netdev - Re: [PATCH v3 net-next 10/12] net: bonding: ensure .ndo_get

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Date:   Fri, 08 Jan 2021 11:25:38 -0800
From:   Saeed Mahameed <saeed@...nel.org>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>, Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        George McCollister <george.mccollister@...il.com>,
        Oleksij Rempel <o.rempel@...gutronix.de>,
        Jay Vosburgh <j.vosburgh@...il.com>,
        Veaceslav Falico <vfalico@...il.com>,
        Andy Gospodarek <andy@...yhouse.net>,
        Arnd Bergmann <arnd@...db.de>, Taehee Yoo <ap420073@...il.com>,
        Jiri Pirko <jiri@...lanox.com>,
        Florian Westphal <fw@...len.de>,
        Nikolay Aleksandrov <nikolay@...dia.com>
Subject: Re: [PATCH v3 net-next 10/12] net: bonding: ensure .ndo_get_stats64
 can sleep

On Fri, 2021-01-08 at 10:57 +0200, Vladimir Oltean wrote:
> On Thu, Jan 07, 2021 at 07:59:37PM -0800, Saeed Mahameed wrote:
> > On Thu, 2021-01-07 at 13:58 +0100, Eric Dumazet wrote:
> > > On Thu, Jan 7, 2021 at 12:33 PM Vladimir Oltean <
> > > olteanv@...il.com>
> > > wrote:

...

> > > 
> > > > There is an effort initiated by Jakub to standardize the
> > > > ethtool
> > > > statistics. My objection was that you can't expect that to
> > > > happen
> > > > unless
> > > > dev_get_stats is sleepable just like ethtool -S is. So I think
> > > > the
> > > > same
> > > > reasoning should apply to ethtool -S too, really.
> > > 
> > > I think we all agree on the principles, once we make sure to not
> > > add more pressure on RTNL. It seems you addressed our feedback,
> > > all
> > > is fine.
> > > 
> > 
> > Eric, about two years ago you were totally against sleeping in
> > ndo_get_stats, what happened ? :)
> > https://lore.kernel.org/netdev/4cc44e85-cb5e-502c-30f3-c6ea564fe9ac@gmail.com/
> 
> I believe that what is different this time is that DSA switches are
> typically connected over a slow and bottlenecked bus (so periodic
> driver-level readouts would only make things worse for phc2sys and
> such other latency-sensitive programs), plus they are offloading
> interfaces for forwarding (so software-based counters could never be
> accurate). Support those, and supporting firmware-based high-speed
> devices will come as a nice side-effect.
> FWIW that discussion took place here:
> https://patchwork.ozlabs.org/project/netdev/patch/20201125193740.36825-3-george.mccollister@gmail.com/
> 

I understand the motivation and I agree with the concept, 
hence my patchset :)

> > My approach to solve this was much simpler and didn't require  a
> > new
> > mutex nor RTNL lock, all i did is to reduce the rcu critical
> > section to
> > not include the call to the driver by simply holding the netdev via
> > dev_hold()
> 
> I feel this is a call for the bonding maintainers to make. If they're
> willing to replace rtnl_dereference with bond_dereference throughout
> the
> whole driver, and reduce other guys' amount of work when other NDOs
> start losing the rtnl_mutex too, then I can't see what's wrong with
> my
> approach (despite not being "as simple"). If they think that update-
> side
> protection of the slaves array is just fine the way it is, then I
> suppose that RCU protection + dev_hold is indeed all that I can do.

To be honest i haven't really looked at your patches, I just quickly
went through them to get an idea of what you did, but let me take a
more careful look and will give my ultimate feedback.