netdev - Re: Correct usage of dev_base

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Date:   Mon, 30 Nov 2020 21:18:01 +0100
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        netdev <netdev@...r.kernel.org>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Jiri Benc <jbenc@...hat.com>,
        Or Gerlitz <ogerlitz@...lanox.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>
Subject: Re: Correct usage of dev_base_lock in 2020



On 11/30/20 8:46 PM, Vladimir Oltean wrote:
> On Mon, Nov 30, 2020 at 08:22:01PM +0100, Eric Dumazet wrote:
>> And ?
>>
>> A bonding device can absolutely maintain a private list, ready for
>> bonding ndo_get_stats() use, regardless
>> of register/unregister logic.
>>
>> bond_for_each_slave() is simply a macro, you can replace it by something else.
> 
> Also, coming to take the comment at face value.
> Can it really? How? Freeing a net_device at unregister time happens
> after an RCU grace period.

Except that the device would have to be removed from the bonding list
before the RCU grace period starts.

This removal would acquire the bonding ->stats_mutex in order to change the list.

 So whatever the bonding driver does to keep a
> private list of slave devices, those pointers need to be under RCU
> protection.


Not at all, if this new list is _only_ used from process context,
and protected by a per-device mutex.

I am not speaking of existing lists that _possibly_ are
used from IRQ context, thus are using RCU.


 And that doesn't help with the sleepable context that we're
> looking for.
>

Again, RCU would not be used at all, since you want ndo_get_stats64()
being called in process context (sleepable context)

And this should be solved without expanding RTNL usage.
(We do not want to block RTNL for ~10ms just because a device driver has to sleep
while a firmware request is processed)