netdev - Re: [PATCH net] net: Fix inconsistent rtnl_lock usage on dev_get

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Date:   Fri, 6 Jan 2017 11:30:09 -0800
From:   Michael Chan <michael.chan@...adcom.com>
To:     David Miller <davem@...emloft.net>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] net: Fix inconsistent rtnl_lock usage on dev_get_stats().

On Fri, Jan 6, 2017 at 10:01 AM, David Miller <davem@...emloft.net> wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Fri, 06 Jan 2017 09:32:56 -0800
>
>> This makes no sense to me.
>>
>> RTNL is absolutely not needed to get device stats.
>>
>> We try to not add RTNL, especially when not required.
>>
>> Sure, RTNETLINK dumps currently hold RTNL, but we had various attempts
>> in the past to get rid of this behavior.
>>
>> If a device driver expects RTNL being locked, it is clearly a bug that
>> needs a fix anyway.
>
> This is extremely problematic when the driver has to synchronize some
> piece of state between the get stats method and open/close.  It is
> exactly the case we are trying to solve in tg3, and lots of drivers
> end up hitting the same exact issue.
>
> If open/close can happen asynchronously to get stats, it is very hard
> to make dynamically allocated data structures or DMA buffers usable
> from the stats call.
>
> Drivers in this situation will just add a mutex specifically for this
> situation if we don't consistently apply RTNL locking here.

The patch doesn't work anyway in the net-procfs code path upon closer
examination.  Because we are using seq_ops and taking the RCU lock at
the beginning of the sequence, we cannot take RTNL.  That means
drivers cannot use mutex as well.

For tg3, I think I will just use tp->lock spinlock to protect
get_stats64 and the freeing of the stats memory.