netdev - Re: Correct usage of dev_base

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Date:   Mon, 30 Nov 2020 22:46:00 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Jakub Kicinski <kuba@...nel.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 Mon, Nov 30, 2020 at 10:12 PM Vladimir Oltean <olteanv@...il.com> wrote:
>
> On Mon, Nov 30, 2020 at 10:00:16PM +0100, Eric Dumazet wrote:
> > On Mon, Nov 30, 2020 at 9:50 PM Vladimir Oltean <olteanv@...il.com> wrote:
> > >
> > > On Mon, Nov 30, 2020 at 09:43:01PM +0100, Eric Dumazet wrote:
> > > > Understood, but really dev_base_lock can only be removed _after_ we
> > > > convert all usages to something else (mutex based, and preferably not
> > > > the global RTNL)
> > >
> > > Sure.
> > > A large part of getting rid of dev_base_lock seems to be just:
> > > - deleting the bogus usage from mlx4 infiniband and friends
> > > - converting procfs, sysfs and friends to netdev_lists_mutex
> > > - renaming whatever is left into something related to the RFC 2863
> > >   operstate.
> > >
> > > > Focusing on dev_base_lock seems a distraction really.
> > >
> > > Maybe.
> > > But it's going to be awkward to explain in words what the locking rules
> > > are, when the read side can take optionally the dev_base_lock, RCU, or
> > > netdev_lists_lock, and the write side can take optionally the dev_base_lock,
> > > RTNL, or netdev_lists_lock. Not to mention that anybody grepping for
> > > dev_base_lock will see the current usage and not make a lot out of it.
> > >
> > > I'm not really sure how to order this rework to be honest.
> >
> > We can not have a mix of RCU /rwlock/mutex. It must be one, because of
> > bonding/teaming.
> >
> > So all existing uses of rwlock / RCU need to be removed.
> >
> > This is probably not trivial.
>
> Now, "it's going to look nasty" is one thing, whereas "it won't work" is
> completely different. I think it would work though, so could you expand
> on why you're saying we can't have the mix?

You can not use dev_base_lock() or RCU and call an ndo_get_stats64()
that could sleep.

You can not for example start changing bonding, since bond_get_stats()
could be called from non-sleepable context (net/core/net-procfs.c)

I am still referring to your patch adding :

+       if (!rtnl_locked)
+               rtnl_lock();

This is all I said.