_len_store(struct device *dev,
>>       if (!capable(CAP_NET_ADMIN))
>>               return -EPERM;
>>
>> -     return netdev_store(dev, attr, buf, len, change_tx_queue_len);
>> +     return netdev_store(dev, attr, buf, len, dev_change_tx_queue_len);
>>  }
>
> Is this protected by RTNL lock? If not what happens if this and do_setlink
> both try to change tx queue length at the same time? Seems we could get
> a race with multiple dev_deactivate/dev_activate sequences in-flight in
> the following 2/3 patch.

Surely it is protected by RTNL...

  96         if (!rtnl_trylock())
  97                 return restart_syscall();
  98
  99         if (dev_isalive(netdev)) {
 100                 ret = (*set)(netdev, new);
 101                 if (ret == 0)
 102                         ret = len;
 103         }
 104         rtnl_unlock();